These are usually called a Magnetic Longwire Balun. Its really an impedance transformer (9:1) to feed a high impedance long wire (~450 ohm), down to a 50 ohm unbalanced coaxial input. I have heard them called an UnUn which seems more appropriate. I almost bought one of these for around £30, but then decided to make one. The toroid is a T130-2 Iron Powder core, with 3 x 9 turns of 18SWG enammeled copper wire,
The following diagrams and tables show some simple 1:1 and 4:1 baluns for use between 1.8 and 30 MHz using twin transmission lines made from enamelled copper wire (ecw). The bifilar windings have been adjusted (spacing) to produce transmission lines with Zo of 50 or 75 ohms as required for best matching. These designs should be suitable for 100-200 Watt operation.
RF transformers should always operate within safe limits of temperature and linearity. The following designs which use toroid cores can be checked for safe operating conditions with the programs included in Programs.
Examples of winding methods a, b, c, d on a rod.
Ferrite Rod: R-61-037-300
Start by winding a layer of ptfe tape on the rod
Cut two 750mm lengths of 1.25mm ecw for a bifilar winding.
Starting at the centre of the rod, close wind half the length of the wires towards one end. Repeat for the other half. 15 bifilar turns (2 wires) should now be centred on the rod, with sufficient end lengths for terminations.
Cut the spacing twine into 750mm lengths and insert between the wires as required by tightly pulling the cord between the turns. This makes a secure finished winding. Start at the centre of the winding and work towards each end.
For a 50 ohm balun put spacing twine between the bifilar turns (winding method c).
For a 75 ohm balun put spacing twine between all wires (winding method d).
Waterproof, terminate and house the finished balun as required.
See test results for the rod balun.
Ferrite Rod: R-61-037-300
Use the same winding method and materials as in the rod balun above.
Cut two 750mm lengths of 1.0mm ecw for a bifilar winding.
Wind 15 bifilar turns centred and close spaced on the rod.
Wind spacing twine between all turns as described above (winding method d).
See test results for this rod balun.
Ferrite Toroid: FT-140-61.
Use the same winding method and materials as in the rod balun above.
Spread the 12 bifilar turns of 1.25mm ecw evenly around the core. This leaves space between the 12 turns.
If the lowest operating frequency is 3.5MHz then 10 turns is sufficient.
For a 50 ohm balun the bifilar pairs are close spaced (winding method a).
For a 75 ohm balun put spacing twine between the bifilar pair(winding method b).
See test results for these toroid baluns.
Ferrite Toroid: FT-140-61.
Use the same winding method and materials as in the rod balun above.
Spread the 12 bifilar turns of 1.25mm ecw evenly around the core. This leaves space between the 12 turns.
If the lowest operating frequency is 3.5MHz then 10 turns is sufficient.
For the 50:200 ohm balun the bifilar pairs are close spaced (winding method a).
For the 75:300 ohm balun put spacing twine between the bifilar pair (winding method
Homebrew A 4 To 1 Balun
Many modern HF transceivers come fully equipped with built in tuners. While these tuners are great for changing bands, the manufacturers left out a very important accessory; the 4 to 1 balun. With out a balun the transceiver can only feed an antenna which uses coaxial cable. While this may be satisfactory for some operators, this is a real problem for those of us who prefer the super low loss ladder line. The only other alternative is to buy an external tuner with a built-in balun which is really absurd after spending the additional money to have one built into the radio. Fortunately, a 4 to 1 balun can be easily home brewed as illustrated in Figures 1 and 2.
Figure 1 shows a bifilar winding on a toroid. The toroid should be type 2 (red) material and can be any of the following sizes but the number of bifilar turns should be adjusted accordingly:
|TOROID||NUMBER OF TURNS||POWER RATING|
The exact number of turns is not critical but the numbers listed in the preceding table should yield optimum results. It is possible to exceed the power ratings listed above but the performance of the balun may be degraded during high SWR causing heating of the core.
Toroids of this type are available from Palomar Engineers, P.O. Box 462222, Escondido, CA 92046 (1-800-883-7020). If enough inquires are made, the author will make a limited number of toroids available at RASON meetings for those who hate mail order. The author will even wind the toroid for the faint of heart.
The balun should be housed in a suitable metal enclosure such as those available at Radio Shack. Use a SO239 or BNC connector for the unbalanced input. Nylon binding posts such as RS 274-662 work just fine for the balanced output.
the SIMPLE 2m Copper pipe "J"
by Dale “Kuby” Kubichek, N6JSX /8 03/2001
Can you use a pipe cutter or a hack saw, can you solder – then here is how to build a SIMPLE ”J” antenna that will more than double your 2 meter (and even 440) performance!
The "J" antenna goes back many years, long before I became a ham. There are a bunch of reasons why you're going to want to build one of these beauties: “J” has the lowest angle of radiation, “J” requires NO ground plane; “J” is very easy and inexpensive to make; “J” has great performance for mobile, marine, or base operations; This “J” design can be used as a dual-band’er - 2m/440. Technical: The basic "J" is reported to have >3dB of gain over a ¼ l ground plane antenna and 6dB over an isotropic (theoretical) antenna. The "J" can be made from almost any material: copper pipe, steel whips, and even 300 ohm TV twin-lead. Technically, the "J" antenna is an end-fed ½ l antenna that uses a ¼ l matching stub. Old-timers call it an "end-fed Zepp", bent 90°. In actuality, the conductor is ¾ l long and the matching section uses the bottom ¼ l. The matching stub creates the tuned ½ l length antenna. Due to the matching section acting as the matching transformer, the ½ l radiator sees the lower ¼ l matching section as an image of a false ground plane. In best terms, the "J" is a balanced ¼ l matching stub feeding an unbalanced ½ l load. The feed-lines to a "J" can be almost anything (ladder line to coax). However, in experimentation, I found RG-58/U coax to be best when used at odd ¼ wave multiples. A “J” is the best for mobile and marine application where you want the most distance across relatively flat ground/water. A 5/8 or ¼ l antennas have a higher angle of radiation and need to be centered on a good ground plane eliminating gutter or vehicle edge mounting to obtain optimal performance. A "J" requires NO additional ground plane. A “J” has an exceptionally low, to nearly flat, angle of radiation of about 0-2 degrees. The 5/8 l has about a 3-6 degree radiation angle and the highest radiation angle comes from the ¼ l that has about 4-10 degrees. These two antennas are usually better for mountain top (a few thousand feet elevated) repeater site use but will fall far short of a “J” in overall flat-land transmitting distance. The pictured “J” is at 60’ on top my tower in Manitowoc, WI. I can now hit repeaters across Lake Michigan, Milwaukee, or Upper Michigan that are well over 85 miles away. Parts: The antenna pictured here is made from one 10’ piece of thick wall ¾” rigid copper pipe, one ¾” copper pipe “T”, one ¾” copper pipe 90° elbow, and three ¾” copper pipe caps, one SO-239 connector, and a 3” piece of 1/8” solid brass brazing rod (from a local welding supply company). These parts, plumbers flux, plumbers solder, and propane torch can be obtained at most hardware stores. Using copper pipe makes it easy to solder and snap to assemble. These materials will withstand a lot of abuse and weather. Total cost of this antenna was about $9.00. I use ¾” thick wall copper pipe due to my ½” copper pipe “J” was bent in a 59 MPH wind gust last year. The ¾” thick wall is much stronger! Theoretically, the ¾” pipe should be slightly more broad banded but I’ve not measured any difference from ½” pipe “J”. NOTE - about dimensions: I’ve seen numerous articles on “how-to” build a “J” antenna with various dimensions. Everyone seems to have the secret of the optimal “J” design dimensions. I’ve made many “J” antennas and nearly everyone I’ve ever made has NEVER operated like the previous built “J” – they all required some tweeking to obtain “my perfectionist requirements” near 1:1 VSWR as possible! However, in general if you follow the dimensions I’ve included here you will have an antenna that will be less than 2:1 VSWR and more like 1.5:1 VSWR across the 2 meter band. Recently, I’ve been experimenting with the basic “J” dimensions; I have found that a 2m J length of 63” really enhances the 440 band operation (63”=2.5 l at 445). The trade-off on 2m is an enhancement of a much wider bandwidth and an overall lower 2m VSWR. My J design dimensioned here is really great for single feed dual band operations!!! Building: Basic dimensions for a 146.000 MHz. ¾” rigid copper pipe “J” The difference of this design over my previous designs is the change to the feed point attachment method. I did not like soldering the coax wires directly to the copper pipe these wires were exposed to the elements. The coax got very brittle, the center dielectric crack, and the coax eventually got water logged. I experimented using a brass brazing rod. I’ve seen designs with the coax center conductor attached to the ¼l element or the ¾l elements. I found the best performance was to attach the coax center/brass rod to the ¾l element solder the brass rod to the ¾l element. Place the SO-239 into position and measure the rod then cut the rod accordingly. Sand off the finish of the backside of the SO-239 and tin this area. Insert the brass rod into the center conductor of the SO-239. Solder the SO-239 to the ¼l matching element. Make sure the brass rod/center conductor is NOT touching the ¼l matching element. Finish by soldering the brass rod to the center conductor. NOTE: BEWARE of your heat used when soldering the SO-239 to the “J” or the center conductor insulator in the SO-239 will melt away or go off center!!! Recently, I came across another very good feed point method for the ½” copper pipe “J” that eliminates the connector strain of the 90° coax loop. This design comes from the ARES group of Auglaize County, Ohio. The Auglaize ARES has installed this type of antenna on most all the Auglaize County Fire Department locations. They state they have made over 60 of these “J” antennas and have even sold them at Dayton. With the construction jigs created by WD8LLN mass-producing of identical “J” ‘s is a snap. In conclusion: I have found that the length of the attached coax does have an affect on the J’s VSWR. Multiples of odd ¼l lengths seem to minimize these coax affects. I have pruned off 3” pieces of coax in the HAM shack to bring the VSWR back to the 1:1 tuning the antenna was setup at. On VHF/UHF the VSWR variances are very susceptible to the consistency of the coax velocity factor and quality. I've used copper pipe “J” in an apartment placing the antennas in the corners of the living room or hanging the "J" from curtain rods behind the curtains. I’ve even made a corner hat & coat rack from a copper pipe “J”. The "J" offers the foundation for a stealth antenna by placing the antenna in PVC with an angled mounting box - the antenna can look like a gas/sewer breather pipe on the roof of CCR restricted house. NOTE: PVC/ABS/plastic will affect the J’s VSWR. The TV twin lead “J” is the “BEST” hidden transmitter hiding antenna I’ve ever used. It can be wrapped around branches of a tree or laid on top tall grass next to a riverbed emitting complex angles of various polarizations that caused extreme multi-path. I’ve enclosed a TV twin-lead “J” inside a black ABS/white PVC pipe and buried the antenna and “T” just under the surface of the ground near a wire fence. The wire fence ran through the Puente Hills; the fence parasiticly re-radiating the 2 Watt signal for considerable distances in either direction, add to this the limited access to the area and the hunters were totally confused for many hours. I’ve taken this same PVC antenna and “T” creation and put it underwater in a creek – now that was fun to watch the hunters not wanting to get wet but wanting to win. (Note: PVC will detune an open air tuned TV twin-lead J.) I take a wire wheel and steel wool to make my copper “J” antennas giving them a near military shine. Then I put multiple coats of Varithane (non-UV type) spray or Marine Spar varnish over the entire antenna - this will keep the antenna bright and tarnish/rust free for years. I even do this to my aluminum beams.
2m ¾” Copper pipe “J” Performance Data
Alternate feed method for ½” copper pipe “J”
The "J" antenna goes back many years, long before I became a ham. There are a bunch of reasons why you're going to want to build one of these beauties:
“J” has the lowest angle of radiation,
“J” requires NO ground plane;
“J” is very easy and inexpensive to make;
“J” has great performance for mobile, marine, or base operations;
This “J” design can be used as a dual-band’er - 2m/440.
The basic "J" is reported to have >3dB of gain over a ¼ l ground plane antenna and 6dB over an isotropic (theoretical) antenna. The "J" can be made from almost any material: copper pipe, steel whips, and even 300 ohm TV twin-lead.
Technically, the "J" antenna is an end-fed ½ l antenna that uses a ¼ l matching stub. Old-timers call it an "end-fed Zepp", bent 90°. In actuality, the conductor is ¾ l long and the matching section uses the bottom ¼ l. The matching stub creates the tuned ½ l length antenna.
Due to the matching section acting as the matching transformer, the ½ l radiator sees the lower ¼ l matching section as an image of a false ground plane. In best terms, the "J" is a balanced ¼ l matching stub feeding an unbalanced ½ l load.
The feed-lines to a "J" can be almost anything (ladder line to coax). However, in experimentation, I found RG-58/U coax to be best when used at odd ¼ wave multiples.
A “J” is the best for mobile and marine application where you want the most distance across relatively flat ground/water. A 5/8 or ¼ l antennas have a higher angle of radiation and need to be centered on a good ground plane eliminating gutter or vehicle edge mounting to obtain optimal performance. A "J" requires NO additional ground plane.
A “J” has an exceptionally low, to nearly flat, angle of radiation of about 0-2 degrees. The 5/8 l has about a 3-6 degree radiation angle and the highest radiation angle comes from the ¼ l that has about 4-10 degrees. These two antennas are usually better for mountain top (a few thousand feet elevated) repeater site use but will fall far short of a “J” in overall flat-land transmitting distance.
The pictured “J” is at 60’ on top my tower in Manitowoc, WI. I can now hit repeaters across Lake Michigan, Milwaukee, or Upper Michigan that are well over 85 miles away.
The antenna pictured here is made from one 10’ piece of thick wall ¾” rigid copper pipe, one ¾” copper pipe “T”, one ¾” copper pipe 90° elbow, and three ¾” copper pipe caps, one SO-239 connector, and a 3” piece of 1/8” solid brass brazing rod (from a local welding supply company). These parts, plumbers flux, plumbers solder, and propane torch can be obtained at most hardware stores. Using copper pipe makes it easy to solder and snap to assemble. These materials will withstand a lot of abuse and weather. Total cost of this antenna was about $9.00.
I use ¾” thick wall copper pipe due to my ½” copper pipe “J” was bent in a 59 MPH wind gust last year. The ¾” thick wall is much stronger! Theoretically, the ¾” pipe should be slightly more broad banded but I’ve not measured any difference from ½” pipe “J”.
NOTE - about dimensions:
I’ve seen numerous articles on “how-to” build a “J” antenna with various dimensions. Everyone seems to have the secret of the optimal “J” design dimensions. I’ve made many “J” antennas and nearly everyone I’ve ever made has NEVER operated like the previous built “J” – they all required some tweeking to obtain “my perfectionist requirements” near 1:1 VSWR as possible! However, in general if you follow the dimensions I’ve included here you will have an antenna that will be less than 2:1 VSWR and more like 1.5:1 VSWR across the 2 meter band.
Recently, I’ve been experimenting with the basic “J” dimensions; I have found that a 2m J length of 63” really enhances the 440 band operation (63”=2.5 l at 445). The trade-off on 2m is an enhancement of a much wider bandwidth and an overall lower 2m VSWR. My J design dimensioned here is really great for single feed dual band operations!!!
Building: Basic dimensions for a 146.000 MHz. ¾” rigid copper pipe “J”
The difference of this design over my previous designs is the change to the feed point attachment method. I did not like soldering the coax wires directly to the copper pipe these wires were exposed to the elements. The coax got very brittle, the center dielectric crack, and the coax eventually got water logged.
I experimented using a brass brazing rod. I’ve seen designs with the coax center conductor attached to the ¼l element or the ¾l elements. I found the best performance was to attach the coax center/brass rod to the ¾l element solder the brass rod to the ¾l element. Place the SO-239 into position and measure the rod then cut the rod accordingly. Sand off the finish of the backside of the SO-239 and tin this area. Insert the brass rod into the center conductor of the SO-239. Solder the SO-239 to the ¼l matching element. Make sure the brass rod/center conductor is NOT touching the ¼l matching element. Finish by soldering the brass rod to the center conductor.
NOTE: BEWARE of your heat used when soldering the SO-239 to the “J” or the center conductor insulator in the SO-239 will melt away or go off center!!!
Recently, I came across another very good feed point method for the ½” copper pipe “J” that eliminates the connector strain of the 90° coax loop. This design comes from the ARES group of Auglaize County, Ohio. The Auglaize ARES has installed this type of antenna on most all the Auglaize County Fire Department locations. They state they have made over 60 of these “J” antennas and have even sold them at Dayton. With the construction jigs created by WD8LLN mass-producing of identical “J” ‘s is a snap.
I have found that the length of the attached coax does have an affect on the J’s VSWR. Multiples of odd ¼l lengths seem to minimize these coax affects. I have pruned off 3” pieces of coax in the HAM shack to bring the VSWR back to the 1:1 tuning the antenna was setup at. On VHF/UHF the VSWR variances are very susceptible to the consistency of the coax velocity factor and quality.
I've used copper pipe “J” in an apartment placing the antennas in the corners of the living room or hanging the "J" from curtain rods behind the curtains. I’ve even made a corner hat & coat rack from a copper pipe “J”.
The "J" offers the foundation for a stealth antenna by placing the antenna in PVC with an angled mounting box - the antenna can look like a gas/sewer breather pipe on the roof of CCR restricted house. NOTE: PVC/ABS/plastic will affect the J’s VSWR.
The TV twin lead “J” is the “BEST” hidden transmitter hiding antenna I’ve ever used. It can be wrapped around branches of a tree or laid on top tall grass next to a riverbed emitting complex angles of various polarizations that caused extreme multi-path. I’ve enclosed a TV twin-lead “J” inside a black ABS/white PVC pipe and buried the antenna and “T” just under the surface of the ground near a wire fence. The wire fence ran through the Puente Hills; the fence parasiticly re-radiating the 2 Watt signal for considerable distances in either direction, add to this the limited access to the area and the hunters were totally confused for many hours. I’ve taken this same PVC antenna and “T” creation and put it underwater in a creek – now that was fun to watch the hunters not wanting to get wet but wanting to win. (Note: PVC will detune an open air tuned TV twin-lead J.)
I take a wire wheel and steel wool to make my copper “J” antennas giving them a near military shine. Then I put multiple coats of Varithane (non-UV type) spray or Marine Spar varnish over the entire antenna - this will keep the antenna bright and tarnish/rust free for years. I even do this to my aluminum beams.
Building A 2-meter J-Pole Antenna*
Cheap and Effective
22 September 2003
As an avid ham radio operator you can’t have too many antennas. On my tower I have an 11-element beam, a 102" CB whip and 2 2-meter J-poles (more to come). They also tune up quite well in the 70cm (440Mhz) ham band.
I needed another antenna so today I built a J-pole.
Use this calculator to figure out your lengths. Just punch in your frequency and hit calculate and Ker-BAM all the dimensions are there. I uploaded this to the FTP server so it’s in there incase this calculator site crashes. I also saved it on my computer so that it’ll still work even offline.
So this J-pole is made for 146 Mhz so all of the lengths pertain to that frequency. It holds a perfect tune throughout the band.
All you need for material is a 10’ piece of ½" Type L (thicker walled) copper water pipe, 1 T fitting and 1 90 degree elbow.
For implements of destruction you will need:
10’ of pipe 2 fittings
Ok, let’s start breaking stuff :o) woohoo!!
I used meters for all the measurements. A lot easier than trying to figure out inches and fractions of inches.
I’ll keep the labeling consistent with that of the calculator. Pay close attention to where the tips of the arrows point in the diagram below. A slight off measurement is not a big deal.
Again, this is for 146 Mhz-
A= 1.47 meters
B= .488 meters (round off to .49)
C= .049 meters (round off to .50)
D= .046 meters
The first step is to cut segment "A".
Stick the T-fitting on the end of the 10’ of pipe and shove it all the way in like so and measure from the inside corner.
I use a pencil to mark the segment letter 1.47 meters is about 58 inches
The penciling helps me not to screw up…keeps things simple. Anyway….
Segment A is now done
The next step is Segment "B". Grab the remainder of the copper pipe use CAUTION not to accidentally grab "A" and cut it…whups.
"B" needs to be .49 meters long. Stick the 90-degree elbow on the end. Measure from the inner corner out to .49 and cut it.
There’s only one more cut to make…again…make sure you don’t cut from "A" or "B".
Segment "D"…yes "D". "C" comes later.
The spacing for "D" needs to be .046 meters. I cut a small piece about .044 long from the remaining pipe. I could slide the elbow in or out as needed to get the right measurement.
Ok, now take whatever’s left of your remaining pipe and shove it in the bottom of the "T" joint. You should have something like this-
Assembled, but not soldered.
Now’s the time to double-check the measurements.
Alrighty…time to glue this puppy together. You get to practice your plumbing skills.
Pull it apart. Be sure not to get "A" confused with the bottom piece.
You’ll now need to sand the joints. This is absolutely CRUCIAL!!! If you try to solder the pipe and it isn’t shiny, the solder won’t even begin to adhere (stick) to the pipe. Cleanliness is the key when soldering. Shine it right up! Sand inside of the "T" fitting and 90 elbow. Sand the outside of the pipe. Then wipe the dust and geeb off with the rag.
You only need to sand about 2" down the pipe
Now you need to apply the flux. Use the acid brush and wipe flux all over where you’ve shined. Assemble the antenna as you go to prevent dirt intrusion. Notice the smear in the right pic…that’s all the flux you need. Just a light coat.
All together with flux
Before you can solder the antenna needs to be elevated a few inches so that you can heat underneath each joint. Make sure that the measurements are still correct and that "B" is parallel with "A". When it’s ready, solder it.
Fire up your propane torch. Pick a joint and start heating from the bottom. Heat the pipe and the fitting. You’ll notice the flux start to liquidize and dribble. This is normal. Hold the solder on the top of the joint. When it starts to melt, remove the torch and quickly run the solder around the joint. You’ll have to do this a couple of times. You might also have to apply a little more heat. The solder will be sucked right into the joint. This is what you want.
Solder everything else the same way.
I suggest letting it cool for a while before continuing.
Ok…now it’s time for Segment "C". You could just attach stripped back coax to the feed point. I don’t like doing it that way. I use a SO-239 connector. The SO-239 is what couples to a PL-259.
To the workbench I went and made up the connector.
I used a piece of 14 AWG solid copper wire and soldered it into the connector. I used a 140/100 watt soldering gun for this.
Then back to the J-pole. Let’s make Segment "C". Measure up from the inner corner of the "T" joint .050 meters and pencil it. Then sand this area and eyeball across to Segment "B" and sand the same area. Sand the wire where it will contact Segment "A". Wipe them off with the rag. I suggest that you measure again and pencil it so that you solder at the exact measurement.
Now, before you go any further. There are 2 ways to attach the connector. Bolt it or solder it. If you’re doing this for the first time or are new to soldering I suggest you bolt it. If you get the connector TOO HOT you’ll melt the insulator right out of it. Just use a small bolt and drill a hole thru Segment "B" at the .050 mark.
I soldered mine….keep in mind I’ve been doing this stuff for awhile.
Sand a corner of the connector and wipe it.
Ok, if you’re bolting the connector go ahead and bolt it. Then you can solder the wire to Segment "A".
If you’re soldering the connector I suggest doing it like the above pic. Wrap the excess wire over Segment "A". This will hold the connector snug on "B".
Spread flux on the wire and on "A". Then solder it just like you did on the fittings.
Then carefully solder the connector to "B". If you melt the snot out of it don’t say I didn’t tell you so.
Just snip off the excess wire
You’ve got a J-pole
In an emergency situation, it is often necessary to squeeze every bit of performance possible out of a 2 meter HT. One way to do that is to replace that little rubber duck antenna with the ever-popular Twinlead J-Pole. This simple antenna lends itself well to emergency use or as a portable antenna for hotel room operations while traveling.
There are several features which make the ubiquitous Twinlead J-Pole antenna a good addition to your emergency grab-n-go kit. When rolled up, it is an extremely compact, pocket-sized antenna. In use, it makes for a very effective antenna and provides about 3 db of gain with a low take-off angle. In fact, when used on your HT, it will dramatically out-perform your rubber duckie. And finally, it can be built in no time flat for a few dollars of readily available materials.
Technically-speaking, the J-Pole is an end-fed, halfwave antenna with a quarterwave matching section to allow feeding with 50-ohm coax. Being a halfwave antenna, it is not dependent on a ground or radials for proper performance. That's also a plus for portable operation.
Here's what you'll need to build one for the 2 meter band:
Here's how you build it:
That's about all there is to it. To facilitate hanging, punch a small hole in the top of the antenna. Use monofilament fishing line or other non-conductive line through the hole for hanging. The J-Pole is very broad-banded, so it shouldn't require any tuning. (An SWR check, however, is recommended.) Just unroll it, hang it up, and communicate.
Over the years, I've built dozens of different ones. Some have utilized commercial parts (modified and raw), some home brew parts, and a few defied description. If I have any regrets, it is that I didn't take pictures of them all.
The basic HF mobile antenna scenario really isn't complicated. You need some sort of mount, a mast, a coil, a whip, and (hopefully) some sort of matching device. The problem is nowadays, you just can't find the individual parts. And when you do, they want your first-born as payment! I'll give you a very good example.
During the late 70s, I worked for CW Electronics in Denver. They were the only amateur radio retailer within 500 miles. We sold just about every kind of amateur radio gear you could name, most of the nonproprietary parts to repair them, the tools to fix them, and everything in between.
One of the requisite parts for any mobile antenna (save for 10 meters) was a loading coil. At the time, the largest manufacturer was B&W (Barker & Williamson). The trade name for their coils was Airdux®. They were made in a variety of lengths and diameters. They ones I liked were the 2000 series. A piece of the 2004T was enough coil to build two 20 meter coils. A piece of 2006T was enough to make a 40 meter loading coil, with enough left over to built a 15 meter one. The cost was less than $8 each. A quick check of their web site will show the current price at $65 each! If you opt for a really good coil, you'd have to use their 4804TL. Which now, incidentally, sells for $500 each! It is no wonder that companies like Texas Bug Catcher opted to wind their own.
So in reality, what this article is about is where to find the parts to home brew your own. If you wish to call it DIY (Do It Yourself), fine as I don't have an argument about that. What's more, to me at least, it doesn't make any difference what "parts" you use. If you designed it, and it works, it is yours! What's more, there is a big measure of pride that comes from rolling your own.
So what you see here is not so much a home brew how to, but a few links and suggestions that will aid you on your quest. Whatever avenue you take, you need to bone up on antenna design. The first place to start is the ARRL Handbook.
If you're resourceful enough, it is easy to make your own loading coils. There are several aspects you need to be aware of. Most important is the material you use for the core of the coil.
Any material in close proximity of the loading coil will reduce its Q. This includes end fittings where the mast and whip attach. For example, the very large end caps used on Hustler's high power coils reduces their Q so much, the smaller coils are actually better in terms of loss.
A lot of home brewers use PVC for coil forms. On the lower bands, white PVC works fairly well, but its dielectric strength diminishes as you approach 25 MHz. The gray PVC is not suitable for any RF application. Lexan® and Delrin® both work well, but are somewhat more expensive.
The hardest part to fashion, if you don't have a lathe, is the end fittings. One alternative is to use brass pipe plugs. The 3/4 inch size will snugly screw into 1 inch schedule 40 PVC. You should cross drill them, and either use a screw clear through the pipe, or you can tap the hole. A 10 x 32 by 1/2 inch works well. The cross drilling should be done about 1/4 inch above the bottom of the pipe plug. The reason for this will become evident later on.
The shoulder inside of a 3/4 inch brass pipe plug is just right for drilling and tapping for 3/8 x 24 threads. The resulting thread is almost a 1/2 inch long which is adequate in most cases. It is best to have a drill press, but if you have a good vise and a decent hand drill you'll be okay.
You should drill the hole using a 21/64 bit, and the plug should be held securely. Remember, brass swarf (the curl of metal that spirals off during machining) tends to bind the bit. High speed and light pressure is the key.
If you use a drill press here is a little secret. Once you complete the hole, don't move the work piece. Remove the drill and insert the tap into the chuck. Then manually (no motor!) start the tap into the pipe plug a couple of turns. This will assure that the tap is in straight. Then release the tap, and finish with the tap wrench.
First, finding the center of the PVC pipe cap is a little difficult. One way to find it is to file a flat on the rounded end. Then use a compass to make several arcs across the flat. Their intersection will be close enough to the center. A 1 inch spade bit will drill a neat hole.
Next, the outside diameter of 1 inch schedule 40 PVC pipe, is slightly larger than 1 inch. So you have to clean up the hole in the PVC cap so the 1 inch PVC pipe will fit through the hole. Make sure the fit is very snug. The cross bolt (or threaded screws if you went that route) should just touch the inside of the PVC cap, and the 1 inch pipe and its brass plug in place, should be flush with the outside.
You want to make careful measurements with respect to the length of the 1 inch PVC pipe, and the 2 inch PVC pipe so that the mate just as described. Although the 2 inch caps are made to accept up to 1 1/4 inches of pipe, it is very difficult to drive them together since the cementing will be done after assemble. In the photo, the center 1 inch pipe is 7 1/2 inches long, and the 2 inch pipe is 5 1/2 inches long, or 1 inch of insertion on each end. Even then you'll need to tap them together. Use a block of wood, or a rubber hammer to do this.
You'll need to drill 4 holes for the wire to snake through. A 1/8 inch drill bit is about right for most coil material. Drill them close to the assembled 2 inch caps as seen in the photo, and one inch apart. You can then angle the drill to make the holes into slots. You can barely see this in the photo.
By the way, you could use 2 inch long 3/8 x 24 bolts and nuts for the end connections. If you go this route, here's a few suggestions. First, file off a flat on the PVC cap with a rasp or coarse file. This makes drilling the 3/8 inch hole a little more precise. Don't use regular bolts. Spend the money, and buy grade 8 bolts. They don't rust as easily, and they are stronger for sure. A dab of JB Weld or DYI epoxy to the inside it warranted for obvious reasons. After the epoxy sets up, you can remove the outer nut to install a wire lug for the connection. Put the nut back on to assure a good connection.
Once you're satisfied that the coil form is like you want it, it is time to cement it together. You can use standard PVC cement, but I find it easier to use acetone. A small glass syringe made for the purpose is available from any good hobby store. A small amount should be applied to the 2 inch pipe seams. You can glue the center 1 inch pipe by squirting the acetone through the wire holes. Enough should be used until it runs out around the seam. Be careful as acetone is flammable, and will mar just about any surface it touches.
The amount of wire is dependent on how much inductance you need. If you've read this far, I'd like to believe you're serious about the project. In which case, you've already read the ARRL Handbook or Antenna book, and know how to calculate what you need. It doesn't have to be exact, but it does have to be close. Here's an on-line calculator you can sue too.
Note the pigtail in the photo. A wire lug soldered on, and placed under the whip or mast will make an adequate connection. If you have to, you can remove a turn or two. In any case, the length of the whip will determine the final resonant point.
The wire you use can be almost anything. The wire in the photo is #12 Thermalese, but building wire may be used. I just wound the coil randomly for the photos. If you need a large inductance you may want to close wind the wire, or do your best to space it evenly as you can. One way to do this is to use trimmer string which comes is several sizes. You just wind it along with the wire in parallel windings, and as taught as you can.
Next, spray on a little high voltage lacquer (about $3 a can at most hardware stores), and just before the lacquer sets up hard (about 10 minutes), remove the trimmer string. A few more coats of HV lacquer, and the coil will stand up to almost any weather.
As I said before, this is a method I have used to construct mobile loading coils. It is not a primer of where to place the coil in the antenna, or how much inductance it must have for some frequency, and over all length; all of that information is in the ARRL Handbook.
I would not of thought of this, but David Stall, AE5DS, did. What you see in the left photo is a special coil form made by Breedlove Machine Shop. It is actually an elongated version of their HV insulator made for use with an auto-coupler. The screws you see are threaded into the end bosses, which are bored for 3/8x24, making them compatible with existing antenna hardware.
The right photo shows the completed coil. The wire used #14 awg, but it looks much bigger. The reason is, David used some nylon rope as a spacer for the coil turns. As long as moisture doesn't get in, that's a good way to keep the turns evenly spaced. The tape is vinyl electrical tape, but Rescue Tape might be a better solution as it seals out moisture better.
One thing to remember if you duplicate this method is to keep the coil turns away from the screws to minimize Q losses, as David has done. The aforementioned on-line calculator can be used to approximate the required turns needed. Once you have that calculation, the requisite length form can be ordered from Breedlove.
All in all, I think it is a novel idea, one that is easily duplicated, and the completed antenna is certainly full of DIY pride!
I can't tell you how many different masts I have used over the years. I've used fiberglass ones with copper wire buried inside of them, steel, aluminum, copper water pipe, stainless steel, chrome moly, and even wood! Lengths varied from a foot or so, to one 10 feet long.
DX Engineering sells ready-made masts in several different lengths, and they are reasonably inexpensive. You can still find Texas Bug Catcher masts, although they're now out of business.
You can homebrew your mast too. Mike Brueggemann, K5LXP, has the answer on his web site. For less than $10 you can buy enough material to build two masts using his method.
While copper pipe works well, DIY (Do It Yourself) steel tubing will work to. Fact is, the 1/2 inch diameter material is will accept a 3/8 x 24 bolt if you cut off the head. The bolt can be brazed in, along with a nut to tighten it down to. A coat of paint, is all it needs. If you look in the Photo Gallery under W5LLD, you'll see one of these. This particular mast is 4 feet long, and the loading coil is from an old ARC5.
This antenna was built by Jerry Clement, VE6AB. Admittedly, Jerry is a first-class machinist, with the equipment, and know how to design an antenna from scratch. He didn't copy anyone else's design, rather he started with a clean sheet of paper. You can get a closer look by clicking on the photo. There are several other views in the Photo Gallery under his call (use the search function in the upper right corner of the album).
The coil form, 4 inch OD by 9 inches long, is made from clear Lexan®, and wound with 14 gauge, tinned bus wire. The tuning sleeve is made from 16 gauge aluminum, including the spun-aluminum bottom section. The finger stock is beryllium copper.
In this photo, the antenna is tuned to 80 meters. Although the overall length, coil diameter, pitch, and wire size are identical to his previously used, commercially-built antenna, 6 less coil turns are required to resonant the antenna. This enhanced performance is a result of the end caps being made from Delrin®, rather than aluminum. There are brass inserts for the mast, and whip connections, however.
Of the literally hundreds of home brew antennas I've seen over the years, Jerry's takes the Blue Ribbon prize. It is, indeed, a work of art. Form follows function as they say, and so does performance in this case.
Before the aforementioned multiband antenna was made, Jerry made a 75 meter single band one. Although he wrote an article about the construction techniques used, it didn't appear in the pages of QST until July, 2011, starting on page 39. Like the multiband unit, it too is a work of art. If you read the article, you'd know why; Jerry has a well-equipped machine shop to work in.
Most folks won't go through the exercise to build their own coils. However, I'd like to think it is a good way to learn about the intricacies of mobile antennas. There is, after all, more to it that just the three parts; mast, coil, and whip.
If you buy all of the parts, drills, taps, and pieces, you'll end up spending about $50. I already had most of the material, and all I had to buy were the 2 inch PVC caps, and the 3/4 inch pipe plug. The grand total was under $4.
I mention this in my Antenna Mounts article, but it merits mentioning here too. Professional Plastics sells all kinds of remnants in just about every kind of plastic imaginable. For example, a 25 pound box of Delrin® remnants sells for $75 plus shipping. The pieces are at least 3"x3", and vary in thickness. The box I ordered had four pieces larger than 6"x6", and most of it was 3/4 and 1 inch material. There was even a few pieces of rod and tubing. It is a home brewer's delight, and worth the money. Remember, Delrin®, in its natural color, is RF transparent up to about 2 GHz. It can be used for all sorts of insulators, braces, coil forms, and mounting brackets. Since it works just like wood, standard shop tools are all you need. As an added bonus, it thread taps very well. However, you should use one number drill smaller than would be otherwise required. This assure a good, solid thread. The only caveat, Delrin® melts easily, so go easy with the drilling, and use Forstner bits when you can.
Occasionally, you can find Parmax®, one of the world's strongest plastics, in remnant form too. It machines and drills much like aluminum, but weighs a bit more. I found a piece about one foot square by one inch thick, for just over $50. It makes excellent replacement insulators for ballmounts.
There is probably more misunderstanding about grounds than there is any other aspect of amateur radio. Some of this misunderstanding comes from the fact we call a whole bunch of diverse things ground. The most basic one is the ground we walk on. It doesn't make much difference if it is asphalt, dirt (earth as it were), or concrete, we collectively refer to it as ground. Electronically, we add earth ground (earthing), DC ground, RF ground, ground plane, chassis ground, isolated ground, and a few others. While they may be similar in basic terms, one type of ground doesn't necessarily act as, or replace, another type of ground. Of all of the various grounds we deal with in mobile operation, the most important one is the ground plane—essentially the missing half of a vertical antenna.
Antenna efficiency should be the goal for every mobile operator. Because ground losses dominate the efficiency equation, decreasing them by just one ohm, can make a significant difference in efficiency. This point should not be under estimated! Put another way, excessive ground losses can turn an otherwise efficient antenna, into an also-ran.
One very important point needs to be made here. A vehicle is an inadequate ground plane at any frequency under ≈100 MHz, no matter how large it is! Rather it acts like a capacitor placed between the antenna, and the surface under the vehicle which is the actual ground plane. Since the surface in question is a poor conductor of RF, ground losses occur. The term ground plane in the following text is therefore a bit of a misnomer, but is used to differentiate it from DC and RF grounds.
One of the most misconstrued concepts about grounds in general, is the thought that an RF ground is the same thing as a ground plane. It is not! The only commonality is this; The coax shield should be connected directly to the antenna's ground plane. It doesn't matter whether the ground plane is RF grounded (earthed) or not. What's more, the ground plane should be directly under the antenna; within inches, not feet! Running a ground strap to the nearest hard point will not negate this premise.
For a better understanding of the need for a ground plane, read the Ground Plane Notes article. And please, don't use the term counterpoise. While similar to a ground plane, it is in fact a different animal altogether.
First of all, there isn't an average vehicle. In fact, between two, otherwise identical vehicles, there can be a great difference in the amount, type, and severity of RFI. For example, the egressed ignition RFI of one may be S9, an the other an S2. Even minor annoyances like fuel pump and AC fan hash vary greatly from model to model. The same can be said of ingress, especially to sound systems. Some of the more common noises are described in the Noise ID article.
Secondly, the suggestion that one specific model or brand is superior to another doesn't take the aforementioned facts into account. This begs the question, is one generally better than another? Well, maybe, but we have to be more specific. In this case we're speaking about ground planes, and to a lessor degree, RFI issues. With that in mind, we can make a few general statements.
As a rule, unibody vehicles exhibit less problems, both with ingress and egressed RFI. This is due mainly to the all-welded body construction. However, most still have sound insulated undercarriages for the suspension, engines, transaxles, exhaust systems, etc., and these need to be bonded to maximize RF continuity, and egresses RFI.
Body-on-frame vehicles tend to have more bolted on pieces, and this is especially true of pickup trucks. No matter where you mount an antenna on a pickup truck, the bed should be bonded to the chassis on all four corners, and to the cab on both sides. If you don't, you'll most likely be plagued with RFI problems, some of which you won't know you have.
One major point to remember, DC continuity is not the same as RF continuity! When you don't properly bond and/or wire your installation, it is possible to create a ground loop in the vehicle's wiring. When you create a ground loop, the resulting affect will often appear to be an RFI issue. Ground loops are the toughest of problems to find and correct.
Although mentioned in the wiring article, it bears repeating here. Under no circumstances, should the body and/or frame be used as the DC power ground return. Doing so on a modern vehicle is a prescription for RFI, and operational problems with the various on-board electronics. Returns should always be directly to the battery or jump points as the case dictates.
The ground plane is one type of ground that needs a different name applied to it, because everyone seems to have a different opinion of what it is or isn't. It isn't a counterpoise, although many folks use the term synonymously. What it really is, is the missing half of a vertical (monopole) antenna. In an HF mobile scenario, the body of the vehicle, and the capacitive coupling to the surface under the vehicle, is acting like the missing half—a lossy one at that! On average, mobile ground plane losses vary between 2 and 10 ohms, 10 through 80 meters respectively. In most installations, the ground loss is somewhat higher due to improper mounting, bonding, and assumed ground conductivity.
The coupling between the superstructure of any vehicle, and the surface under it, is not consistent. As a result, there will always be standing waves between them. These standing waves are, in essence, the main cause of the ground losses in the first place. Please note, we're not talking about the standing wave ratio (SWR) of the antenna! It should also be noted that you can't measure these standing waves directly. It should also be noted that these standing waves are frequency dependent. However, they may indeed cause more ground loss on say 20 meters, than they do on 40 meters. This is in opposition to the normally excepted theory that ground losses always increase with a decrease in frequency. Looking at this in another way, you cannot assume any measurement you cannot directly measure!
The individual parts—the antenna, the vehicle, and the surface—should be viewed as a system! Change one, and you change them all. This fact is why proper bonding and, mounting are of prime importance. The issue is to make all of the bolted on pieces as RF congruent as we can. Here's another way of thinking about the system!
Let's assume our antenna represents a 50 Ω load (which is seldom does), and our transceiver outputs 100 watts. From Ohm's Law, flowing into the antenna will be 1.4 amps of RF current, at ≈70 volts. This same current, and voltage must flow in the missing half of the antenna, the ground plane as it were. Since both our antenna, and the ground plane are lossy (resistive), some of the current flow will be dissipated as heat, and will not be radiated. Quite obviously, we want to minimize these losses. Incidentally, excessive ground losses also contribute to common mode current, and other RFI issues. It should be obvious then, that minimizing system losses is a worthwhile endeavour.
Whether you use a monoband antenna, of a remotely controlled multiband one, once properly matched it will have a relatively low SWR. Once tuned, few mobile operators keep an eye on the SWR while underway (and for good reason!). If you did you would notice that the SWR changes over a rather wide range depending on the surface we're driving on. One would correctly assume that changes in ground conductivity under the vehicle affect the ground loss figure, whatever that may be. However, there is another factor at play. As mentioned above, there is capacitance coupling between the surface, and the body of the vehicle. Changes in ground conductivity also chances the amount of this capacitance. It has the same effect a cap hat does, but in this case, it is at the base of the antenna, not the very top. As a result, the resonant frequency of the antenna changes. Whether the resonant frequency increases or decreases, depends on what the conductivity under the vehicle was when the antenna was first resonated. It is this capacitance change which causes most of the SWR variations we see, rather than the actual ground loss figure, whatever it is.
The best place to mount an HF mobile antenna, is in the center of the roof. This places it as far away from the surface the vehicle is sitting on, and as far away from the vertical surfaces of the vehicle as possible. With respect to system losses, any other position on the vehicle will exhibit more loss. And contrary to popular belief, DC or RF round straps will not negate this premise!
As stated, low mounting heights increase ground losses. The reason is, a goodly portion of the return current is forced to flow in the lossy surface under the vehicle, rather than through the vehicle's less lossy superstructure. How much affect this has on efficiency depends on a lot of factors, especially the quality of the antenna itself (coil Q, overall length, etc.). Since ground losses dominate the efficiency equation, a ground loss change as low as one ohm can make a significant change in efficiency; in some cases, as much as 15 dB or more. One key to increased efficiency, read that as low ground losses, is placing as much metal mass under the antenna as possible, as depicted in the pictorial below right. That certainly isn't the case in the upper right photo!
This is a good point to bring up a hotly debated issue about mounting mobile antennas down inside the bed of pickup trucks. Unless the mast of the antenna is very close to the pickup bed wall (well less than an inch), the reduction is performance is very minimal. The reason is, the amount of capacitance between the bed wall, and the antenna's mast, seldom exceeds a few pF. Even on 10 meters, this amount of capacitive loading is almost immeasurable without sophisticated lab equipment. It certainly can't be measured with an inexpensive antenna analyzer. Where you can get into trouble is mounting the antenna's coil too close to sheet metal. In this case, the reduction in performance is easily measured, even with an SWR meter in some cases. It is always best to keep the coil as far from sheet metal as you can.
Let's dispel another notion about pickup truck beds. Some would have you believe, that mounting an antenna above the bed opening on outriggers actually reduces ground losses. These pundits argue that the bed becomes a resonant cavity, with a very low impedance. This is pure junk science at its best! Remember, it is the metal mass directly under the antenna, not along side, that counts!
The next two sections talk about ground straps, bonding as it were. In this context we're taking about braided strapping. It can be commercial braided strap like the Electric Motion material shown at left, one inch wide braided battery cable, or the shield off discarded RG8. What ever it is made out of, there are a few things to remember.
First, straps much be kept short! That is to say, inches not feet! You often see ground strap running from antenna mounts down to the frame of the vehicle, and sometimes they're several feed long. From an RF standpoint, they do nothing. Yes, they provide as DC ground, but they're no substitute for a properly installed antenna mount. Think about this. A 4.5 foot long ground strap is a perfect 6 meter antenna!
The width of the strap is important too. Remembering that RF flows on the outside of the conductors, the total surface area is important. Obviously, braided cabling have much more surface area than a round wire. The larger the surface area, the lower the impedance, which is exactly what we're striving for. Here's a good rule of thumb to follow is. For each foot in length, the width should be one inch wide. In other words, the longer the strap, the wider it had be in order to keep the impedance low.
Speaking of low impedance; wide, solid copper strips have the lowest impedance of all, however, they're not very flexible to begin with, and they tend to work harden. As a result, solid copper strips shouldn't be used if the inter-connection needs any flexibility; door straps for example.
If your antenna mount is securely fastened to the frame or body work of the vehicle, and the coax is securely fastened to the mount, no additional strapping is needed. If you added a strap, and it cured an RFI or SWR problem, then something else in your installation was amiss.
There is only one RF ground (if we can call it that) we need to deal with, and that's a proper return for the coax shield. It should be very close to the base of the antenna, coincident to any matching device used (coil, UNUN, etc.), and most importantly, as close to the metal mass of the vehicle as possible. This negates antenna mounting atop long posts, extended brackets, clamps, and luggage racks.
There is one more thing to consider when the coax shield connection is raised above the ground plane. Not only are ground losses increased, the amount of RF flowing on the motor control leads also increases, as do common mode currents flowing on the coax. Therefore, the requisite RF chokes may need to have an impedance in excess of 10 k ohms. There is more information on this in the Antenna Controller article.
One of the most misunderstood aspects is the notion that DC grounding an antenna mount will magically act as, or replace, a ground Plane. It will not! The only way a ground strap could act as, or replace, a ground Plane is to make it long enough to be radial; a ridiculous notion! While the strap may indeed DC ground the antenna's base, and it just might RF ground it too depending on its length and width versus the frequency of operation, it is by no means a replacement for a ground Plane. Nor is it a substitute for proper bonding.
Another misunderstood aspect is adding DC grounds to the various transceiver parts in an effort to control RFI and/or high SWR. If a DC ground (in this case RF ground as well) addressed your problem, then something else in your installation is incorrectly installed or mounted.
As mentioned above, proper bypassing of the motor leads of remotely tuned antennas is an absolute necessity. This is covered very well in my controller article. If you don't properly bypass (choke off) the RF from the motor leads, all sorts of maladies will occur. This includes erratic operation, RFI ingress, and a whole lot of frustration. By the way, these chokes much be mounted outside the vehicle, as close to the antenna base as possible. Remember, all the cabling before the choke is part of the antenna, and will radiate.
Unadulterated fact: If your HF installation doesn't require any matching (input SWR under 1.6:1), you either need a better antenna, or a better mounting scheme, or both!
Matching a mobile antenna to the requisite 50 ohms is a requirement for several reasons. For example, modern solid state radios are designed to reduce their output power when the input SWR reaches ≈2:1. Some will handle a little more, some a little less. The SWR doesn't have to be flat, so anything below 1.6:1 is close enough.
One very important point needs to be mentioned before proceeding. If you're using a remotely controlled HF mobile antenna like a Scorpion, the motor leads (and reed switch leads if used), must be properly choked. If they're not, you'll have terminal problems (that's no pun!). Do yourself a favor, and read this article: Antenna Controllers.
If you don't use the reed switch, here's a hint or two. Attach a lug to the leads and connect them to the antenna's mast. This saves you from having to choke them off with a ferrite core. What ever you do, don't ground them! And don't let them float (hanging in the wind). Doing otherwise will affect the input impedance of the antenna, making it nearly impossible to match.
Further, improper choking of the motor leads will also affect the impedance of your antenna. Therefore, the motor leads should be disconnected at the antenna before attempting to adjust any matching method. Once matched, if reconnecting the motor leads changes the matching point and/or SWR (no matter how small the change is), it is a good indication that the motor lead choke impedance is too low. Again, read this article: Antenna Controllers. As pointed out in the article, the chokes supplied by every antenna manufacturer, with two exceptions, are totally inadequate for the purpose.
By the way, antenna manufacturers often tell their customers to cut their coax feed lines to a specific length in order to get a good match. All this does is mask the problem, by moving the SWR node to a different position along the feed line. While this may appear to fix the problem, it doesn't fool most automatic controllers. The truth is, if the antenna is properly matched, it doesn't make any difference how long the feed line is.
In the following sections, it is necessary to know the exact resonant point (X=Ø) of the antenna we're trying to match. This fact alone, should not infer that exact resonance is a requirement; it isn't! Rather, in this case, it is only a means of arriving at the end point (i.e.: match). Once the matching is complete, whatever the SWR is (assuming it is under ≈1.6:1) is irrelevant. It should also be mentioned, that some transceivers can generate excessive IMD levels the (FCC mandated ones) when the SWR is over 1.8:1 or so—yet another reason to properly match antennas.
Properly matching the antenna impedance to the line impedance is important, especially if you're using an automatic antenna controller. And, to repeat, if your HF antenna doesn't require matching to provide a low SWR (with the possible exception of 10 and 12 meters), then you need a better antenna, a better mounting position, or both! Further, proper matching reduces the chances of RF flowing on the outside of the coax feed line (common mode currents).
Literally thousands of articles have been written about antenna impedance matching. Whatever method you choose to achieve a match is fine, as long as the antenna element is DC grounded! There are two reasons for this.
First, DC grounding helps control static discharges from the antenna, thus reducing some of the received hash we all put up with. Secondly it is a safety issue. If the antenna were to come into contact with a live overhead power line, DC grounding will help prevent damage to your equipment and perhaps to you as well. There are several ways to accomplish DC grounding, and an RF match at the same time. These are discussed below.
Some neophyte amateurs are under the assumption that a DC grounded antenna (element) won't work, but this is not the case. Just because we DC ground the antenna element, doesn't mean it is RF grounded too. They also assume that grounding the antenna's mounting structure will assure a low SWR and/or is a replacement for an adequate ground plane. Neither of these assumptions are true.
There is another related issue which needs to be mentioned at this point, and that is lightning safety. Contrary to popular belief, inductive shunt matching, does not increase the likelihood of lightning damage, should a strike occur. If anything, it reduces the possibility.
Due in part to popular press, way too much emphasis is placed on achieving a low SWR. Adding insult to injury, the methodology most amateurs use to check (or set) their SWR is incorrect. A fact which will soon become glaringly evident.
At resonance, the input impedance of a decent-quality, correctly-mounted, HF mobile antenna will be about 25 ohms. By definition, the resonance point is where the reactive component equals zero (X=Ø, or +Øj if you prefer). Since the requisite impedance of our feed line is 50 ohms, the resulting SWR would be 2:1, as read at the radio end of our coax run. However, if you adjust the antenna to a frequency lower than the true resonant point, the indicated SWR will decrease, perhaps to 1.5:1. For this reason, you should use an antenna analyzer with a reactance readout, when adjusting any antenna matching coil.
Again, you should look for the lowest reactance (X=Ø), not the lowest SWR when adjusting any matching device. Once the matching device is properly adjusted, the minimum SWR point on your transceiver (or external SWR meter) will be very close to the actual resonance point of the antenna.
Just to make sure this point is as clear as possible... With respect to the input impedance of an antenna (mobile or otherwise) that is other than 50 ohms, when the frequency if moved away from the true resonance point, the resistive component increases faster than the reactive component. In other words, the apparent SWR decreases, however, the antenna is no longer in exact resonance!
You can demonstrate this for yourself by adjusting your antenna analyzer to the lowest reactance (X=Ø, or as close as you can get to it), and noting the SWR. Then, adjust the analyzer's frequency until the SWR is at its lowest, and note the reactance. It will mimic the chart shown upper left (the reactance is shown in red, and the SWR in blue).
Again, to make sure this point is as clear as possible... When we're dealing with an antenna whose input impedance is other than 50 ohms, the lowest SWR is not the resonant point! Since the primary use of an antenna analyzer in a mobile scenario is to adjust the requisite matching coil, knowing this fact becomes imperative.
The two photos (a MFJ-259B) show a 40 meter resonant antenna before (left), and after (right) proper matching. It should be noted the right photo shows a few ohms of reactance, due to inaccuracies of the meter. Make note of the frequency shown on the 259B. This is indicative of what you'll see while you're in the process of of adjusting an antenna matching coil. It is included here, because (as noted above) adjusting a remotely tuned HF antenna's matching coil is the prime use for an antenna analyzer in a mobile scenario.
The reactance readout on an antenna analyzer may not be exactly zero at its lowest obtainable setting. This is due to several factors, not the least of which is the basic accuracy of the instrument in question. It may also be due to a nearby broadcast transmitter, and here's how to check. With the MFJ-259B connected to your antenna, push the mode switch until the frequency counter displays. If the SWR meter significantly deflects, you probably have BCI. MFJ does sell an optional BCI filter unit for the 259B which eliminates the problem.
Inductive matching does DC ground the antenna. If you're planning on using a remotely tuned antenna like a Scorpion or one of the many other remotely controlled designs, and an automatic antenna controller like the Better RF unit, then inductive matching is your only choice if you're seeking, true, fully automatic operation.
Several important caveats needs to be inserted here. Inductive matching works by borrowing a small amount of capacitive reactance from the antenna (by tuning the antenna slightly above the actual transmitting frequency). This borrowed capacitance, and the shunt matching coil's inductance, form a highpass, LC network which transforms the antenna's low impedance (typically 25 ohms or so) to that of the 50 ohm feed line. Installed and adjusted properly, shunt matching will provide a decent match (<1.6:1) over several octaves. Enclosing the coil, even in plastic, will affect the frequency versus reactance of the coil, effectively reducing its bandwidth. Further, the coil must be as clear of surrounding metal as possible. For example, factory supplied shunt coils are often mounted against the antenna's mounting bracket. For best results, these coils should be relocated. You should also avoid any commercial units which surround the mast. Lastly, some commercial matching coils short out a portion of the coil, to achieve a match. As with antenna loading coils, short-tapping reduces the effective Q of the coil, which further increases matching losses. Obviously then, open air, shunt matching coils provide the best match, and least loss of any other matching methodology, bar none!
At left is a photo of the MFJ-908 L match unit. Building one rather than buying one won't save you any money, but you might learn how they work if you do. The ARRL Antenna Handbook is a good place to start.
However, you typically don't need a switched inductor like the aforementioned MFJ unit (see Odds & Ends below). Instead a simple inductor, like the one shown in the right photo (or the upper right pictorial), will suffice. One end is attached to the antenna feed, and the other end is connected to ground. The ground end of the coil should be collocated with the coax shield ground.
The coil at right has 9 turns, is 1 inch inside diameter, and wound with #14 Thermalese® (enameled) wire. The coil's form factor should be kept close to 1:1 (length to diameter). Long skinny coils do not work nearly as well. You can also use building wire, but it is a little harder to work with. In actual use, the turns are spaced a little further apart to adjust the inductance. The coil needs to be about 1 uH, but in the real world, the value may be between .5 uH and 1.5 uH depending on the actual input impedance at resonance.
There is a specific procedure which must be followed if proper adjustment of a shunt matching coil is to be achieved. To alleviate problems, I have moved (and expanded) the procedure to this article, Antenna Coil Adjustment.
Several commercial versions of the screwdriver antenna have machined-in matching coils, which is fixed in value, and therefore cannot provide an ideal match over the entire resonant frequency span of the antenna. In fact, it is often suggested that a specific length of coax be used to feed the antenna, in order to provide a low SWR. Doing so only masks the real problem; an incorrectly sized (impedance value) shunt coil. Although these antennas can be modified to use an adjustable coil, it's best to avoid the buying the problem in the first place.
You can also use an UNUN (UNbalanced to UNbalanced) RF transformer. Like the LC network above, it provides a DC ground, and the requisite impedance match. The overall system losses are low, so the UNUN can be mounted near the radio rather than at the antenna. An MFJ-907 unit shown at left.
Keep in mind, if you use a remotely controlled antenna, you will have to change taps between bands. In most cases, you can use one tap for 80 and 40, another for 20 and 17, and straight through for 12 and 10. If you don't like the idea of changing taps, then make one of the aforementioned inductors.
If you want to built your own UNUN, the schematic is at right (click to enlarge). An F114-67 ferrite core, a few feet of #14 enameled copper wire (enough for 9, bifilar turns), a small box to mount it in, and you're home free. I prefer to cover the core with 3M #27 glass tape as it makes winding easier, but isn't required.
I don't recommend capacitive matching for two reasons. First, the method doesn't DC ground the antenna. Secondly, as the frequency increases, the reactance (in ohms) decreases, which means you have to use a different value capacitor for each band, and sometimes within a band.
If you're already using capacitive matching and don't want to bother changing it, at least add a 10K resistor across the antenna terminals (high power will require several resistors in series). While this will not protect you from live overhead wires, it will help tame the static discharges. A correctly sized RF choke will to the same.
By the way, MFJ offers two sizes of capacitive match boxes; one rated at 300 watts, the other one 600 watts. Both models are stressed at half their power rating.
Just for the record: At least one (dedicated to mobile operation) publication, incorrectly states that a shunt matching coil requires adjusting the coil's reactance when changing bands. Adding insult, these publications state this is not the case with capacitive matching. In fact, the exact opposite is true. If you properly adjust the shunt matching coil, it is possible to operate from 80 through 10 meters without further adjustment.
Antennas with extended coverage down to 160 meters, may require an MFJ-908 L or similar switched inductor. The reason is, a 160 meter loading coil is about 5 times larger (in inductance) than an 80 meter loading coil. All else being equal, the coil losses will more than double, and therefore the input impedance will be close to 50 ohms (no matching needed). There's a hidden factor at work here too. A 160 through 10 meter remotely controlled antenna, will exhibit significantly more coil loss at any frequency, than an 80 through 10 meter antenna. Potential purchasers should be aware of this fact.
As an alternative, you can make your own switched shunt inductor. The one pictured at right is from Myron Schaffer, WVØΗ. The coil is similar to the one described above. The switch shorts out the bottom 4 turns, reducing the inductance from 2 uH, to .7 uH. This method costs less, and is just as effective as a purchased one, albeit requiring a bit of tweaking to get the correct inductances.
I cover this in detail in my coupler article, but it needs repeating here. Either an internal or external auto coupler may be used to match a mobile antenna's input impedance to 50 ohms. And, they can be used to extend the bandwidth of a monoband antenna. However, using one with a remotely tuned antenna presents some operational problems. If you use one, keep the following in mind.
The antenna in question should be adjusted to resonance (lowest SWR is close enough) before the auto coupler is turned on. This is especially important with external couplers with their greater matching range. Under the right circumstances, failing to tune the antenna close to the operating frequency can cause the RF voltage to be high enough to arc over most base insulators, and might even exceed the ratings of the coax between the coupler and the antenna.
As mentioned above, another good reason to DC ground a mobile antenna is for lightning protection. It won't necessarily protect your radio from damage, but does offer a level of personal protection. Don't laugh, I've been hit several times! The last time was May 25, 2007. The corona ball now has a new hole, the the whip nearly burned in two, and the matching coil was damaged. The radio was on at the time, and suffered no damage.
There are lots of CB antennas out on the market today, all kinds of different shapes and sizes. Because of this competitive market, companies are always trying to put out a better product for a cheaper price. Commercially-made CB antennas are usually easy to put up and maintain. However, you might want to try your hand at making an antenna. You can make an one out of readily-available parts that will work as well or better than some commercially-made CB Antennas.
You will need an SWR meter to check out the Antenna after building it.
This CB antenna consists of a driven element and four radial wires that act as a ground. The driven element receives the transmit energy from the rig.
104" (264 cm.) [I 02' (259 cm.)] piece of aluminum pipe or conduit
¾" - 1" diameter
Two U-bolts, same size as pipe
One sheet metal screw
Four "egg" ceramic insulators
408" (10.22 m.)[400'"(10.2 m.)] of 16 gauge wire
Rope, enough to guy the ground radial, depending on the height of the antenna
A couple of two-by-fours
Silicone sealer to cover coax connection
The inside conductor of the coax is connected to the aluminum pipe by means of a screw into the bottom of the pipe. See the detail drawing on this page. Coat this connection with sealer or cover it with tape to protect it from corrosion.
All vertical antennas need to be grounded in some way. A mobile CB antenna uses the car body as the ground. On this CB Antenna, the four radial wires are used as the ground. This is called the ground plane of the antenna.
The braided wire which forms the outside conductor of the coax is soldered to all four radial wires. The wires must be exactly 264 cm (104') long [259 cm(102") long] (¼ wavelength).
Remember that the inner conductor and outer braid of the coax must not touch each other, nor can the radials come in contact with the driven element. The radials slope down at about a 45 degree angle in different directions, and are tied to the insulators. Rope or nylon cord is then tied to the insulators and used to hold the radials out. They can be attached to anywhere convenient; trees, a fence, house, etc.
If you are short on room for such a radial system, you can use 104" (radials) and 102" (driven elements) pieces of aluminum tubing, or suspend the wires on PVC pipe, bamboo, or 1" x 2"' wooden sticks. It's a must to check the SWR when done. It should be lower than 2, and ideally lower than 1.5 or 1.3.
A ¼ wave ground plane CB antenna made from wire can be suspended from a tree. We have talked to stations over 40 miles away using this antenna up about 30 feet high, running a mobile rig for a base.
For a quickie CB antenna, a vertical dipole (1/2) wave can be made right from the coax itself You take your coax and very carefully, without nicking the braided shield, strip 102 inches of the outer insulation jacket off one end. After removing the outer jacket, start bunching the shield down the coax from the end. Now, where the outer jacket and the shield meet, separate the braided shield enough to get the inner conductor out through the hole in the braid. Pull all of the inner conductor through and stretch it and the braid out. Be careful not to skin any of the insulation off the inner conductor. Now attach an antenna insulator to the end of the inner conductor. Measure the braided shield. Cut it off at about 106" and attach an antenna insulator to the end. The total length of the inner and outer conductor should be about 17 feet (1/2 wave). You can haul it up to any height you want with a string or rope attached to the insulator on the center conductor. It's a good idea to coat the end of the coax cable where it separates with some kind of waterproof sealer. This keeps water from seeping into the coax, which could cause a high SWR.
Just attach a coax connector on the end of the coax going to your transceiver and you are on the air. The SWR should be 1.5 or better, if cut to the proper length. It'll get out about as good as a ¼ wave ground plane CB radio antenna if you get it up high and in the clear. Do not hoist this antenna up next to a metal pole, because the metal will interfere with the antenna's operation and cause a high SWR.
A mobile CB antenna can be used as a base antenna by mounting it on the top of a metal pipe. The metal pipe serves as ground connection for the antenna, taking the place of the body of the vehicle. Remember to run a separate ground wire to a proper grounding rod for lightning protection.
This is an easy antenna to build and find the parts for. If you follow these instructions and have it come out looking like these pictures, it should have a very low SWR and appreciable gain.
You can use any number of pieces of aluminum pipe so long as they are ridged and fairly thick-walled so as not to become bent and broken in a strong wind. The pieces should be gradually smaller, one being able to fit inside the next. Cut two 2' grooves with a hacksaw down the outer pieces of aluminum and put a hose clamp around them. Now adjust the antenna to 22 ½ feet (6.85 meters) and tighten the hose clamps down to hold it all together. This 22 ½-foot vertical element can now be mounted with two U-bolts on to the 2" x 6' board. This board should be treated or painted to protect it from the weather.
Put a bolt through the piece of wood a few inches below the vertical element. Here you should fasten one end of the 6 foot (183 centimeters) piece of copper wire or tubing, the outside braid of the coax and each of the 104" (264 cm)[102" (259 cm)] long, stranded wires. These are called the ground radials and should be tied off with string (not wire) at a 45 degree angle away from the bolt. The ground radials and the braid from the coax can be soldered together or can be crimped together with a crimp connector that fits the bolt. The other end of the 6' copper wire is bent and fastened to the vertical element. The end of the center wire of the coax is then twisted onto this in such a way that it ca be slid up or down along the copper wire and soldered after adjustment.
The SWR of this CB antenna is adjusted by sliding this connection. You do this by keying your rig up on channel 20 and sliding this connection up or down until you have the lowest SWR. In our experience, an SWR of 1:1 or 1 was easily reached on channel 20 with a low SWR throughout the 27 MHz band.
Be sure to cover the end of the coax real well with a moisture proofing sealing compound so no moisture can get in it.
While the next project won't necessarily save you any money, you might want to give it a try if you are interested in understanding more about how fiberglass whip antennas do their thing. We have talked 30 miles mobile-to-base on one of these home brew fishing pole antennas using a $20 barefoot rig.
One 7-foot fiberglass fishing pole with hollow base One piece of steel rod 4 to 6 inches long-right diameter to slip into base of pole One ¼" x 1" machine bolt (threads to match mount) 12 feet of enamel #18 gauge wire Some good epoxy glue
Antenna Mount Parts
One 114' x 21/z" bolt and nut (same threads as bolt on base)
One longer-than-usual ¼" nut
Two plastic insulating washers
Three metal 5/8" diameter washers
One large terminal lug
First you've got to get yourself a fishing pole, 6 to 9 feet long. If you already have an old one lying around, you can clip off the line loops and cut the handle off.
We did some shopping and found that a finished fishing pole as long as we wanted was at least as expensive as a newly-manufactured CB antenna. But then we discovered a sporting goods store that sold unfinished fiberglass poles 7-feet long for $6. The kind we found was a black hollow tapered pole with about a 1/8" inside diameter at the base.
The next step is to hook something to the pole so you can screw it to a mount on your vehicle. The way we did it was to get a piece of scrap steel rod near the inside of the base of the hollow pole. (If the pole is not hollow you will have to figure out another way of hooking to it.)
Grind a slight taper to match inside taper of the fishing pole. Braze bolt to rod. Use 1/4" x 1" steel bolt. Epoxy the steel rod inside the base of the fishing pole.
There are many commercially made CB antenna mounts that you can buy at electronic parts stores that could be used to hold your fishing pole ear. If you elect to buy one of these mounts, get one with a spring so that if the pole encounters a stray tree branch, it can bend instead of break! If you are going to use this antenna with a store bought mount, the threads on the bolt on the bottom of the antenna should mate with the hole in the top of the mount's spring.
Tuning the Fishing Pole Ear
Now it's time to get into the electrical part of the antenna. The fishing pole is not the antenna; it's just a prop that holds the wire up. So we need to wind a wire around the fishing pole in such a way that will make it tune the 27 MHz CB band
If your antenna is shorter than 9 feet long, you can tune it by winding a coil around the pole. On our 7-foot ear, we found that it was necessary to make a coil of four turns ¼" apart about 2'- 3' up from the base of the pole. Tightly wrap the wire in a spiral up from the base of the pole. Make the distance between windings as wide as possible below and above the coil. If you follow these particular dimensions you should be close to being tuned up. To really make the ear a perfect match, however, you'll need to use an SWR meter to check the SWR. You may have to modify the coil spacings or add or subtract a turn from the coil to get it just right.
Different lengths of CB Antennas will also work, but different coil windings will be necessary, so if you have a different length pole you'll have to use a meter to tune it up. You just have to dive right in and try different numbers of coil windings. The longer the pole, the less center coil windings; the shorter, the more windings necessary. It takes some playing around and trial and error.
We used enamel coated wire, the kind used in motor windings, generators, transformers, etc. It's best to use enamel-coated wire so that the coil turns can't possibly short to each other.
At the bottom, wrap the wire around the 1/4" bolt. Be sure to scrape the enamel coating off the wire and clean the bolt for good contact. Solder that wire to the bolt or use a nut to hold the wire onto the bolt so that it makes a good electrical connection.
We covered some CB antennas we made with a thin coat of fiberglass (which you can tint any color you want). They looked pretty good but the fiberglass chipped off some of the antenna tips because they were mounted fairly high up and got tangled in the trees. You can prevent chipping by putting some shrink tubing over the tip. Shrink tubing would be another possible way to hold the wire on the fishing pole. It's available at most electronics shops.
If you are really adventurous, you can also build your antenna mount. The main point to understand when making an antenna mount is that the radiating element is not supposed to ground out to the body of the vehicle. This means that the bolt the antenna hooks to must be insulated from the metal body of the vehicle.
Your homemade mount must use good insulating washers, because if the bolt shorts to the vehicle body it could possibly blow out your rig's final RF power transistor.
One source of home brew insulating washers is the main output terminals of junk alternators or generators. The washers should be made out of some type of plastic. The best kind of insulating washer has a shoulder around its hole. You drill a hole in the vehicle body, big enough to allow the shoulder to fit through. This holds the bolt away from the metal body.
If you can't find a plastic washer with a shoulder, it is possible to use two flat plastic washers with a little piece of plastic tubing in place of the shoulder. Make sure the plastic is tough enough to not get cut by the edge of the hole in the vehicle body.
What you will need:
20 feet of 12AWG solid wire.(with or without the insulation)
2 'egg' insulators.(Radio Shack P/N # 271-1234)
A tape ruler, wire cutters, and black electrical tape.
A lenght of coax.(RG-8 RG-58 or RG-8x)
Solder and a good iron.
First you will take the wire and cut two 8 foot 5 inch lenghts.
Next strip 1 inch of the insulation off the end of each wire.
Then cut off about 3 and a half inches of the black insulation from the coax.
Pull the center conductor ,with the foam, threw the braid. Now you should have the coax split in two.
Trim off 1 inch of the foam to expose the center wire of the coax.
Solder one of the 12 gauge wires to the braid and the other to the center conductor.
You can use the tape and wrap it around the solder connections and down the wire a little bit.
This is to keep the water out of the coax. You can also use silicone chaulking if you wish.
Now at the end of the wire ,loop the wire around the egg insulator and wrap it securly so it will not come loose.
Do the same to the other wire.
Now thats it for the antenna all that's left is to mount it.Use the rope to pull it up into a tree.
After you have it installed and if you find that the S.W.R. is a little high you can lower it and trim off some wire from the ends of the dipole. Cut it in 1/2 inch increments. Remember thats its easier to cut off then it is to put
BY ARNIE CORO
Host of "Dxers Unlimited"
SEND YOUR COMMENTS, QUESTIONS AND IDEAS DIRECTLY TO ME AT: firstname.lastname@example.org
Yes, this is a "helically wound" wire antenna. It can be built in a few hours... it will take longer if you really want to make it look pretty... UGLY versions can be assembled in minutes. It MAY be used without an antenna tuner, BUT... it works best when you do USE a TUNER.
Here are the easy-to-follow, step-by-step building instructions for Arnie Coro's "Broomstick Special:"
2. Diameter of broomstick is not critical; anything from about 1.5 centimeters or better will work (this means that PVC tubing of about 19mm or 3/4 inch is ideal)
3. Prepare a base to hold the broomstick or PVC pipe vertical... Use a wide base, with enough counterweight attached to keep the broomstick vertical (I use mine next to the bedside radio, have convinced the wife that it is "modern art"
4. Obtain an aluminium disk of no less than 15 cm (6 inches) diameter. I prefer using a disk of around 30 centimeters (12 inches) but this is not critical. DO USE THE DISK... as it provides a capacity hat termination and helps reduce NOISE PICK UP
5. Obtain enough No.16 PVC plastic covered household wire; this is the ideal choice, but if you can't find it, then you may use No.16 or No.18 enamel covered copper wire (the one used for winding motors and transformers). If you can't find No.16 PVC covered wire, then your second best choice is No.18 "speaker wire"
6. Connect one end of the wire to the aluminium disk, and start winding a uniform coil using the "broostick" as the coil form. YOU WANT A NEAT JOB! Turns should fit tightly one next to the other... the "broomstick" will be filled with the wire forming the coil... When you arrive at the bottom end, make a termination> I use a long bolt with nuts and washers to which I tie the end of the wire, and another wire that goes to the antenna tuner. This wire that goes to the tuner can be from 1 meter to 3 meters long (from 3 to 10 feet) but DO KEEP IT AS SHORT AS POSSIBLE.
7. After the antenna is built, you may want to protect it with tightly wound PVC plastic tape over the wire. For EXTRA protection, you can paint the whole antenna with several coats of SPRAY ENAMEL...
8. The antenna works best near a WINDOW!!! Or better yet, you can install it in your balcony or garden... BUT DO KEEP THE CONNECTIONS TO THE TUNER SHORT
9. The antenna is RESONATED with your antenna tuner.... YOU MAY USE IT WITHOUT A TUNER but results are not going to be as good as when the antenna is connected to the receiver via a well-designed antenna tuner
10. YES... YOU MAY USE IT FOR TRANSMITTING.... BUT... according to recent medical research information, keep it as far away as possible from your body!!!
11. The helically wound "broomstick" is a lot of fun to experiment with... It works best when you provide a ground connection to the antenna tuner - receiver combination. One way of providing an "artificial ground" is to connect a length of wire of no less than 5 meters as a "counterpoise," that meaning that you can let the wire hang around the room's floor or garden. Using the "broomstick" with resonant radials turns it into an excellent amateur radio antenna for a specific band... For example with 4 radials cut for the 15 meter amateur band and a 4 feet high broomstick, (about 1.5 meters)
I can work a lot of stations on 21 mHz, something I do often to demonstrate to visitors what can be achieved with simple homebrew antennas, even when you don't have a lot of space.
12. QUESTIONS.... to: email@example.com
6 February, 199
The dipole can be fed directly with coax, or a balun can be used. It can also be fed with twinlead or ladder line. It is important to seal the insulating dowel with epoxy to keep it from rotting. I used the hose clamps to attach the coax to the antenna.
Caps should be placed on each end of the antenna to keep out moisture. I found that the threaded 1/2" PVC caps work well on 1/2" conduit. They can be just placed on the ends but I fixed them on with epoxy. Make sure that this is done after shortening the antenna for best VSWR match.
The length for each leg of the dipole is found using the formula L=234/F, where L=length in inches and F=frequency antenna is to be used on. If the antenna is too short, it can be lengthened by increasing the space between them on the insulator and lengthening each side of the coax or ladder line feed on the dowel.
This antenna must be mounted so that it is insulated from ground. Mine is mounted on a 4X4 and attached to a 2X4 that is buried two feet into the ground. I used U bolts to secure the antenna to the 2X4. This antenna could also be mounted horizontally off the side of a tower.
This antenna could also be built for the 12 meter band, and possibly even for the 15 meter band. However, most conduit you find in hardware stores is sold in 10-foot lengths, and a 15 meter antenna will need to be around 11 feet long. Some method lf lengthening the antenna will be needed. It could be done by attaching another foot of conduit to each end by using connector pieces for conduit.
It must be noted that copper tubing can also be used in place of conduit and may actually work better due to the improved conductivity of copper over the conduit. Copper tubing is often used in J-pole construction and there's no reason it couldn't be used here. It is, though, softer than conduit so it may not be as tough, but it should be able to last many years.
If copper pipe is used, it is advisable to paint it.
A 2 element beam antenna for 10 meters band
Living in a condo has many advantages, none of which is being able to mount a tribander on a 60-foot tower. So I make do with a long, thin random wire that works nicely as long as the New England wind, snow and ice don't conspire to give my hamming a holiday (which happens more often than I'd like). And although it's somewhat directional on the higher bands, I haven't figured out how to rotate 200 feet of wire without the neighbors becoming suspicious. One answer is to operate mobile. A bumper-mounted vertical is fine for casual operation, but it leaves a lot to be desired when mountain-topping for rare DX. With the solar maximum just around the corner, I decided that a portable 10-meter beam was necessary.
The beam had to fit in the trunk of my Subaru (limiting the largest component to about four feet in length) and had to be easy to assemble and erect on site by one person. In this article I'll describe the antenna and provide some construction tips that may help you avoid some pitfalls if you take on this worthwhile project.
From past experience I know that TV masts make good booms for smaller antennas. They're lightweight, strong and readily available at most Radio Shack and home stores. The light-duty stuff is plenty strong and comes in five-foot lengths. That was my starting point.
At 28.4 MHz, for an antenna made of tubing and not supported at the ends, a half wavelength is 491.8 divided by 28.4 MHz, or 17.3 feet. To accommodate my "Subaru factor," four feet divided by 2 x 17.3 feet produces a boom length of 0.116 wavelength, a size that gives a nice gain and a feed-point impedance that can be easily matched to your coax line.
The TV mast (with the crimped end lopped off) fits in my trunk and allows two elements to be mounted 4 feet apart and fed with RG58 coax. So far so good. I would be building a two-element beam.
Now, how to mount the elements to the boom and the boom to the mast (another 5-foot TV mast section)? In the past I had used a U-bolt and clamp arrangement, but this technique requires care in keeping the elements parallel to each other and to the ground. This is fine for permanent installations, but not something to he bothered with while operating portable.
I decided on right-angle pieces permanently mounted to the boom (see Figure 1). I used 1-inch x 2 1/2-inch aluminum angle scrap because it "looked about right." Your local hardware store has aluminum angle in various dimensions and lengths. I cut six, 3-inch pieces of angle to make the U-bolt mounting brackets -- two to hold each element and two to hold the mast.
Drilling the two holes in the angle's smaller dimension--the part that attaches them to the boom--isn't critical as long as you drill the holes on the boom the same distance apart. The angles will be permanently mounted to the boom using 2-inch bolts, nuts and lock washers. Later, when you mount them, be careful not to crush the tubing. It's not terribly strong, but it is lightweight. We're going for portability here!
The holes in the larger dimensions should be tailored to allow the mounting of the 1 1/4-inch U-bolts for the elements and the 1 3/4-inch U-bolts for the mast. Because the element mounts must be as parallel as possible and the boom mount must be at right angles to them for maximum efficiency (and so your antenna doesn't look like it's under the influence), make the boom holes with a drill press if possible.
Now for the Elements
Most beam antennas are made with aluminum tubing because it's strong, lightweight and available in sizes that "telescope" into each other. The telescoping feature is important. It helps in transportation and makes tuning the antenna a snap.
The beam's driven element should be 17.3 feet. The length of a reflector for a two-element Yagi with 0.116-wavelength element spacing should be 18 feet 1/2-inches.
I needed 35 feet of tubing (plus some to fit inside each telescoping joint for support). Because the tubing comes in 8-foot lengths, this worked out to five lengths of assorted sizes. The three telescoping sizes available at my local hardware store were 1 inch, 7/8 inch and 3 /4 inch--perfect! Because the 1-inch section was going to be the center part of the two elements, I picked up U-bolts and nuts while I was there. You'll also need eight hose clamps sized to fit your tubing.
This is how the material was cut up. One 1-inch tube was cut in half, yielding two four-foot lengths. The two 7/8-inch tubes were cut in half to yield four four-foot lengths. One 3/4-inch tube was cut in half to yield two four-foot lengths. From the remaining 3/4-inch tube I cut off an 8-inch piece (for later use in the gamma match) and cut the remaining length in half to yield two lengths a little over 3.5 feet each. I then took the 1-inch tube and cut a slot in each end to a length of about 1 1/2 inches. Pushing the tube endwise into a band saw makes a really nice double-slot arrangement. I did the same at one end of each 7/8-inch tube. When the elements are assembled, hose clamps will pinch the slots closed and keep the element sections in place (see Figure 3).
Slappin' Together Time
In the garage I erected a 3-foot tripod. I then cut a small piece off the un-swaged end of the second five-foot TV mast (so it would fit in the trunk) and installed it into the tripod. I mounted the boom on the mast with U-bolts and clamps and attached the two 1-inch tube sections to each end of the boom with U-bolts and centered them for balance. I then slid the un-slotted ends of the four 7/8-inch tubes into the ends of the 1-inch tubes, holding them in place with hose clamps. I inserted the the four remaining tube sections in place (using the two shorter 3/4-inch tubes on the driven element). You won't believe how big a 10-meter beam seems when it's inside a garage!
Some Last Element Details
I drilled a hole at the center of the driven element and installed a bolt to attach the shield of the coax. I drew a ring around both 1-inch tubes with permanent markers to show the exact center for easy assembly. I used black when marking the driven element and red on the reflector. That way, in the field I wouldn't have to stop to figure out what was what (that's also why I cut slots into only one end of some of the element sections).
Feeding the Antenna
As you may have noticed, this antenna uses "plumber's delight" construction. The driven element isn't split into two legs like a conventional dipole. In this case, the driven element is one piece, and everything is shorted to the boom, to the mast and to ground. To top it off, the whole mess is fed with unbalanced coax. So, how does it work? Like magic! And the magic words are gamma match. There are actually several ways to feed a plumber's delight antenna, but the gamma match is probably the simplest.
How a gamma match works is beyond the scope of this article.* In short, the braid of the coax is connected to the center of the driven element (since this is where the voltage null occurs in a half-wave conductor). The center conductor of the coax is connected to the same driven element through a capacitor (eliminating the short circuit) some distance away from the center. In the old days we used tuning capacitors from discarded AM radios. Tuning caps are as scarce as hen's teeth nowadays, so I decided to try a technique I'd come across in the 1974 ARRL Antenna Book** incorporating the capacitor into the structure of the gamma match.
Building the Gamma Match
I took the 8-inch piece of 3/4-inch tubing that I had set aside before and cut it to 5 1/16 inches. I cut a piece of 5/8- inch plastic tubing to a length of 5 inches and cut the 1/2-inch aluminum rod (tubing will work) to 24 inches. Sliding the plastic tubing onto the 1/2-inch rod until their ends were flush, I now slid this assembly into the 5 5/16-inch tube until 1/2-inch of the plastic tube was left exposed (see Figures 4 and 5). I now had a capacitor!
I drilled a hole near the end of the 5 5/16-inch tube and installed a small bolt for the center conductor of the coax. This assembly was mounted to the driven element so that the larger end (the one with the bolt) was directly under the center of the element and the two tubes were four inches apart center-to-center.
The gamma match is held on by an insulated strap at the end closer to the center of the driven element and by a conductive aluminum strap at the other end. The locations of the straps aren't critical at this point. The straps themselves can be made of any sturdy insulating and conducting materials. I used flat plastic stock and flat aluminum stock (1 inch by 1/16 inch cut to 3 inches worked fine) held in place by copper clamps (designed to hold copper pipes to a wall). These clamps are shaped like a letter C, come in all sizes, already have holes in them for attaching to the straps, are easily bent to fit around the 1 inch element, and are inexpensive. Mechanically, everything looked good! But would it work?
Tuning the Antenna
I was fortunate that it was a beautiful summer day and that I had my wife, Donna, AA1DQ, to help me. I disassembled the monster in the garage and reassembled it on the lawn by myself to test my simplicity theory. Everything went together nicely in about 15 minutes. I attached the braid of the coax to the driven element and the center conductor to the gamma match. The fact that it was only four feet off the ground would have little effect on the tuning, although the overall performance would be affected by the high angle of radiation. Leaving the hose clamps over the element slots loose, I adjusted the driven element length to about 17 feet and the reflector to about 18 feet 1/2 inch.
I would make the adjustments while Donna, visible through the shack window, keyed the transmitter and recorded the SWR readings. It goes without saying that visual (or some other positive) contact is imperative for safety. She could see that I was clear of the antenna before keying the transmitter.
I find that it's best to keep a written record when tuning an antenna (even if it's only a dipole) so that I know where I am and which way I'm going. I make a chart with frequency on the Y axis and antenna length on the X axis. I then enter the lowest SWR point (resonance) at the appropriate X-Y position. As I change the length I can easily see what's happening.
If you find that the SWR at your chosen frequency is unacceptable, begin adjusting the gamma match by sliding the center bar in or out.. If you can't achieve a match, slide the entire matching section toward or away from the center of the driven element. As a last resort, adjust the driven element length. This will also have an effect.*** Remember to keep records. Otherwise you may get your adjustments all out of whack and won't know where you are. When you're done, tighten the hardware on the gamma match, as it will not be moved again.
After only a few adjustments I could obtained a 1:1 match at any chosen frequency (28.0 to 28.5 MHz). A match of 1.3:1 was attainable beyond these frequencies (up to 28.6 MHz). I set it to 28.4. Your mileage may vary.
I was overjoyed. Using the very unscientific method of comparing S-meter readings, the beam showed very good side rejection and a respectable front-to-back ratio. I marked the element sections at their contact points with a ring using the same permanent markers. When erecting the system I could simply slide the sections to the rings and tighten the hose clamps. There was nothing left to do but try it out on Beseck Mountain. Along the way I got some foot-long metal tent pegs to hold the tripod steady.
The antenna has been used several times mountain-topping and contesting. It performs well and can be erected by one person in about 15 minutes. It was well worth the effort. I have since gotten another section of mast and, with two people, it can easily be put up at 10 feet.
I haven't experimented with the reflector length yet to see the effect on the gain and the front-to-back ratio. As they say, "If it ain't broke, don't fix it!"
* A gamma match - This allows connection of a coaxial cable to the driven element of a beam antenna. Because the RF voltage at the center of a � wave dipole is zero, the outer conductor of the coax is connected to the element at this point. The inner conductor, carrying the RF current, is tapped out on the element matching point. Inductance of the arm is tuned out by means of a capacitor. More information on gamma matches is available in the current edition of the ARRL Antenna Book. Also see ARRL/TIS page on Gamma match (FREE Adobe's Acrobat Reader required).
** The ARRL Antenna Book, 1974, pg. 205, fig. 9-7
*** I comment here on the sequence of the tuning process. The sequence shown above is that presented in QST. My manuscript had a different sequence, but I was persuaded by the editor to change it. Normally the capacitor should be tuned first, but as you will find if you construct the antenna, the fit of the the inner rod, the plastic tubing (the capacitor's dielectric) and the outer tube is quite tight. When making this adjustment, you want to be sure that the plastic tube and center bar slide together, as a single unit, within the outer tube, insuring that the plastic tube and bar ends remain flush. For that reason, I slid the outer (conductive) connecting strap first, then played with the driven element length next. I was fortunate in that I didn't have to mess with the capacitor. I hope you are as fortunate.
Bill of Materials
(2) 5-foot light-duty TV mast
(1) 1-inch x 8-foot aluminum tube
(2) 1-7/8-inch x 8-foot aluminum tube
(2) 1/4-inch x 8-foot aluminum tube
(1) 1/2-inch x 4-foot aluminum rod
(1) 1-foot section of 5/8" clear vinyl tubing
(1) 2-foot aluminum angle
(2) 1-3/4-inch U-bolts
(4) 1-1/4-inch U-bolts
(8) Hose clamps to fit on 1-inch tubing
(6) 2-inch bolts & hardware
(2) 1 1/2-inch bolts & hardware
(4) 1/2-inch bolts & hardware
(1) 3-foot tripod
(3) 1-foot metal tent pegs
This article is reproduced here with the express permission of the American Radio Relay League and QST and Al Alvareztorres, AA1DO. Magazine. It may be reproduced in it's entirety as long as you make no profit and credit is given to QST
< 102 ft. >
| 34 ft matching section *
* Adjusted for velocity factor of feeder, if necessary.
* HINTS, ETC. *
This is my first antenna project. I built mine with insulated 12 ga. wire I had laying around. Initially I had "homemade" open wire feeder for the matching section. The antenna worked fine on 20 and up, but turned my tuner into a "self lighting" meter unit on 40 and 80 (smelled kinda funny too). Originally I blamed my budget tuner, but after replacing the open wire (because of storm damage), it now works fine on 80, 40, 20, 15, and 10. I don't have WARC capabilities and can't report performance on those bands.
The replacement matching section is 300 ohm twin lead with the length adjusted for it's velocity factor ( 34 ft * .82). The top is kinda touchy when it comes to trimming. My antenna is hard to get at so i shortened up 1 ft first try and moved the resonant section from well below the band (SWR was 1.6 at lower band edge on 20m) to above 14.2 (SWR 1.3). Light trimming is suggested. Due to the normal high SWR, others have suggested not using a coax feeder with wimpy dielectric, particularly if running substantial power.
So how's it work?
I LIKE it.
One antenna that works well on all bands. I've received good reports, voice and cw, on all bands running a barefoot Yaesu FT 101B. Surprising number of contacts from QTHs that would seem to be unfavorable because of my antenna orientation. My next G5RV is going to be a half size unit wit a different orientation and extending the 300 ohm line all the way back to the tuner. (G5RV recommends the open type feeder, by the way). If you build (or have built) a G5RV and have anything to add about this versatile antenna let us know.
Why is an Magnetic Loop antenna so special, this antenna is picking only the MAGNETIC part of the ELEKTRO MAGNETIC radio wave. The big advantage of this antenna is that the electric interference from the big city (streetlights, television's , cars etc...) have no influence on the received signal. With the loop you can hear other stations that you can't hear if you use a DIPOLE, with a dipole the stations are buried in the noise.
This is the first loop I build from a article in the QST from February 1996, it's 30 Inch-diameter, and it's designed by G2BZQ/WØ for 80 M.
The first one turn loop that I built was made from 75 Ohm TV Coax and with a small explanation in the RSGB handbook for radio amateurs. I used the outer screen from the coax and the results of the loop where good.The next loop I built is a octagon loop in 15mm copper tube with a circumference of
4.8 meter (16 feet).
The frequency range of this loop goes from 14 MHz to 7 MHz and works fine. The biggest problem is the tuning capacitor, if you transmit with a power
of 100 W you need a capacitor with a voltage rating of 5000 Volt.
A capacitor which can handle this voltage is hard to find over here and if you find one they are very expensive.
The first capacitor I built was a design from GW3JPT from a article in the RADIO COMMUNICATION from February 1994. It is a split stator capacitor with a capacitance of 140 pF and with a voltage rating of 6000 Volt.
The capacitor is remote tuned with the use of a small BBQ spit motor.
The second capacitor I built is my own design and it's a butterfly capacitor because the losses are lower than a split stator.
The capacitance is 5-65 pF and the voltage rating is 7200 volts. I used
it for the small loop with a dia. of 800 mm (2.66 feet) and the frequency range of this loop is from 28 MHz to 14 MHz. The Aluminum plates of 1 mm
for the capacitors are cut with a JIG SAW.
Most asked Questions:
I`d like to talk a little more on your setup. it seems like something whichI could get together if only some more data was available. do you have anynotes etc still laying about since its build ?
The theory for calculating the loop is very simple.The circumference of a magnetic loop is 1/4 wave of the designfrequency.
Example for 14 MHz.
300 / 14 MHz = 21.428 m is 1 wave
21.428 / 4 = 5.357 m is 1/4 wave circumference
5.357 / 3.14 = 1.706 m diameter.
The recommendations are that you can tune the loop from the design frequency to the frequency divided by 2 to keep the effiecency acceptable.
14 MHz / 2 = 7 MHz
I made the small loop (800mm / 31.5 ") from soft copper tube on a role that you can buy in a plumbershop and it's easy to make a nice circle if you draw on the ground a circle with a rope and a piece of chalk.
For mounting the loop to the hardboard I used plastic clamps
that they use for mounting copper tube on the wall.
why is a butterfly capacitor better?
For high voltages and currents the use of Capacitors with wiper contacs is not recomended. That's why they use capacitors in serie's.The pro for serie's capacitors is that the voltage rating is doubled.The anti is that the value of capacitance is divided by 2.
For the split stator capacitor the 2 capacitors are connected in series
by the shaft (bleu) and the red spots on the first drawing are losses.
For the butterfly capacitor the 2 capacitors are directly connected
in series by the rotors and gives less losses.
Do you know of anyone that has built a similar loop that
outperformed a garden variety dipole?
Compare antenna's is very difficult , sometimes I have for 60 % better signals in RX and TX on the loops then on the dipole.
In Theorie is the performance of a magnetic loop - 0.4 dB lower then a dipole or a vertical .
I have over here a homebrew trap dipole from 40-20-15-10m and the height aboveground is only 7 m(23 ft), for a good performance on 40 m the dipolemust have a height of 1/2 wave above ground ( 66ft). I don't have a radiation angle on 40 m and it's only good for contacts in Europe and not good for DX, now the 1.5 m loop tuned to 7 MHz with a effiecency of 38 % (38 w ERP ) and a angle radiation of about 20 degrees performs
better than the dipole because the vertical magnetic loop only 1 M above
ground as a angle radiation and the dipole don't.
Another advantage is that the reception on a loop is mutch better,
on 20m I have with the dipole S5 noise from the big city, if I switch to the
loop I have S1 noise and hear stations who are buried in the noise when
I use a dipole.
Coupling loop dimensions?
I find that the best way to feed the loop is with the shielded 1/5 Faraday loop made from coax RG213 or RG8, I tried the gamma match bud I had problems to keep the VSWR low on all Bands, the shielded loop gives on all bands VSWR 1.1 and reduce more noise pick-up then the gamma match.
I found out that if you use a 1/5 Faraday loop, that the loop is to big, making the loop smaller with 0.5 inch by the time in circumference and checking with a field strength meter you can see that the radiated power increase.
The place off the feeding loop is placed at the electrically neutral point on the loop and that is 180° from the capacitor and I have the best results with the feeding loop close to the ground and the capacitor far from the ground.
I was wondering if you worried about the resistance of the mechanical joints (copper pipe bolted to the capacitor) significantly reducing your radiation efficiency as I think the radiation of these antennas can get as low as .01 ohms
Soulder or weld the capacitor plates is always the best, but I'm afraid if you make the spacers and the plates in ALU that with welding everything is gona bend from the heat and I know from practice ( I work in a maintenance workshop ) that welding ALU is coarse. Another possibility is using all brass or copper and solder, there are hams that using double PC board for the plates.
I made a QSO in phone with Florida, RPRT 5-5 and the other station used a vertical antenna, with the small loop (800mm and theoretical effiecency 41 % on 14 Mc) vertical in the garden and the states side is through the house. I was very happy with the results , so I think that a capacitor
maded with torqued compressed joints is good enough for using 100 W.
Have the dissimilar metal joints weather well?
To keep the oxidation low on the dissimilar metals I used a thin coat of vaseline after assemble the capacitor and with the tupper ware a like plastic box it is good protect against all wheather conditions.
How to find the radiation angle of the antenna?
Can it be found practically?
Finding the radiaton of a magnetic loop is very easy, with a TL-lichgt tube you can see it, with abt 10 w power on the the loop with the TL-tube in the plain of the loop at right angle to the circle you see the tube lightning, there where the the light is the farest on the tube thats the radiation angle.
When you refer to washers, nuts and rods you use the term "M6".Please forgive my ignorance, but to what does "M6" refer? Does this mean 6mm?
M6 is (M=metrical) and 6 is indeed 6 mm threaded rod and
you can compare the size with W1/4" (6.35mm) .
A very easy to build Piston Capacitor.
How to build your own Butterfly Capacitor.
The best material for the front and the back is CLEAR PVC 3 or 5 mm thick as alternative you can use GREY PVC or 2 sheets pcboard together with the copper removed .
The best material for the washers, nuts (M6) and threaded rod (M6) is brass or stainless steel,( NON MAGNETIC MATERIALS for the losses).
For the spacing of the vanes you can use 2 washers M6= ( 6Kv) or a nut M6 =( 12 Kv) if you use aluminum plate 1 mm thick.
If you use a nut then the best thing to do is remove the thread by drilling withØ 6.2 mm.
The effective area for the vanes is 11.7 cm² and with the formula
for 2 washers = (0.0885 x 11.7 cm²)/ 0.1 cm = 10.35 pF for 1 air gap.
for 1 nut = (0.0885 x 11.7 cm²)/0.2 cm = 5.17 pF for 1 air gap
If you you make a capacitor with 2 washers as spacing and you make
5 rotor vanes and 6 stator vanes then you have 10 air gaps.
10.35 pF x 10 = 103 pF + 10 pF stray capacitance = 113 pF / 2 = 56 pF
The final result is a capacitor with a value from 5 - 56 pF.
I am not going to take credit for this antenna, I am just passing it along.
This simple to make antenna could be used by hams who have HOA's and other antenna restrictions. I used mine while I was camping.
The Joystick Antenna has been around for years. As you can see by the diagram it is a very simple antenna to build. One important part of the antenna is the ground radial. I only laid down one radial when camping as I did not want wire running all over the campground where my young daughters would have been running around.
I made contacts on 80, 40, 20, 15 and 10 meters with this antenna while camping. As I said it might be useful to those who have restrictions. No it isn't a yagi, or a 1/4 wave vertical. But it does put RF into the air.
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