Contents: Basics; Commercial Coils; Homebrew Coils; Masts; Odds & Ends;

Basics

I'm been thinking about this article for a couple of years. I've hesitated because too few mobile operators actually build their own mobile antennas. 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's 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 GLA Systems (Texas Bug Catcher) have 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's 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. What ever avenue you take, you need to bone up on antenna design. The first place to start is the ARRL Handbook.

Commercial Coils

There are two manufacturers known to me that make mobile loading coils. Unless you are an antenna design engineer, you have just one; GLA Systems. Just in case you are interested, the other is HiQ Antennas. If you opt to have them make a sealed mono band coil for you, you must give them all of the necessary parameters. That is, the diameter, turns per inch, size wire, total inductance, type of end caps, and a wad of money. Quality costs! If you opt for the lessor expensive, open air design, then again GLA Systems is your company.

The GLA Systems coils are fully assembled, and the recommendations on their web site with respect to which one to buy for which frequency is close to being correct. The one thing you have to remember is this. Large coils work fine on the lower frequency bands. They can be short tapped to reduce their inductance (operate on a higher frequency), but the losses will be greater than a properly size one for the higher bands.

The reason for this is rather esoteric. In simple terms, large coils have a lot of distributed capacitance. This means their self resonant point is low. Attempting to use them over their self resonant point greatly effects their efficiency.

You might be saying at this point, that companies like Hustler make mobile antenna loading coils. That's true, but most folks don't opt to buy their coils, and then make their own whips and masts. There is, however, a hidden agenda For example, it is possible to use a 15 meter loading coil on 20 meters by making the whip length somewhat greater. This does increase efficiency, as well as length. If that is your course, I applaud you for your resourcefulness.

Homebrew Coils

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 homebrewers 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's 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.

The photo shows one endcap of the coil fully assembled into the 2 inch PVC pipe cap. Here are a few tricks of the trade you need to know.

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, high res photos of the coil are in the Photo Gallery under Hardware.

Once you are satisfied that the coil form is like you want it, it's 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. 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. Just make the windings are snug as you can. Once you've got it where you want it, a wee bit of acetone, PVC cement, or even superglue will hold them in place. I wouldn't do this step until you're sure where the resonant point is.

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.

Mast

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 and TBC both sell ready-made masts in several different lengths, and they are reasonably inexpensive.

If you want to homebrew your mast too, Mike Brueggemann, K5LXP, just might have the answer. 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. If you look in my Installations article under W5LLD, you'll see one of these. A coat of paint, is all it needs. This particular mast is 4 feet long, and the loading coil is from an old ARC5.

Odds & Ends

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.