The photo shows one end cap of the coil fully assembled into the 2 inch PVC pipe cap. Here are a few tricks of the trade you need to know.Antennas, Home Brew
Last Modified: August 17, 2010
Contents: Basics; Homebrewing A Coil; Other Ideas; Masts; What Can Be Done; Odds & Ends;
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 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'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. 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'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 end cap 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, 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'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. 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.
By the way, here is a handy, on-line calculator for figuring the inductance of a given-sized coil.
If you want to get wild, here's a different slant on remote controlled antennas. The coil shown at right (click on it for a full-sized view), is an old TBC 640. Installed in the housing are two 15 kV latching, SPST relays. Although they are 24 volt units, they switch easily with 13.8 vdc. They are polarity sensitive so correct wiring is essential. Since they are latching, control switching is very simple. However, because they are mounted high above RF ground, you have to do extraordinary RF bypassing on the control leads. In my case, both mix 31 ferrites, and parallel caps were needed. The relays do reduce the coil's Q, but since it fairly high to start with, the hit isn't significant.
Yes, it's ugly, but it works are good as any HF mobile antenna I have used. If I were to do this over, I would have used SPDT relays which would lend itself (with careful planning) to a 5 band antenna with just 2 relays.
As alluded to above, some folks opt to use perhaps a 15 meter loading coil, but add length to achieve resonance on a lower band; twenty meters in this case. It's difficult to use a 20 meter coil on 40 meters, because the overall length would be over 16 feet!
Sometimes you can find old Master Mobile coils on ebay. Although old, most are still usable or at least adaptable to other means of support, like a homebrew 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 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 two other views in the Photo Gallery under his call.
The coil form, 4 inch OD by 9 inches long, is made from clear Lexan®, and wound with 14 gauge, tinned buss wire. The tuning sleeve is made from 16 gauge aluminum, including the spun-aluminum botton 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 resonante 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.
Since the above photo was taken, Jerry has added remote control to the antenna utilizing a linear actuator, as shown at right. To say the least, the completed antenna is a work of art!
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.
Before you plan, or complete, any antenna installation, do yourself a favor, and read my Grounds, RF & DC article.
The next article you need to read is Antenna Mounts.
The next article after that is Antenna Matching.
Bonding is also very important. Bonding maximizes the performance of any antenna system, and minimizes the chances of of RFI ingress and egress.
If you have RFI issues, and you don't know what they are, read my Noise ID article.
If you're plagued with static, you might want to read my Static Control article.