“Hey Jim, I’m headed for Oshkosh this year and need a base station antenna for my handheld when I park in the homebuilt section.”
“That’s cool. All the mail order catalogs have them anywhere from $85 to $250. Buy one of these and I’ll see you at Oshkosh—and by the way, come to my forum.”
“Jim, Jim, I can’t afford that kind of money for a simple antenna. What can I gin up? It has to pack up simple ’cause old Brother Rutan didn’t leave a lot of baggage space in my airplane and it has to be compatible with the BNC connector on my radio.”
I guess it always comes down to making it better, cheaper, and easier to use when it comes to pleasing my homebuilding brethren and sistren. OK then, let’s see what we can do about this problem.
I solved it neatly in the February 2013 issue of Kitplanes® Magazine, for those of you who want to strap it onto the prop. But there are mechanical and electrical reasons why you may not want to use this design. Besides, that dipole in a tube radiates half your power directly into the earth ground beneath the antenna and I’d like to see what I can do about fixing that. To boot, the February sucker has to be about four feet long and you need to be able to pack the antenna into the tail cone if you have access to that part of the airplane.
We talk a lot about the elements of an antenna. Simply put, the elements are the metal parts that both receive a signal and transmit your power out. One little something that antenna gurus keep as a deep dark secret is what is called reciprocity. In plain language, reciprocity says that it makes no difference to the antenna if it is being used for receive or transmit; if she hears well, she talks equally as well.
And yes, the metal does make a difference, but just barely so. Whether I use solid silver (the best antenna material known to man) or iron baling wire (pretty poor stuff) for the elements, it takes delicate instruments to measure the difference between the best and the worst metals used for the elements. That means that we pretty much have leave to select whatever metal we want for the elements.
Radiating wire and pin mounted into the BNC connector body. Fill the connector-wire end with RTV or hot glue.
There are other constraints on the elements. They need to be easy to solder to if we are going to use soldered connections, and they need to be very flexible if we are going to wrap this thing up for transportation. And cheap. Very cheap.
Copper is the second-best metal to make an antenna from, followed by gold, aluminum, and iron in that order. The easiest to work and the cheapest is copper. Cheap copper? You bet. Copper house wire (Romex) is less than a nickel a foot if bought new, and it is much less than that in the scrap pile. It is flexible and solderable, two qualities that make it ideal for a cheap portable antenna.
For some esoteric reason far beyond the scope of this article, antenna elements need to be roughly a quarter of a wavelength long, and a wavelength is determined by the frequency for which the antenna is designed. Without too much handwaving, let me just state that a quarter wavelength for thin #14 house wire is about 2800/f, where f is the center frequency of the antenna. If we choose the Oshkosh ATIS (125.9 MHz.) as the center frequency, then our elements will be a little longer than 22¼ inches each. Neat. Two pieces of Romex two feet long will give us six elements (a white, a black, and a ground in each piece of Romex) and we only need four. We’ve got two extra pieces for spares.
Now comes the requirement for BNC connectors on the cable. And from what do we make the mounting plate for all these components?
If we use a small (any size you want) piece of scrap aluminum for a mounting plate, you’ve got perhaps a nickel worth of aluminum. Punch a half-inch diameter hole in the center for a BNC UG-492 connector (Jameco Electronics #311351, $3.25) and mount the four radial ground plane elements at the corners of the plate with 4-40 screws, lock washers, and nuts. Grind down one small end of the fifth (radiating) element to fit into a BNC UG-88 male connector pin (Jameco #355099, $0.99), and there you have your cheap ground plane antenna for about $5 in parts.
Disconnect the BNC antenna assembly from the plate, wrap it around the plate along with the ground plane wires and that is your portable antenna.
“Um, cheaper, Jim?”
Sigh. Never satisfied, eh? OK, so replace the aluminum mounting plate with a scrap of copper PC board material and drill the corners 1/8-inch to fit the copper ground plane wires. Solder the copper ground plane wires directly to the PC board material. That gets rid of about two-bits worth of 4-40 hardware.
“Um, cheaper, Jim. Like maybe for a dollar or so?”
Really running this project to the limits, aren’t you? OK, forget about the UG-492 connector and the UG-88 connector. Buy a UG-1094 connector (www.mpja.com p/n 0507RC for $1) and solder the radiating element directly to the center pin of the connector. You have now built your antenna for what I think is about $1.10 in parts. Here is the caveat for this antenna. Nothing is detachable.
The connection between the radiating element and the connector is the weak point of this assembly. When you wind the antenna up, if it is going to break, here is where it will break. Be gentle with this connection or use a bit of RTV on the pin-wire joint to keep the joint from work-hardening.
The antenna mounted onto the PCB plate. The ground plane wires are soldered to the copper on the bottom side of the plate. The connector is soldered onto the plate; you can use the lock washer/mounting nut if you like to make the radiating rod removable. The refinement where the radiating wire is soldered to the connector center pin has a strain relief made from a paper clip wound into a helix and soldered to both center pin and radiating wire.
How well does this antenna work? We have an antenna test called VSWR (pronounced “vizz-war”) that can tell us exactly that. We also have a computer simulation that mimics antenna performance very well. This program tells us that the VSWR of the antenna will be below 1.5:1 (excellent) from 122 to 128 MHz. and well below 2:1 (very good) across the whole 118-137 MHz aircraft COM band.
The portable antenna mounted to the portable wooden mast and held on with portable nylon tie-wraps. Total assembly time—under one minute.
What’s VSWR? Why does it matter? How well did this antenna fit the theoretical simulation? Why do you bend the ground plane down at a 45° angle? Why are com radios 50Ω and TVs 75Ω? Can I make a nav antenna this way? How do I modify this antenna for the amateur radio 2-meter band? The 150 MHz public service band? Why do some antennas point up and others point out? Will Lassie find Timmy down the well? Stay tuned for the months to come.