Incredible isn’t it? But can it really work?
“It must be a compromise antenna, or very low efficiency.”
“Why don’t you just put up a good old-fashioned dipole?
“One square foot? Are you crazy?”
No, we’ll yes, I may be a bit crazy but believe me, the performance of my “one square footer” is no compromise. The efficiency isn’t too shabby either. I’ve logged contacts all over the world with it.
See, I told you so
Oh, and I’m a contrarian thinker; trust me it helps. One square foot is an area of 12 inches on each of four sides, or 130 feet long by one tenth of an inch wide. As in a long piece of wire – like a dipole.
And now for something completely different
But, this post isn’t about dipoles. I have built another antenna that occupies about half as much space again – actually more than one and a half square feet.
Walkies and Talkies
Unlike the dipole I can take it for a walk. It was much more fun to take my dog (SK) for a walk but this antenna comes a close second.
So what’s so special?
Well, apart from the walkie-talkie thing (or in my case “walkie-Morsie”) this antenna is really just a plain vanilla, short loaded vertical – with one small variation.
A vertical antenna needs a ground plane, or radials, or a counterpoise or something else to form the “other half”. I chose the planet. I’ll explain in a moment.
There is a group of hams called “HFpack” who specialize in operating while walking with their radios – and antennas – on their backs. What do HFpack enthusiasts do for the “other half” of their antennas?
What a Drag
The answer is they use a “drag wire”. Imagine 15 feet of wire bouncing along the trail behind you. If my dog weren’t a silent key she would be chasing those drag wires, then digging her heels in with the wire end in her teeth and doing a splendid job of de-tuning the antenna.
Change the World
Nah, I have a better solution. It’s called a Ground Tuning Unit or GTU. It’s a little box that resonates planet Earth to match my antenna.
The GTU connects to my aluminum hand cart which provides a capacitive link to ground. Inside is a series connected variable capacitor and inductor. A current sensing circuit with a meter is used to see when the ground current is at a maximum for optimum tuning.
While operating as a pedestrian mobile station with my short vertical whip (with coil for tuning 10-40m) also mounted on the hand cart, I can tow my entire station around and never occupy more than one and a half square feet. Try that with a dipole! Oh, wait …
Other GTUs employ a variable inductor as well as a variable capacitor. I had a different idea but it involved a little math. My favourite bands for outdoor operating are 20m and 40m. I calculated the range of resonant frequencies I could get with known values of capacitance (min and max) and a fixed inductor. It turned out I needed two values of inductance to achieve resonance throughout the range of frequencies I wanted to work. So a tapped inductor was used with a switch on the front panel to select high (marked as “I”) or low (marked as “i”) inductance. It works.
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