A Glimmer of Hope

By JonathanGuthrie, 16 August, 2013

You may recall that I'm trying to build a compact whole-band antenna for 80 meters, and the simulations I've done show some odd results that I'd like to resolve. To that end I've built a scale model that should work for both 10m and 12m. Unfortunately, the scale model doesn't work and it doesn't work in interesting ways. In particular, it looks like the antenna wire wasn't cut to the right length. I've been wondering what mistake I could have made that would give me the results, but I hadn't been able to come up with anything.

However, day before yesterday I was reading some things and the concept of "velocity factor" came up. The speed of light in a wire is less than the speed of light in a vacuum, and you have to account for that fact when you cut the wire or the antenna won't work. The velocity factor of a wire or feedline is a what you multiply the electrical length of the wire by in order to get the mechanical length. Normally, the velocity factor is about one, so the correction is small, so it's not worth worrying about since the antenna's environment also changes the wire's electrical length and nobody's going to model, say, the nails used to make the roof that an antenna might be real close to. The thing is, an insulated wire has a much lower velocity factor than a bare wire, and both sets of radiators/resonators that I've made for the 10m/12m model used ordinary insulated house wiring.

At that point, I realized that a number of the other antennas I've made that didn't work (chief among them being a couple of "Texas Potato Mashers" for 70cm satellite work) were also made from the same house wire. So, the problem may be that the wire is electrically too long, which neatly explains certain of the phenomena that I've seen. So, stay tuned. I'm going to be checking out this theory over the next few days.