Fencepost Antenna Experiment
Len, WA6KLK, dropped by this weekend with the elmer spirit… and a pile of aluminum tubing, plus a base bracket with a built-in SO-239 socket.
It took only a few minutes to piece together a few short segments and cut off the bent tip for a quarter wave 10M vertical. A brandy cork fit well in the tip to keep the water out, and it was ready to bolt to a fence post.
We picked a post that had a unique location: it’s 300 feet above the valley with a steep 45 degree slope on its southern side. Then as part of the experiment, we used the field fencing itself as the counterpoise. We weren’t entirely sure how it would tune.
After tweaking the length of the main element an inch or so, the antenna analyzer gave us a 1.7:1 SWR at around our target frequency, 28.4 MHz. And, because of the large diameter tubes, the bandwidth was reasonably flat over the entire 10M band.
Len suggested we add just a single radial, cut approximately to the band and hung out over a sawhorse. With that single change, the SWR dropped below 1.3:1!
When you think about it, it’s really quite remarkable. The antenna had the entire field fence to use as a counterpoise, terminated in the ground itself, but a roughly tuned wire tied in-parallel with the fence gave us a better SWR. I think it’s just a bit like those fan antennas where current flows into whatever element resonates best.
Encouraged by that change, we cut a second radial and dropped the SWR below 1.2:1.
Of course, this was all an experiment. We know this isn’t an ideal setup, because:
- The antenna is only a quarter wave, so no gain.
- It’s close to the ground (although on southern side, it drops of quickly.)
- The coax is 110 feet long to reach it.
- It could use many more radials.
However, the project was a learning experience, and I appreciate Len taking the time out to share some of his knowledge of antennas. I’m still working on my understanding of the currents generated inside the coax shield, and how to best radiate or terminate them, as well as how to choke return currents that can pass down the outside of the coax and re-radiate unwanted signals. This project helped make the theory real for me.
Sure, the jury is still out on whether this will be an effective antenna for 10M or not. This Wednesday, we’ll give it a trial on the 10M local net (28.405) at 8:20 PM. Please tune in.