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Fencepost Antenna Experiment

Fencepost 10M Antenna

Len, WA6KLK with fencepost 10M 1/4 wave Ant. 300 ft drop to the right.

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.

 

fencepost ant SWR

Fencepost Ant SWR less than 1.2:1

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.

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Categories: Antennas Tags: , , ,
  1. 2010/11/07 at 2:16 PM

    Whoops, a typo in Len’s call in the first line. The photo caption had it right, but I didn’t notice the other one. Sorry about that! It’s fixed now.

  2. Len WA6KLK
    2010/11/07 at 6:18 PM

    A few comments about this experiment.

    First, it was a fun day and allot was learned and observed. Thanks to Carl for having me there.

    Second, some might think “Why didn’t they just re-tune the antenna –lengthen or shorten it — when using the fence wire as the counterpoise, like using a car as a counterpoise and tuning the antenna for minimum SWR???” Well, with just the fence as a counterpoise, the 2 or 3 to 1 swr range was from over 30 mhz down to almost 24 mhz. Lengthing or shortening the active element just moved that range lower or higher, and did nothing to the swr. It seems as tho the antenna should have been more resonant, but I don’t know why it was not. Fencing had a good long run in each direction and was also six feet high with openings both vertically and horizontally about every six inches. The bottom wire of the fence was buried in the dirt and had been for some time. The added near quarter wave counterpoise — about 105 inches long — really made a difference. The fence was well connected to the ground side of the coax and antenna base. Just one of those things that an engineer with a lot of time will have to figure out. I know what we did and what worked. PS: the SWR was checked at the antenna base and also at the transmitter output.

    Third, it would have been interesting to raise the antenna to about twenty feet off the ground as a true ground plane. It was not possible at this time to do such, AND this was an experiment to see what might happen. Also was thinking that this method could be used by others if they could not really find a place for an antenna.

    Fourth, it was considered that another way to run a small antenna on ten meters would be to use a CB mag-mount antenna cut down to ten meters. Placed on a small piece of thick steel and bolted to a metal rain gutter might also work. This is more like a car mount. Have not tried that yet but if it does work, then it’s a cheap and easy way — if the antenna will handle 100 watts — to have a ten meter antenna that will not readily be seen by others.

    Fifth, a ten meter dipole is only about 102 or 103 inches on a side. It can be mounted in about any configuration of horizontal to vertical and does not have to be a great distance above ground. Just a few more ideas and bits of information.

    Would be nice to find others in the local area that have a ten meter rig but no antenna. Think we can help them and give them another band to operate on also. Besides local work, there is some cross country and DX to be found in the daylight hours.

  3. 2010/11/07 at 6:42 PM

    Good additional notes, Len. Thanks for posting those.

  4. 2010/11/09 at 3:02 AM

    Wow………..What a great article. Thanks 😉

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