Fence to protect Vertical Antenna - will it interfere?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB8LFA, Jul 31, 2010.

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  1. KB8LFA

    KB8LFA Ham Member QRZ Page

    I have a Hustler 6BTV w/64 ground radials going out 65' in most directions (33' in some). I need to put a fence around the antenna itself (not all the ground radials). I need to do this to protect it from animals that on occasion are put in the field where the antenna is (mainly sheep).

    The easiest (and strong enough) way of doing it would be to just use 7' metal chain link fence posts and a simple galvanized garden type fence. The sheep will not bother it as it's a barrier and they are not very ambitious. My concern is that it will interfere with either the main antenna or ground radials.

    1. How far away from my antenna does the fence need to be (48" tall). I want to fence off as little as possible, say a 4'x4' box around the antenna but do not want to cause issues with the antenna.

    2. Will the metal fence cause weird things to take place with the ground radials running under the fence? The ground radials have been in place and stapled to the ground for about 3 weeks now. The grass has grown quite a bit. I can still see them if I search for them, but you really have to look.

    I am open to other suggestions as well.

  2. K4SAV

    K4SAV Ham Member QRZ Page

    I did a quickie EZNEC simulation to see if I could estimate what a fence would do. I made a square, 4 ft high fence, 4 ft on each side and placed a vertical in the center. The largest effect was on the highest band, in this case 10 meters since more of the antenna is below the level of the fence. The fence absorbed about 15% of applied power on 10 meters. That went down on the higher bands (4.5 watts on 15 meters). I had to guess at the impedance of the fence posts to ground, but I checked a bunch of different values and there was not a lot of difference in absorbed power for different values of fence to ground impedance. The gain loss on 10 meters due to the fence was about 1.2 dB. That's a little more than would be accounted for in the 15% of power absorbed, but the fence causes the feedpoint impedance of the antenna to drop some and that results in increased ground loss. I don't think the impedance drop will be enough to give SWR problems. It was a small change.

    Rescaling the model to a 7 ft high, 7 ft on a side fence, resulted in 2 dB loss on 10 meters.

    Jerry, K4SAV
  3. KB8LFA

    KB8LFA Ham Member QRZ Page

    Wow, that's pretty cool Jerry. Could you save that simulation and attach it to this forum post? I've attempted to use EZNEC before but was never able to set something up that resembled my configuration.

    I'm now wondering if I should find another solution to this problem. The sheep will certainly do damage to the unprotected antenna so it has to be protected somehow.
  4. WD5ABC

    WD5ABC Ham Member QRZ Page

    Instead of protecting the antenna, can you mount it on a pipe about 6ft off the ground so they can't get to it? Then either elevate the radials or let them angle down to your fence 4ft or so away from the pipe and buried the rest of the way out? I'd have to model doing the radials that way but it might work and the fence wouldn't be interfering with the antenna at all. It could even be part of the radials.

    Kerry, WD5ABC
  5. M0WAN

    M0WAN Ham Member QRZ Page

    Or use wooden fencing panels secured to wooden posts. Properly installed, this will keep most animals out and have negligable effect on the functioning of the antenna.
  6. G0GQK

    G0GQK Ham Member QRZ Page

    Why does it have to be steel ? Why not wood ?

  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    That was my first thought, also.

    I've put up "protective fences" around verticals before; always used wooden posts and either wooden pickets or PVC (or whatever molded plastic material those are!) that come precolored and look just like wood until you touch them. Those are strong, light and cheap and keep animals out just fine.
  8. KB8LFA

    KB8LFA Ham Member QRZ Page

    It does not have to be steel, it's just that I have a lot of it laying around and I was hoping if I got far enough away I could just use what I have laying around.

    After this discussion, I'll just make a wood fence around the antenna base. Just hoping to save a buck or two I guess.
  9. KA3TQG

    KA3TQG Ham Member QRZ Page


    Brother use wood !!! Goats may chew it but sheep should not...
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