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Author Topic: Isolation of antenna conductor from nearby objects  (Read 261 times)
AA8RF
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Posts: 67




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« on: May 11, 2003, 09:36:13 PM »

I've never read anything really good on the subject of keeping the antenna conductor away from nearby objects beyond the basics. The basics I have seen are that the antenna obviously has high current and/or volatges along it and you want to keep it in the open as much as possible, away from people and conductors.

My experience so far has been limited to dipole variations secured at the ends with normal insulators, yagis & log periodics, and verticals.

I am playing with an indoor loop in the attic, using uninsulated aluminum electric fence wire. To start I loosely set up wire along the wood rafters and touching the non-conducting insulation. As expected it performed poorly. But then I got to thinking, why does uninsulated wire touching insulators perform worse? My guess is that it is because the wood and insulation are not as good insulators as ceramic, and there is a lot more contact area. Detuning the antenna is not a problem since it is a random loop with an auto-tuner at the feed point. I think signal bleed off is more the issue but would like to know more.

Next I tightened up the antenna and suspended it so that it is touching at only 6 points other than free space. The stand offs are only nylon cord so they are still not high quality, but there is a lot less area touching at least. Better performance, especially on 20 M (the loop is about 80 feet in length).

I am curious whether it is worth moving to better insulators. At some point I will probably go to an outside design for the loop but the attic design makes it easier to experiment.

I am also curious as to whether I would do better with insulated wire given the multiple contact points. I have used insulated wires for dipoles and they seemed to work about as well after compensating for the length shift.

-Jim
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RobertKoernerExAE7G
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Posts: 1435




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« Reply #1 on: May 12, 2003, 04:32:33 AM »

Sounds like a good experiment, 80 feet of insulated wire can't be expensive.

If nothing else, you could run 80 feet of insulated and 80 feet of un-insulated wire for two layers of wire and see if that makes a diff?

In your attic, I doubt if you have to worry about the insulators getting whet?  You could cut up a plastic bottle, or scraps of PVC to see if diff insulator material will make a diff.

Other metallic objects couple with your antenna, detuning it.

Have FUN
Bob

PS: you can download a free trial version of antenna modeling software, EZNEC.com
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N3BIF
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Posts: 1190




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« Reply #2 on: May 12, 2003, 11:46:08 AM »

     No matter how dry, there is still ever changing moisture in the wood, particularly in an attic which is presumably not climate controlled.
      I would definitely go with the insulated wire, I have loops, dipoles, Vees etc and I never use insulators just my tie off cord, I never have to worry about the wires touching anything.
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AA8RF
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Posts: 67




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« Reply #3 on: May 12, 2003, 10:23:08 PM »

Good advice - the next step will be insulated wire and back to back tests.

I am new to loops but the loop plus the tuner seems very versitile, tuning up at least on 160 through 6m.

-Jim
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AA8RF
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Posts: 67




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« Reply #4 on: May 14, 2003, 10:54:15 PM »

Similar performance flipping from one to the other routed on about the same path. For grins I hooked them in series making a large, 2 turn loop. Worked considerable better than either.

Goes back to that old rule, "more antenna is better".

-Jim
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