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Re: LF: RE: Antenna tests on 136k and 503k - Results

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Subject: Re: LF: RE: Antenna tests on 136k and 503k - Results
From: John Andrews <[email protected]>
Date: Sun, 09 Dec 2007 14:26:33 +0000
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Dave, all,

That's an excellent bit of work. Very depressing for city dwellers...

Actually, when carried to the extreme, it's good news for city dwellers. Since the principal losses have to do with vegetation, and that may be either non-existent or many stories below, antennas stretching between city buildings can be quite efficient.

I've given the example before of a 350 foot single wire T on 580 kHz with a 30 foot downlead. It was supported on the ends by two 10-story buildings, and the feedpoint was on the roof of a 4-5 story building in the center. There was a LOT of copper cornice work in the area, and it had been carefully bonded together, and tied to some heavy copper cables which ran to the basement of the center building, thence to the water mains. The feedpoint resistance actually dropped over the years as air-conditioning equipment and piping was added on the roof. The last measurement of impedance was 2.3 - j240 ohms, as I recall. The antenna was taken down about 20 years ago.

As Jim points out, the loading coil controls the efficiency in such cases. If memory serves the tuning setup was about 50% efficient in the above example. The simplest coupling arrangement is a single coil, grounded at the bottom, with a coupling tap. It can be analyzed as a high-pass L network.

Anyway, excellent work, Jim. I hope your results will be recorded on a web site somewhere.

John Andrews, W1TAG



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