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LF: PF per Meter dependant on wire size?

To: <[email protected]>
Subject: LF: PF per Meter dependant on wire size?
From: "James Moritz" <[email protected]>
Date: Tue, 30 Mar 2010 19:56:11 +0100
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Dear Paul, LF Group,

I have attached an excerpt from Terman's "Radio Engineers' Handbook", which gives formulas and tables for calculating capacitance of multiple parallel wires.

A more general way of calculating capacitance of antennas is to use one of the NEC-based antenna simulators. Doing an "SWR" simulation using "perfect" ground and lossless wires gives you an impedance consisting essentially of the capacitive reactance plus the radiation resistance. I have found this gives results that are reasonably close to reality, and can be used for any complicated collection of wires. Also it gives you a way of optimising the radiation resistance.

As far as Paul's antenna goes, obviously what is possible will depend on the location of the mast and its surroundings, other antennas, and so on. A straightforward way of increasing C would be to connect additional sloping top-loading wires at the insulator at the top of the tower, and fan them out over as wide an angle around the tower as possible, to maximise spacing and so capacitance. Having the top loading wires coming close to ground level would be detrimental for radiation resistance, so it would be better to make the added wires shorter than the existing wire, or better still if possible, support the ends higher off the ground. Remember that basically you want to have the greatest possible average height of the wires to maximise Rrad, as well as a long wires to maximise C, so a trade-off is likely to occur.

In my "tree current" experiments, I found that roughly twice as much current flows to ground through the tree trunks at 136k as does at 500k, for a given antenna and antenna current. This is consistent qualitatively with the reduction in Heff, and increase in Rloss that occurs at the lower frequency. This would suggest that trees would have an even greater effect at lower frequencies like 9kHz, although there is probably a lower cut-off frequency below which the current distribution is determined entirely by the proportion of total antenna capacity between the trees and the actual ground, and the resistance of the trees is much lower than the capacitive reactance of the tree path.

Cheers, Jim Moritz
73 de M0BMU

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