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Re: LF: Aerial conundrum.... ZL2CA's antenna

To: [email protected]
Subject: Re: LF: Aerial conundrum.... ZL2CA's antenna
From: "James Moritz" <[email protected]>
Date: Wed, 31 Oct 2001 14:08:38 +0000
In-reply-to: <[email protected]>
Organization: University of Hertfordshire
Reply-to: [email protected]
Sender: <[email protected]>
Dear Alan, Bob, LF group,

Distributed circuits like the spiral top load can have many modes of resonance, so one might expect complex behaviour. I remember having a difficult time trying to predict the resonant frequency of multi-turn loops at HF for this reason. One way to isolate different effects going on with the spiral top-loaded antenna would be to temporarily connect a shorting wire to the beginning and end (and preferably the middle too) of the spiral inductor/capacitor. One would then have a straightforward top-loaded antenna of the same dimensions for comparison measurements. I suspect the effect of adding radial loading wires would be to make the spiral behave more as a lumped loading coil, since it's capacitance to ground would be a smaller fraction of the total antenna capacitance. Radial wires would bring the antenna closer to resonance at LF, reducing the overall impedance of the antenna, which would be a good thing from the efficiency point of view, always assuming they are not buried in trees, or lower than the rest of the antenna, in which case their effect might well be detrimental, since they would be the highest voltage parts of the antenna - the usual problem exists of trying to minimise the number of variables in an experiment.
I'm still not clear what the configuration above ground of ZL2CA's
antenna was at the time he did his impedance measurements - if it
was 8m high then as it is now, the estimated loss resistance of
around 12 ohms at 136k is extraordinarily low compared with other
antennas of similar size. It would certainly be interesting to know,
since this represents a substantial improvement in efficiency, a
factor of 3 better than mine, for example.
Cheers, Jim Moritz
73 de M0BMU

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