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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