Jim M0BMU and other readers,
I don't think I have a "special antenna" but I have been thinking as to why
it is a low resistance system.
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.
One possible reason for my low loss is that one part of the top loading is a
delta loop that goes over my house roof, and the roofing material is
corrugated iron. Many years back I decided to bond adjacent sheets with
stainless steel self tapper screws, and earth the whole roof, so as to guard
against intermodulation and harmonics from HF transmitting with 400 watts
PEP. The earthed iron sheets are far more conductive than soil, so under
that part of my top loading I probably approach "metal ground plane
conditions". However, it also is very likely it gives me a significant
penalty in effective height. The other side of my top loading is reasonably
in the clear, and has fairly good earth radials under it as they were easier
to run, as most of the ground is lawn. At the time of the impedance
measurements I had a multi-wire fan, with 9 or 10 wires in the fan, to give
high capacitance and all wires were at good height. However, it used the
street frontage pole originally put there for telephone and mains power
reticulation, but I found it very convenient to bang a nail in the top and
support LF top loading. As I mentioned earlier, I received a "Dear John"
letter about my use of the pole, so I decided to remove my wire, which had a
drastic impact on my top loading. Now my resistance increased with the
reduced top loading.
So in summary, I'm probably doing well on low losses because of the earthed
roof, but on the other hand I would rather have an antenna than a low loss
capacitor on the end of the transmitter, so there are swings and roundabouts
as to resistance and effective height.
73, Bob
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