Dear LF Group,
Thanks to those to commented on the antenna loss measurements.
I added the data from ZL2CA, and also some provided by G3NYK
to the graph of results from my own antenna (see attachment). The
empirical formulas for the loss resistance of the 3 antennas work
out to:
G3NYK (6m high, 30m long inv. L): Rloss = 4733/f^0.86 (for points
below 600kHz only)
M0BMU(8m high, 40m long inv. L): Rloss = 1550/f^0.75
ZL2CA(Don't know, but must be quite big!): Rloss = 142/f^0.50
So as the antennas get bigger, the loss resistance gets lower and
falls off more slowly with increasing frequency. This is reasonable
compared with big commercial antennas, which have very low
Rloss (a few ohms or less) which rises with increasing frequency.
The fact that loss resistance decreases as the antenna gets higher
is consistent with the majority of loss occuring in the ground under
the antenna - for a given antenna voltage, as the antenna gets
higher, the E field at ground level will get lower, reducing the
dielectric loss in the ground. This also explains why the (much
higher) commercial antennas are not as significantly affected by
this type of loss. It also ties up with G3AQC's "footprint" idea -
increasing the area covered by top loading will also reduce the
field at ground level, and result in lower losses.
It seems most people do not use a loading coil to resonate the
antenna whilst measuring the loss resistance using bridge
methods; the reasons I did were:
- The loading coil filters out most of the signal from the broadcast
stations near my QTH - otherwise, these can put a watt or two into
the unfortunate bridge detector, even though the antenna is not
resonant.
-Having the antenna resonant means you are measuring just the
resistance, rather than the resistance riding on a much larger
capacitive reactance. This means that, for a given amount of
resistance unbalance, the voltage at the bridge detector will be
much higher, and so the SNR at the null will be better.
-Likewise, the effect of imperfect balance in the bridge transformer
is less important when the antenna is resonant and purely
resistive. This is especially so for errors in the phase between the
voltages across the two bridge transformer windings, which will
effectively transform part of the reactance into resistance.
Cheers, Jim Moritz
73 de M0BMU
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