Dear LF Group,
First, the measurements on G3XDV's antenna - The SNR in a 25Hz
measuring bandwidth and 9.3km distance was probably about
50dB, so noise was not really an issue. I checked the calibration
by an independent method after each measurement. So I am
confident that there was a slight increase in FS, and that it was
near 0.9dB
A decrease in current from 2.7 to 2.2A would result in FS
decreasing by 1.8dB if the antenna remained the same. So overall,
there was an improvement of 2.7dB from the radiating point of
view, although offset by increased losses, apparently. If ways
could be found of reducing these added losses in a practical way,
this would be a significant improvement in LF antenna technique.
What ZL2CA says about increasing TX power to make up for
reduced antenna efficiency is only true up to a point, as some of
us have found already. There comes a point where the voltage on
an antenna becomes so high that breakdown occurs. Since for a
given antenna, the voltage is directly proportional to the current,
this places an upper limit on the ERP for a given antenna.
Once this limit is reached, there are two ways to go - one is to
increase the effective height of the antenna, and so obtain more
radiation for a similar current/voltage, the other is to reduce the
voltage by adding more top loading, basically increasing it's area
(or footprint, as G3AQC would say). Doing either of these things
also tends to reduce the losses, reducing the power required in the
first place.
So the size of the antenna restricts both the power that can be put
into it, and the efficiency of radiation from it, so placing a definite
upper limit on ERP. Most of us are not lucky enough to be able to
change the height or area occupied by our antennas at will, so
maximising the ERP of our stations depends on finding the best
compromise between antenna configuration, and transmitter power
with the volume of space we have available. Since our
circumstances are all different, the best compromise is likely to be
different too.
By the way, I find that the loss resistance of the antennas I have
used tends to increase by a factor of 2 or 3 when frequency is
reduced from 136k to 73k - in indirect support of Rik's idea about
ground loss being reduced at increased frequency.
Cheers, Jim Moritz
73 de M0BMU
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