Hi Alan and all,
However in practice
the capacity of the antenna continues to increase at about 5 to 6 pF per
metre. This turns out to have two useful effects. First is reduces the size
of the loading coil required to resonate the antenna, and hence reduces
tuning losses. Secondly it reduces the current density flowing into the
ground. Measurements show, and they are supported by some theory from Alex,
that if you double the antenna capacity you half the ground loss. Since ERP
is current squared, halving the ground loss doubles the antenna current,
giving a four-fold, 6dB, more ERP ( a cheap "amplifier").
IMHO that is misleading. If you halve the ground loss (and also halve the
loading coil loss resistance), you could have twice the antenna current,
only if the transmitter can deliver it at the original feedpoint voltage.
But most of us have PAs with limited output power, and would need to
adjust the matching transformer for the new impedance (reduce turns ratio
by sqrt(2)). So, we would "only" gain 3 dB more ERP from the improved
antenna.
73,
Stewart KK7KA
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