Hi Bill,
> I think bandwidth *is* a good indicator of mobile antenna efficiency,
> provided you're comparing the same configuration and changing only the
> coil.
If I have a fixed antenna design and add the same effective inductance with
two different loading coils, one coil with considerable shunting capacitance
and the other without, it is quite possible to have LESS bandwidth with the
LOWER Q inductor! It works this way on paper, on models, and in the real
world. Of course if the antenna system changes, the effects are much more
profound.
For example a typical 160 meter mobile coil with 40pF shunt capacitance in
the coil design and 80pF of antenna load capacitance matched to a 50 ohm
system would have a 1.5:1 bandwidth of 20kHz.
An exactly identical antenna system having 5pF of shunting capacitance in
the coil design would have 27kHz bandwidth, even though this wider bandwidth
coil would have significantly LESS loss.
This is why looking at bandwidth alone tells us nothing about efficiency,
even if the SYSTEM physical size and construction outside the loading
inductor is fixed and only the effective Q of the inductor is changed.
We can never use bandwidth changes to predict system efficiency, unless we
are positive ONLY the loss resistance has changed and none of the reactance
values in the system have changed. Bandwidth is a very poor way to estimate
efficiency, even in a fixed size system.
73 Tom
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