> complete azimuth coverage. Given those limitations (again
> very
> common to city dwellers), I think that there is some
> worthwhile
> advantage in chosing top-loading over base loading.
So long as loss in the loading system doesn't increase more
than loss by ground current reduction decreases, that's
true.
> I would agree that with a good ground system (30 to 60 1/4
> wave
> radials with fairly uniform angular spacing), the choice
> of
> loading technique won't make that much difference.
It's a mixture of several things and we can't focus just on
one thing. It's difficult to put in a few words when the
system has different current in every part of the system.
If the ground resistance is pretty high (as in a system with
a very small or lossy ground, like a mobile) the radiation
resistance or current distribution can be the most important
thing. That changes as the ground improves.
For example moving to an optimum top load in a 1/8th wave
vertical creates nearly uniform current distribution in the
antenna, and that approximately doubles radiation resistance
over the triangular distribution of a base loaded vertical.
See the graphs at
http://www.w8ji.com/radiation_resistance.htm
That means losses in the earth would be reduced by as much
as 50% if they were all due to ground current at the antenna
base. That's a big change if the ground losses totally
dominate the whole system and most of the transmitter power
was being turned to heat. The ERP might go up around 3dB.
With a typical mediocre ground the change would be much
less. With a very good ground losses wouldn't any noticeable
amount at all from the change in radiation resistance.
It always helps to put the perspective change into real
numbers, and we know it is less than 3dB from the change in
radiation resistance in a current fed antenna.
It's certainly possible if we use a bad loading design to do
more than 3dB of damage. The GAP vertical is a good example
of loss. They worked pretty hard to get the 10dB or more
loss on the lowest bands (the Titan was measured about 16dB
below a 1/4 wave vertical on 80 meters). Still, many people
are happy with that kind of loss! 16 dB loss makes <3dB loss
look unimportant.
73 Tom
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