> Are these problems (if any) worsened by using BOTH
antennas at the same time
> on different bands with two different radios? Both
4-squares would be using
> elevated radials. Would using elevated vs ground radials
make any
> difference for better of worse?
Bob,
What you are doing is the same thing as co-locating two BC
stations, which is something I have done as a consultant.
Besides the higher loss of an elevated radial system (expect
at least a 3-4dB loss if you use four elevated radials
compared to 30-50 radials on the ground or in the air) you
have a significant new problem. The radials radiate fairly
strong nearfield electric and magnetic fields.
I think mixing multiple interlaced or overlapping radials
that radiate and couple to each other and everything around
the antenna is a HUGE can of worms, even if you believe
strongly that a few radials are as good as 30 or more. (From
experience, I certainly don't.) You also have problems with
radiation from feedlines when you use a small radial system,
like four or so elevated radials. All this interaction is
begging for problems, and now we add the element
interaction!
You can successfully decouple the 160 antenna from the 80
meter by adding a series/parallel resonant trap system at
the base of the 160 elements. This would have to be a
shunt-to-ground L/C combination that is parallel resonant on
160, with a series-to-ground 80 meter resonance. I've built
systems like this for BC stations, and they aren't that
complex. The real issue is bandwidth and power rating
because voltages can be high. The network would be a
parallel L/C 160 network with a series inductance for 80
meter tuning. Three components.
The function of that network is to ground the 160 elements
for 80 meter signals while parallel resonating on 160 so it
is open circuit on 160.
The 80 meter elements might or might not be an issue on 160,
depending on how the phasing network in combination with the
feedlines and 80-meter equipment in the house appears on 160
to the 80 meter elements. It is quite possible to have
interaction with some equipment/feedline combinations. All
of it is correctable.
You would also have to isolate the 80m rigs from 160
ingress, the 160 rigs would likely be taken care of by the
antenna networks...but there still might be some cleanup
there. If you don't have 50-80dB isolation you probably will
have a mixing product of 80 and 160 meter signals radiated
at levels that might go for some distance. So I'd be sure to
include bandpass filters in the lines anyway, or at least
test for F1-F2 mixing besides normal overload.
Complex but all workable.
73 Tom
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