For the Stew Perry, I constructed a 4 foot diameter
"shielded" loop out of coax, as seen in various handbooks.
The only difference was that I used a feed method of my
own design because I didn't like any of the ones in the books.
The purpose of doing this was to orient the loop broadside
to the transmit vertical to null out the transmitter so
I could connect the loop to the second radio for SO2R operation on 1
band. It worked very well in this respect, having at least 60 dB of
nulling when VERY carefully aimed. I was able to make dozens of S&P
QSO's with only slight interruptions of the run radio CQ'ing.
However, what surprised me was that it was a darn good receiving
antenna in its own right. At my QTH, any vertical of any height
is virtually useless as a 160 meter receive antenna. The loop is
supposedly vertically polarized. Yet it received as well or
better than my full sized dipole and much better than the
transmit vertical. On CE/K7CA, the copy was excellent, although
not quite as good as a beverage. I could also copy other DX such
as JA with it. Many coast to coast QSO's were made with it.
I cannot explain why this loop should work so well, especially
since it was near power lines, which the other antennas were
not. I suggest that everyone should build some sort of loop
to try it out. If it works, use it.
I didn't connect a preamp to the loop because I was worried
about the transmitter overloading it. Nevertheless, I was
able to hear OK with just the radio's internal
preamp turned on. The main problem was that there was little
AGC action, so I had to ride the AF gain a lot.
BTW, one of the other posts suggested that RF noise gets into
the ground system of the vertical from power lines.
I'm sure this is possible, but you are not out of the woods
just because you eliminate it.
My vertical is over 1000 ft from any power line and is plenty noisy when
connected to a battery operated receiver at the base of the antenna.
As is the vertical on the mobile station.
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