Ohh, OK, a shunt C. I missed that. My instincts tell me we
have to be careful with a shunt C when the network has a
series C in the output, like a T network. But maybe I'm
wrong.
> Maybe I wasn't clear here either: "drop the counterpoise"
> does not mean "omit the counterpoise". It means connect
> a wire to the cold end of the counterpoise and let it drop
> vertically from there to the ground. BTW, when your
> antenna impedance is 1000's of ohms, you don't need much
> of a counterpoise, especially if you feed the
antenna/counterpoise
> through a balun to keep RF out of the shack.
Actually you do need more than a single wire counterpoise if
you want to keep the antenna doing nearly all of the
radiating, and not have the counterpoise a big part of the
radiating system. Every bit of current leaving the feedpoint
into the antenna has to flow into the counterpoise whether
it is an intentional or accidental counterpoise.
All those end-fed half-waves that say they don't need
radials or counterpoises? Not true! You can bet the feedline
and any supporting structure is carrying considerable common
mode current and radiating.
Of course 1000's of ohms antenna impedance helps reduce
current, but the problem is a single wire dropped near the
building is right on top of every in the building, and it
has too high an impedance (maybe 50 ohms or more) to sink
current away from other ground paths.
Even the feedline of a 1/4 wl groundplane with four radials
radiates from lack of a perfect counterpoise. Even though
the counterpoise impedance is fairly low in the GP compared
to the CM impedance of the coax, enough current flows to be
a major headache at times.
Bottom line is if we can't get a substantial spread-out
area for a counterpoise near noise sources or consumer
devices, it will contribute to the system noise and maybe to
RFI. That's true even if the antenna impedance is 1000's of
ohms. Look at what happens in a J-pole!
73 Tom
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