> Would you then create a group of 1/4 wave radials tied
together at the
> center, but floating above ground as the antenna is
grounded to the tower?
When speaking of radials, I was talking about traditional
Marconi antennas, but I do have an AV640 as a backup antenna
myself.
I'm going to assume all AV640's are still like the early
pre-production antenna I have. The small counterpoise is
isolated from ground. There is a matching transformer and
some decoupling inside the feed box. The decoupling is
marginal on 40, and gets better as we go up in frequency.
There really isn't a need or a way to add radials. The 640
acts like an off center fed dipole on lower bands. Adding a
ground system will mess it up since it depends on the
"ground plane" being electrically small to reach resonance.
What this does is make the counterpoise to mast voltages
very high (I measured several hundred volts between the
counterpoise and ground on 40 meters with 1000 watts). The
toroid used to decouple the feeder can't adequately choke
off common mode currents on lower bands. Beside affecting
SWR and pattern, feedline shield currents can lead to RF in
the shack.
In many cases, especially at low power, we get away with
poor feed systems like this. We might never notice a
problem. I noticed it here because my antenna was on a mast
outside my shop and had the feeder coming right into the
room . When I used my antenna RF got into everything, even
the electronic ballasts in the shop lights. Of course noise
from the light ballasts got back into the antenna. I
increased feedline isolation near the feedpoint before the
cable reached the supporting mast with an additional high
impedance common mode choke. Made the antenna useable in my
situation. My lights with electronic ballasts don't shut off
when I use it now.
73 Tom
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|