I have been using the Hy Gain AV 640 (http://www.hy-gain.com/products.php?prodid=AV-640
) for over five years. It's mounted at 15'. I have been extremely happy with its performance. See W8JI's comments below.
"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
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 W8JI"