Thanks Tom,
This AV640 is on a 32 foot Aluminum tower on the west end of the shop that
is tied into a ground system. There is about 75 to 80 feet of LMR-400 that
runs through conduit from just behind the station, up over the south big
door, back down and then half way across the west wall about 3 feet off the
floor. It goes through the wall and into a metal box and through about a 12
inch long inch and a half thin wall conduit.From there it's just a drip loop
to the tower.
I don't appear to have any RF getting back into the shop, but the entire
interior of the shop is bonded barn metal so it makes a pretty good shield.
The antenna works well (a relative term) on 40 and I have run the legal
limit into it from a Henry 2K4. It also appears to work well (for a
vertical) on 6-meters. I've heard a few stations on 15 and 10 but not much
as I've remarked, I'm usually out there around this time of night (currently
2:30 AM), but the darn thing is just plain deaf on 20 and I'd really like to
use it on 20.
This is not a "serious" station, but consists of my old Hallicrafters stuff
and the little Yaesu FT897D currently barefoot, but soon to drive the 2K4. I
listen to it while working on projects, the airplane, or programming. If a
good opening comes along, I come inside and fire up the station in here, but
typically I'd use the one out there, if I could hear, or be heard.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
>> 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
>
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