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JIMMIE · July 28th 06, 10:39 PM posted to rec.radio.amateur.antenna

wrote:
Hello, I have a vertical antenna that is tuned via taps on a coil at
the base. As I was moving the tap the other day I had the idea...why
not replace the tapped coil with a roller inductor? This would
certainly make tuning easier and as long as the roller is protected
from the elements should work as good as a fixed coil. Is there any
reason why this wouldnt' work? Has anybody else modified or built an
antenna using such a system? (For the record, the antenna is a Hy-Gain
AV18VS. I used to use it only on the low bands and as such rarely had
to move the tap, but now that I have an amp and this is the only
antenna I have that can handle power, I've been using it more often and
on more bands).
73, Craig KB8FGC

Yes it has been done but not without its problems. Stopping the coil in
the right place, roller contact with the coil... can all be a problem.
I had good luck using a tapped coil and using relays to switch the taps
much as is done with an auto tuner. Some radios would allow the coil(s)
to be switched automatically with the help of a little extra equipment.
Just using manual control is not too bad. I think I saw a circuit to be
used with ICOM radio that controls relays by utilizing a DC voltage
that changes with band selection/frequecy. This was fed to a group of
comparator circiuts that drove the relays. I know of at least one
person who has tried this and it proved to be a good route for
lightning to get in his radio so my preference is the manual switch
controling relays. I dont know how this would work with your radio. The
only one I am familar with using this arrangemet was a 30+ ft whip
using a big flat metal roof as a couterpoise.

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