Topband: Shunt Fed Tower
k9ay at k9ay.com
Tue Dec 20 11:26:38 EST 2005
> All of my reading materials are at my station and to save a trip want to
> ask > if someone has a general idea off the top of their head what will be
> required to feed (for 160M) a 60 foot 25G tower that has a small tribander
> at the top and another tribander about half way up.
> I wonder whether the tower is electrically long enough to get by with a
> simple shunt feed even with the wire going to the top of the tower.
> Thanks...Stan, K5GO
At 60 feet, a shunt wire should go all the way to the top. Two wires spaced
the width of the tower face will improve the bandwidth, especially if they
are installed more than a foot away from the tower (they make the tower
'fatter'). The second tribander halfway up is a wild card for matching,
since it adds loading that may reduce the current above it. Routing the
shunt wires past it can be problem, too, unless it's fixed on EU or JA like
some guys do.
A number of years ago, I had 65 ft. of 25G with a small tribander and shorty
40 on top -- I shunt fed it with two wires to the 60-ft. level. A single
capacitor in series gave me 2:1 or better VSWR over the lower part of the
band, which my rig handled OK. An L- or T-network would have been better.
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