Hi Lane,
Depending on the desired freq of operation the
shunt will be attached at different places
anyway. On 1/4 wave shunt fed broadcast towers,
the shunt is usually about 1/3rd the way up
(notice I said 1/4 wave AND usually AND about).
Unless someone here can model your situation and
give a better idea I would have to say that
randomly picking a spot and trying it will be
about the easiest thing to do. Probably start at
the top and see what it looks like. If you have
an antenna analyzer it will make things easier.
Also, with most shunt fed towers around 1/4 wave
tall (electrical height at desired freq being the
deciding factor) if the measured resistance is too
high then the shunt is too high.
FWIW, when we tune AM broadcast shunts or
unipoles, we normally tune them for 50 ohms and
then cancel the inductive reactance with a simple
series vacuum variable cap. Sometimes the
reactance becomes unmanageable and the resistance
has to be set somewhere other than 50 just to keep
the reactance within reason. In this situation a
T-network tuner is employed to tune to 50 ohms 0j
to make the transmitter happy.
Good Luck,
Lane Zeitler wrote:
>
> Gents,
> A friend of mine here in San Diego wants to shunt feed his 50 ft self
> supporting tower. Not much on top of it except a Cushcraft A-4S with the 40
> meter add on kit. reflector and director are attached directly to the boom,
> not insulated. I will be doing the manual labor for him since he is getting
> up in years and not able to do the work. How far up should I attach the
> shunt?? There is a pipe abt three feet above the tower at which point the
> beam is bolted. Can anyone give me an idea?? I would rather not just
> randomly pick a spot.
>
> thanks
>
> Lane Ku7i
>
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--
Later,
Kevin
WD4RAT
mailto:kkidd@kkbc.com
KK BROADCAST ENGINEERING
http://www.kkbc.com
AM GROUND SYSTEMS CO.
http://amgroundsystems.com
51 Ridge Ln
Lawrenceburg, TN 38464
Toll Free: 877-766-2999
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