Mike;
As Steve says, you will need a huge choke in the braid to have much effect.
What you are trying to do is form a potential divider, with your choke at
the top and 'the impedance-to-ground of the base of the whip' at the bottom,
to attenuate noise from the shack getting onto the whip.
My suggestion, which is much easier, is to go for an isolating transformer
rather than a braid choke. Put two equal windings on opposite sides of a
small toroid so that the stray-capacitance is kept to a minimum. Don't put
the transformer into a metal box with coax connectors on each end! Another
thing to realise is that if your isolation transformer is really working,
then you MUST install a ground connection at the base of the whip. If you
put the isolation transformer at the whip end of the feeder, then the whip
cannot use the feeder braid as a ground return so it really does need a
separate connection to ground. Remember this if you plan to put the whip in
a tree! I would suggest installing the transformer at the shack end of the
feeder run for this reason, but other people may not agree with me. Try it
both ends and see what happens.
If the whip has a pre-amp inside it which needs power, then you have another
problem - how to get power to it without re-introducing the noise up the
power cable. That's a more difficult one to solve. The quick answer is to
run the whip pre-amp from a battery. The proper answer is to run AC power
through a low-capacitance isolating transformer and build the whip PSU at
it's base.
73
Peter G3PLX
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