Jeff,
Here is my experiences with feedline common mode problems. Others may have
different experiences/opinions. Your mileage may vary!
I have yet to find an instance where a feedline choke of any description would
replace a proper isolation transformer. I went through this the hard way. When
you have a proper isolation transformer such as described on Tom's web page or
mine, and have it properly installed, you just can't get any better. Don't
waste time on superstition and witchcraft.
This applies equally to Beverages as well as the hard to feed Flags and
Pennants. At the AZ QTH, the AM broadcast crap was strong enough to light bulbs
at the base of the TX antenna. Properly installing isolation transformers at
each RX antenna feedpoint completely eliminated all common mode problems from
both the AM broadcast and the in house computers. Of course this won't help bad
coax (broken shield or loose connectors) or re-radiation problems. It would
seem these problems are sometimes mistakenly identified as common mode feedline
issues. Be careful with your assumptions.
I'm afraid that you might be stuck with superstition and witchcraft for those
audio applications. I have never found a magic bullet for those problems. Every
one is different. Every one is a struggle. Ferrite has never worked.
I'll be watching for you on the band.
73,
Larry - W7IUV
http://w7iuv.com
Jeff Maass <jmaass@columbus.rr.com> wrote:
My applications are:
1) For keeping RF out of audio lines in a high RF multi-
transmitter station. I think I understand this one,
although I'm interested in seeing K9YC's article to
learn to better select ferrite materials.
2) Reducing the effect of the feedline on the effectiveness
of a Beverage or pennant/flag antenna. From Tom's
description above, ferrite is pretty much useless in
the case of my pennant/flag (long feedline, poor ground
at both ends), but we are told in the various pennant/flag
articles that a choke at the feedpoint is pretty much
essential to proper operation.
So, who has *practical* advice of how to apply a choke
for flag/pennant antennas? I've been winding 15-20 turns
of RG8X in an FT240-77 core.
Next week I'll be flying down to Curacao for a few weeks, and
I'll have some time to try out some more receiving antennas.
73,
Jeff Maass K8ND
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