On 1/15/2012 7:39 PM, Jim Miller Waco Texas WB5OXQ wrote:
> I believe this is too close to the tv.fm antennas and equipment.
> When operating on 75 AM last year rf got into the speaker wires on the
> surround sound system and blew the outputs on a 1800.00 receiver.
You do NOT need to move your antenna, you need to fix the RFI problems
in your stereo rig. See my RFI tutorial for an explanation of how it
happens and detailed advice on how to fix it.
http://audiosystemsgroup.com/RFI-Ham.pdf
Two things to do immediately -- replace the speaker wiring with twisted
pair, and wind each speaker cable about 12 turns around a #31 toroid.
See Appendix One for part numbers and advice on where to buy them. You
may also need chokes like this on one or more of the audio cables, but
try these two things first.
In Chicago, I ran 1kW on 160, 80, and 40 to a dipole that ran about ten
feet from the roof TV antenna, and never had a TVI problem. I DID have
RF pickup on speaker lines, and I also had Pin One Problems in some of
the amplifiers, and one of those Pin One Problems fried a lovely 15-in
JBL theater sub-woofer while I was working a 160M contest. I came
upstairs after the contest to find the woofer surround in shreds.
Another good move would be to add a coaxial ferrite choke at the antenna
feedpoint to keep RF off that transmission line. Contrary to popular
belief, you ALSO need a choke on parallel wire feedline. And a badly
unbalanced antenna like a Windom is the WRONG thing to try if you're
trying to keep RF off the feedline. The unbalance creates massive common
mode voltage, which is likely to fry even a VERY GOOD choke if you run
much power.
To understand the issue with common mode current, see my Power Point
slides on Coax Chokes, also on my website.
http://audiosystemsgroup.com/publish.htm
73, Jim K9YC
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