[TowerTalk] My dipoles

[Jim Brown]

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