From: ehare@arrl.org (Ed Hare - KA1CV)
Newsgroups: rec.radio.amateur.misc
Subject: Re: locating TV antenna near ham antennas
Date: 29 Sep 92 13:37:24 EDT
Organization: American Radio Relay League

In rec.radio.amateur.misc, dale@sequent.com (Dale Mosby) writes:

>Unfortunately her antenna must point through my antennas toward
>the local TV stations.  My guess is that the best thing is to place
>her antenna at the far end of her house to keep it as far away as
>possible from my antennas, even though the feed line becomes much
>longer.  Is this true?

For the most part, this is true. What ultimately counts is signal to noise
ratio. Your amateur signal represents the noise. That will fall off as 
the square of the distance (approximately -- it is not 100% true in the
near field). The signal will fall off linearly with the length of 
the feed line.

>Next question is whether I run coax from the antenna or use 300 ohm
>twinlead.  I feel that the coax will be more resistant to the RF
>from my transmitting, but will have higher loss than the twinlead,
>especially for the UHF.  I don't have any experience with TV
>twinlead -- will that pick up TVI?  What is best for this situation?

Both coax and twin-lead can pick up interference, although coax may be
more resistant. I can't really address the loss issue without knowing
the length of the line. 

>Final question -- should I just install a high pass filter from the
>start, or wait and see if there are any problems?  The reason for 
>installing from the start is so if it causes some drop in signal
>that isn't noticed later.  I haven't experimented with any of these
>so I don't know how much attenuation they provide to the signal they
>should be passing.

I would probably start with the high-pass installed, and have an FT-140-43
or FT-240-43 ferrite core (or two) handy to use as a common-mode choke.
A description how to use a common-mode choke, plus lots of 
sources for high-pass filters, common-mode chokes, ferrites, etc, plus
a lot of good tips about EMI control are free for the asking from ARRL HQ.
Send me your postal address (there is non-electronic material) and
ask for the "RFI Package." If anyone else wants it, I would appreciate
it if you would also give me a description of your EMI problem.

>I believe that 80 meters is causing most of the TVI that I
>do generate, possibly due to high SWR.

Acutally, a high SWR in and of itself WILL NOT cause a TVI problem. This
is a common myth that has sent many a ham down the wrong trail. The problem
with 80-meter TVI is usually a suspectible VCR or the fact that the TV
feedline length is often resonant on 80-meters. (Or any of a number of 
other factors.)

In summary, if you have TVI, ask for the RFI Package. Also, contact your
ARRL Section Technical Coordinator for help. Page 8 of QST lists the
Section Managers -- usually including phone numbers. The SM can 
quickly put you in contact with the Technical Coordinator.

73 from ARRL HQ, Ed


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Ed Hare, KA1CV                  ehare@arrl.org
American Radio Relay League
225 Main St.
Newington, CT  06111            There is no limit to what you can 
(203) 666-1541 - voice          accomplish if you don't care who
ARRL Laboratory Supervisor      gets the credit. - Harry S Truman
RFI, xmtr and rcvr testing  
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