From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: rec.radio.amateur.misc
Subject: Re: locating TV antenna near ham antennas
Date: 30 Sep 92 03:49:56 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <1992Sep29.060450.7022@sequent.com> dale@sequent.com (Dale Mosby) writes:
>
>The person next door is evidently using "rabbit ears" for the TV,
>but while watching my tower go up mentioned one day that she would
>like an outside TV antenna.  I offered to install it for her (after
>I assured here that our housing development really did allow
>outside antennas. :-)
>
>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?

Pointing through your antennas is NOT GOOD (tm). If you can possibly
shift the mounting point to look beside your antennas rather than
through them, do it. This is more of a ghost issue than an RF pickup
issue if you operate HF. The TV beam has no gain at HF. You want the
antenna as far from your near field as possible, but the near field
of a transmitting antenna extends out more than a wavelength at the
transmitting frequency. So you'd like to mount the TV antenna at least
80 meters from your antenna. That's not practical in your situation
so just mount it at the best physical location and try to avoid ghosting
caused by the metal of your antennas.

>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?

*Properly* mounted twinlead is no worse than coax for picking up 
unwanted HF radiation. The balance of the lines should cancel
differential currents and leave you to deal with common mode
currents. Proper installation can be difficult, however. Coax
will be subject to common mode currents as well, but is physically
easier to mount. Twinlead has lower loss at TV frequencies than
coax, but since your neighbor uses rabbit ears now, that's not
a concern. Use coax.

>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.

Yes, include a good high pass filter from the start. Be careful here,
many high pass filters for coax on the market don't break the common 
mode path and are worthless. A trick I often use is to get a high pass 
filter for *twinlead* and install it in a coax line using baluns. This 
breaks the common mode path and offers high pass filtering. A really 
cheap way to do this was posted recently. Simply attach a balun to each
of the cables and about 2 feet of twinlead to each balun. Now tape
the twinleads side by side to each other. There will be enough coupling
to pass TV signals, but HF won't get through.

>I don't think they are experiencing many problems now, but I suspect
>that I'll be be getting on the air more once I finish up this antenna
>work.  I believe that 80 meters is causing most of the TVI that I
>do generate, possibly due to high SWR.

High SWR has nothing to do with TVI in a properly engineered system.
The devils you want to control are fundamental overload and harmonic
generation. Make sure your antenna and not your feedline is doing the
radiating, this is a matter of balance. Make sure there are no rectifying
junctions in the near field. Use a low pass filter, a properly designed
transmatch can serve as a low pass filter. Don't care about SWR on your
feedline, care about matching the impedance to the transmitter. That's
what transmatches are for. Use a good RF ground system or a counterpoise
to maintain balance.

Most hams would be better off if they threw their VSWR meters away
and concentrated on the fundamentals of antennas and feeders. It's
easy to get a low VSWR reading with a lousy antenna system, and it's 
very possible to have an efficient clean antenna system that shows a 
very high VSWR.

Gary KE4ZV