On Mon, 20 Sep 2004 23:01:30 -0400, "Leland C. Scott"
wrote:


"Dave Hall" wrote in message
.. .
On Mon, 20 Sep 2004 00:33:28 -0400, "Leland C. Scott"
wrote:


"Glenn S." wrote in message
.. .

Which omnidirectional base antenna will put out the least TV
and telephone interference in the neighborhood?

It doesn't depend on the antenna almost at all. The main cause of
interference is from three sources. One a dirty transmitter, the second
is
from receiver front end over load, and the third is from RF getting in to
the electronics directly.

That's not entirely true. The antenna DOES have a part to play in the
whole mess. Some of the "stick"-type antennas, such as the A-99, have
poor decoupling and this allows for significant coaxial shield
radiation. Some of these antennas also concentrate a good portion of
their near field radiation in places where it would exacerbate front
end overload or couple R.F. into house wiring.

An otherwise clean transmitter coupled to an antenna with the above
characteristics can cross the line between no RFI and significant RFI.
In some cases, this can be mitigated somewhat by moving the antenna
(usually raising it) to another area, where it's radiation will not
couple as much R.F. into neighboring premises. A better solution would
be to run an antenna with ground plane radials, such as a Sigma 5/8
wave or similar.

That would be true of course for his own situation, the classic RF in the
shack problem, but he specifcaly asked about interference to the local
neighborhood if you re-read his post. In that case I doubt the antenna type
will have any significant impact.

If you live in a fairly densely populated area (1/4 acre lots) the
near field will affect the neighbors as much as your own shack. I've
known people who have switched from an A-99 to a 5/8th wave ground
plane and have solved RFI problems. Nothing else changed, just the
antenna.

Antennas do not "create" harmonics or other spurious signals. But some
may be more "friendly" to radiating a second or third harmonic. Their
near field radiation pattern and coaxial decoupling contribute to the
near field strength of a signal. Due to the characteristics of some of
the most infamous antennas, they may radiate the same amount of R.F.
into nearby structures while running 4 watts, as another type of
antenna may do with 100 watts or more.

This just illustrates that there are many facets of a radio station
that need to be looked at when troubleshooting RFI. Everyone is aware
of "dirty" transmitters, and most people are aware that consumer
electronics are not particularly well designed with RF immunity in
mind. But once you've proven that the transmitter is "clean", and you
still have an RFI problem, you have to consider other options. Your
mission is to minimize the amount of RF which gets into neighboring
homes. If raising or moving the antenna to a place where it couples
less RF into a home may just do the trick. Changing the antenna to a
better design may accomplish the same thing. Good RF grounding may
also minimize RFI. There is no "one size fits all" solution.

Dave
"Sandbagger"
http://home.ptd.net/~n3cvj