In article ,
"Richard" wrote:

I'm trying to receive some a distant FM stations, but I have locals 0.1Mhz
away that block the desired station out. I could try to use a big gain
directional yagi, but perhaps the solution is to make use of a null.

Putting the undesired signal in an antenna null is a great idea,
provided the undesired signal is coming from one direction only. If
there is multipath propagation, that may not be true. How are you going
to null out several different bounced versions of the signal
simultaneously?

In North America FM broadcast channels are spaced 200 kHz apart rather
than the 100 kHz spacing you indicate. But even then there can be
adjacent channel problems.

One signal which I would like to listen to comes from a low powered
repeater station (43 watts effective radiated power) on 91.1 MHz with
antenna at an altitude of 1316 m and about 80 km south of where I live;
there are a few low hills in the way so the path is not line of sight
but the signal strength, in the absence of interference, is quite
adequate. High altitude can help make up for low power.

There are two interfering signals. One is on 90.9 MHz, has antenna at
about 800 m altitude, has much higher power, and is less than 20 km away
but is behind a nearby low ridge so that the signal is diffracted and
also arrives from lots of directions due to bounces. The other
interfering signal is on 91.3 MHz, is of even higher power than the
nearby one, has a much lower antenna since it is located on an island
with no nearby high points, and is located slightly closer than the
desired station. It too arrives from lots of directions at once.

I used to be able to receive the 91.1 MHz station quite well, even when
using a whip antenna on an ordinary kitchen radio. (Actually it was on
91.3 MHz then; it moved 200 kHz lower when the 91.3 MHz station came on
the air.) Reception using a good Yagi antenna connected to an excellent
stereo FM tuner became difficult when the higher frequency one came on
the air, and became impossible when the nearby lower frequency one came
on the air. So now I listen to the main transmitter on 88.5 MHz, perhaps
200 km away, whose repeater I used to receive very well, but I listen to
it via our cable connection. I believe that the cable company's
reception antennas are up on one of the mountains to the north where the
nearby interfering station's antenna is located. I don't know whether
they actually pick up the 88.5 MHz main transmitter signal or the closer
91.1 MHz low powered repeater signal, but up where their receiving
antenna are, both paths should be line of sight or nearly so. When I am
in my car, using an extremely sensitive and quite selective Sony
receiver, reception of the repeater station becomes possible when I
drive perhaps 20 km closer to it than my home, but by that time the main
station's signal is also strong enough for quite acceptable listening
quality.

David, ex-W8EZE

--
David Ryeburn

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