Richard Clark wrote:
On Sat, 14 Feb 2004 14:21:12 -0000, "Richard"
wrote:
Anyone know of any antenna system that would produce a real good null in
the FM band? Would that be to try for a cardoid pattern? Or is there
something better than that?

Hi Richard,

That would be useful (the cardioid) if the signal strengths are good
enough to follow the interfering signal down into the null. To put
the interfering signal into the null means degrading the other 10 or
more dB.

Otherwise, look for a yagi with about 30 degrees of beam width (in the
vertical polarization).

Thing is, these stations would be quite weak, so I could do with gain, so
perhaps a good yagi is the way to go after all. .Like you say, rotate until
the offending station goes down in the null of the antenna.

I think also you are saying use vertical polarization, because that
orientation always produces the sharper pattern. I never thought about that.
I was though intending to mount vertically as it happens. Thanks.

On Sat, 14 Feb 2004 14:44:08 -0000, "Richard"
wrote:
I think also you are saying use vertical polarization, because that
orientation always produces the sharper pattern. I never thought about that.
I was though intending to mount vertically as it happens. Thanks.

Hi Richard,

I was careful to mention vertical polarization, because barring some
across-the-pond differences, that is what your broadcaster uses. If
you attempted to listen to them with horizontal polarized antennas,
you would suffer what is called "cross-polarization." The
consequences of this are signals that are down 20dB or more. If those
signals are circular polarized, then the vertical or horizontal would
work equally well (as long as that beam width was half the difference
of the two bearings).

73's
Richard Clark, KB7QHC

Hi Richard,

Horizontal polarisation from UK TV/radio TXs seems to travel a lot further
on the same power than vertical. The same has been found over distances
using 2m/70cm.

"Richard Clark" wrote in message
...
On Sat, 14 Feb 2004 14:44:08 -0000, "Richard"
wrote:
I think also you are saying use vertical polarization, because that
orientation always produces the sharper pattern. I never thought about
that.
I was though intending to mount vertically as it happens. Thanks.

Hi Richard,

I was careful to mention vertical polarization, because barring some
across-the-pond differences, that is what your broadcaster uses. If
you attempted to listen to them with horizontal polarized antennas,
you would suffer what is called "cross-polarization." The
consequences of this are signals that are down 20dB or more. If those
signals are circular polarized, then the vertical or horizontal would
work equally well (as long as that beam width was half the difference
of the two bearings).

73's
Richard Clark, KB7QHC

Richard wrote:
"Anyone know of any antenna system that would produce a real good null
in the FM band?"

Direction finding relies on sharp nulls. The small loop works well with
vertically polarized ground waves, but small loops don`t work well in
nulling out horizintally polarized waves simultaneously with a null in
vertically polarized waves .

The lack of a simultaneous small loop null in both polarizations was the
cause for development of an improved Radio Direction Finding (RDF)
antenna, which was patented by F. Adcock in 1919. The story is found in
the 19th edition of the ARRL Antenna Book on page 14-5.

FM broadcasts typically contain both polarizations, so what`s needed is
an antenna which nulls out both polarizations.

The Adcock antenna has been found to prooduce good nulls under sky-wave
conditions (containing both polarizations) at HF when loops produced
poor nulls.

Instructions and directional patterns for the Adcock appear in the ARRL
book. All that`s necessary is to approximately scale the Adcock for the
frequency of the null.

With the interfering station in the null, the desired station may
capture the FM detector.

Best regards, Richard Harrison, KB5WZI