"Lostgallifreyan" wrote in message
. ..
Frank Turner-Smith G3VKI wrote in
:

This cross
polarisation provided about 26dB protection against co-channel
interference.


That's a useful figure. I asked a few weeks ago about the prosects of
wiring
an external vertical dipole for FM VFH broadcasts, via a MAR6 based
amplifier boosting by maybe 20dB, to an internal horizontal dipole to
overcome local digital hash from nearby flats that gets in to degrade the
signal from a portable radio with a telescopic whip. As it is the SNR
rather
than the raw strength which is an issue, an ideal situation would be to
allow
thwe whip to be in its resting horizontal, cotracted position, while still
allowing clear use of radios carried around the flat while I work.



Theory say infinate, but in practice it won't hapen. Just too many
reflections, especially on the low bands where the signal reflects many
times over a long distance.

Here are some numbers if there are no reflections to upset theory.
deg differance in dB

20 54
30 1.25
45 3
60 6
70 9.32
80 15.2
90 infinity

This also holds up for right hand and left hand circular.

There is another modification if the antennas are seperated vertically or
horizontal. You usually get lots more isolation if the antennas are mounted
one above the other instead of horizontal.




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