On 31/10/14 18:40, Lostgallifreyan wrote:
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 prospects of wiring
an external vertical dipole for FM VHF 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
the whip to be in its resting horizontal, contracted position, while still
allowing clear use of radios carried around the flat while I work.

Various possible problems have been discussed, and I haven't pushed for this
with a trial, but if 26 or more dB are cut in the difference between antenna
based on polarisation, and the amp boosts only by 20dB, it seems that
feedback can be avoided, AND also the risk of interference to other FM VHF
radios in other flats. (Which might even benefit, if my own would.)

This is the first time anyone's mentioned a figure for isolation (for want
of a better word) between similar dipoles based on 90° difference in
orientation, so I'm taking this moment to reopen the subject in passing...

26dB cross-polarisation protection was the "rule of thumb" figure in use
by the BBC service Planning Section in the 1970s when planning TV relay
stations. I know, I was there.
"Active Deflectors", relaying TV signals down into valleys without
changing channels, have been doing exactly what you describe for many
years. For safety's sake, keep the TX and RX aerials as far apart as you
can.
--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
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