On 31/10/14 19:05, Ian Jackson wrote:
In message ,
Lostgallifreyan writes
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...

I've often heard this '26dB protection' quoted, but I'm sure that it's
just a 'wet finger in the air' figure. Even if it is sort-of a 'typical
average', at any location it could equally turn out to be almost
anything between 'not a lot', and a lot more than 26dB - mainly
depending on reflections.

Indeed. The figure applies under "free space" or "line of sight"
conditions. Reflections or "grazing" can reduce this amount.

--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
Thunderbirds are go.