"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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"Ralph Mowery" wrote in news:V5-
:

You usually get lots more isolation if the antennas are mounted
one above the other instead of horizontal.


Nothing I can do to bring that about, nice idea though it is. The external
one is about 12 feet off the ground, and the inner one will be wherever it
gives best results, likely about 6 feet. Much less difference there than the
distance between them across ground.

This is a building surrounded by buildings, even the yard is partially
enclosed by walls two stories high. When I try it I won't be expecting any
ideal theoretical figures to match, not even remotely. I just hope that the
grounded solar panels out there will be more likely to deflect unwanted
signals between the dipoles than to aggravate things, because they form the
bulk of material in direct line netween dipoles, assuming the internal one
goes where I need it most. I'll set it up horizontally because it seems
likely that would be better than vertical like the outer one, no matter what
happens.

On 31/10/14 21:22, Ralph Mowery wrote:
"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 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.



Theory say infinite, but in practice it won't happen. 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 difference 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 separated vertically or
horizontal. You usually get lots more isolation if the antennas are mounted
one above the other instead of horizontal.


Are you sure about the 20 degree difference? Should it have been 5.4dB?

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

"Frank Turner-Smith G3VKI" wrote in message
...
On 31/10/14 21:22, Ralph Mowery wrote:
Here are some numbers if there are no reflections to upset theory.
deg difference 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 separated vertically or
horizontal. You usually get lots more isolation if the antennas are
mounted
one above the other instead of horizontal.


Are you sure about the 20 degree difference? Should it have been 5.4dB?


Yes, fat fingered the thing and missed putting in the decimal point.

:-)



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