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.

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 isaolation (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...

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
Thunderbirds are go.

Frank Turner-Smith G3VKI wrote in
:

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.


Nice. That looks like bearing out my hopes for the scheme after all. The
distance isn't huge, but I've already decided to aim for the internal
horizontal dipole to be central to the flat, so I can use the smallest boost
that will do the deed.

Small extra question.. Would I get away with a simple low loss coax to dipole
with ferrites on the coax to prevent leaky signals running outside on the
screen, or should the internal dipole get a balun or other appropriate
treatment for a transmitting antenna, despits the tiny signals and extreme
locality involved?

Frank Turner-Smith G3VKI wrote in
:

For safety's sake, keep the TX and RX aerials as far apart as you
can.


I can quantify that relatively, which may help.. The distance between the
internal dipole to the radio will be about a tenth of the distance between
dipoles. Would that offer enough scope for avoiding feedback while boosting
20dB in the antenna amp? (If not I estimate I can get useful help with 10dB).

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 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.

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 isaolation (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.
--
Ian

Ian Jackson wrote in
:

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.

Good point. In optics reflections wreak merry hell with polarisation (often
intentionally for good purpose) so I assume that it's the same with radio.
Even so it has to beat putting both in same plane, and I'll use no more boost
than will reduce local RF noise acceptably. Actually I have a bunch of PV
panels and such out there on grounded mounts that any major reflections that
occur won't make it into the flat, it's more likely there would be a few
narrow opportunities for a direct signal to get past rather than many
reflections getting here.

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.

"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.




---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

"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.