"Nigel Feltham" wrote in message
...
Arfa Daily wrote:


"Jeff Liebermann" wrote in message
...
On Sat, 18 Jul 2009 09:54:43 +0100, "Arfa Daily"
wrote:

Circular polarisation for US DBS, Jeff ? Over this side of the pond,
it's
vertical and horizontal polarisation for alternate channels or whatever,
but
with the same selection / LNB powering voltages, and additional
switching
by
22kHz tone, the same.

DirecTV and Dish are both circular polarization.
HughesNet internet is linear (vertical or horizontal) polarization.
FTA (free to air) transmissions are all linear polarization.
Broadcast downlinks (FSS) are all linear polarization.

http://en.wikipedia.org/wiki/Low-noise_block_converter
"Most North American DBS signals use circular polarization, instead of
linear polarization, therefore requiring a different LNB type for
proper reception. In this case, the polarization must be adjusted
between clockwise and counterclockwise, rather than horizontal and
vertical."

This explains it in detail:

http://www.abadss.com/forum/102-faq-how/5880-understand-lnbs-standard-linear-circular-universal-lnb-lnbf-stacked.html



That's interesting. I wonder what the perceived advantage of using
circular over linear polarisation is ? I can see that polarisation twist
from passage of the signal through the atmosphere would be eliminated,
and
also skew caused by your E-W position with respect to the bird. Do you
think that's the reason ? I didn't realise that a c.p, LNB was
effectively
just an l.p LNB with a signal 'de-spinner' (lump of ferrite ?) in the
feed. As long as DBS has been going in Europe, which is quite a lot of
years now if you go back to the old analogue birds, c.p. has never been
used, which is why I'm not too familiar with it as applied to microwave
signals.

Not completely true - the UK's old BSB satellite service used circular
plolarisation (quoted as right hand circular which I'm assuming means
clockwise) until the system was killed off by SKY's dodgy dealings - they
persuaded the UK government that BSB would need 2 satellites so there
would
be a backup and that they'd need to use the more expensive D-MAC standard
to get good enough picture quality.

The result being it cost a fortune to get new box hardware designed and
put
2 satellites up just to broadcast 5 channels (which couldn't be expanded
without launching another 2 satellites) when at the same time SKY were
exempt from UK satellite broadcast restrictions due to uplinking from
Luxembourg and were allowed to rent space on an Astra satellite with space
for 16 channels and using cheaper PAL receivers that were already designed
and available.

When BSB closed down (after only 6 months of broadcasting) the 2
satellites
were sold to Scandinavian broadcasters who switched to D2-MAC and used
them
until 2003.

Yes, all quite true. I'd forgotten about the BSB service, it's active life
being so short ...

And to Jeff. The 'multi' LNBs that I was asking you about are not multiple
singles on a bracket on one dish, but all inside one standard sized LNB
package, so it's one lump with either two or four F connectors on its back.

Arfa