"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.

Many of the LNBs here are now multis to allow use of receivers with dual
independant tuners, and the multi-room service which allows two dual tuner
receivers to be connected to the same dish. Do you have multis like this
that still work with c,p, ?

Arfa