On Mon, 20 Jul 2009 10:02:15 -0400, Meat Plow
wrote:
Thanks for the explanation Jeff. I thought a dual LNB was actually two
LNBs with different polarizations.
Nope. I finally decided to look inside a DBS LNB and see how they
switch from circular polarization. I couldn't find anything on how
the polarization switching works using Google or anything specific
with a patent search. So, I tore an LNB apart.
There are two probes located 90 degrees from each other along the
circular waveguide. There's also what I guess is a 90 degree 12GHz
phase shifter between these two problems. With the RF amp input on
2nd probe, only CP in the direction coming from the first probe to the
2nd probe will pass. The 90 degree rotation of the signal in the
waveguide coincides with the 90 degree phase shift in the phase
shifter. Going the other direction, the 270 degree rotation of the
signal in the waveguide, cancels with the 90 degree phase shift in the
phase shifter. Move the RF input to the first probe, and it will pass
in the other CP direction. (Disclaimer: This was done without a
schematic and might be totally wrong).
While digging, I found some more detail on control voltage for the
newer LNB's, that work with both CP and linear polarization:
www.intersil.com/data/an/an1161.pdf
12V - Horizontal, 18V - Vertical
13V - Circular right-hand, 20V - Circular left-hand
The major use for this arrangement is frequency re-use. The satellite
can transmit simultaneously on all 4 modes.
But as far back as I was paying
attention, the DISH discs were always tilted even before they offered
HD and it's always been a question floating around inside my head as
to why DTV were not tilted and DISH were way before HD. Maybe DISH has
two LNB devices anyway and looks at two different birds? Or is it
linier vs polarization circular at issue?
Bingo, you got it. With linear polarization *AND* a single LNB, you
must have the tilt correct. With circular polarization *AND* a single
LNB, the tilt is not important. Of course, with multiple LNB's, tilt
along the ecliptic is required.
Incidentally, I've seen ads for transparent DBS dishes, but only for
the UK.
http://www.maplin.co.uk/DiscontinuedModule.aspx?ModuleNo=47234
http://www.pulsat.com/satellite/site/details.php?product_id=330
Nothing available across the pond. What inspired transparent dishes
in UK?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558