Quote:
Originally Posted by sv1000
Its my understanding that all non-fta sats (dss) transmit using some sort of "circular" method and there is never a need for tilting a LNB (and nor can you and if you can, it won't improve signal strength or quality). Am I correct ?
The exception might be a specific Echostar satellite (not 6 or 7) that needs a KU band lnb and presumidly needs the lnb to be rotated ?
Same for what ever satellite a star-choice service in Canada uses ?
Am I wrong ?
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This is an interesting question, to me at least. Yes, many of the DBS sats (I'm used to seeing the DSS term applied mainly to
DTV, but DBS is a more generic term) use circular polarization, and most FTA signals are found on the linear polarized satellites. Linear polarization has the polarization of the signal either vertical or horizontal. The transmiting and receiving elements are usually just metal rods that are either horizontal or vertical, similar to TV antennas that have horizontal polarization and FM antennas that are usually vertical I think, like a car antenna. Circular polarization is basically a condition where the plane of polarization of the signal is rotating like a left or right handed spiral.
Most of the sats used by DTV are circular (exceptions being things like the channels on G3r which are transmitted on a linear satellite). Most of the sats used by Dishnet and Bell ExpressVu are circular, the Star Choice things are linear. Some of the Dishnet stuff is on linear sats, like the Echo-9 satellite is really the same satellite as IA13 and is a linear satellite. Also there are some sats, like AMC 15 and 16, which seem to be advertised as being capable of being either linear or circular.
The reason I find this interesting, is that to me, the signals put out by AMC15 and AMC16 do not seem to be pure circular to me. On most of the "pure" CP sats, when I use my linear feeds for reception, I can usually get some of the transponders, but usually only on one of the 2 polarizations, and as expected the signal strength and quality is reduced. However, on AMC15 and 16, I can get almost ALL the transponders on both polarities with my linear feed. My suspicion is that these hybrid satellites aren't really putting out pure circular polarization.
I've built and used a few types of CP antennas. There are some called quadrifilar helix that look like a twisted kitchen whisk and other antennas that look like cork-screws, that I think these put out pure CP, however there are some other antennas, that seem to simulate CP by phasing the signals into crossed linear elements. These can be made to be either circular or linear depending upon the way they are fed. My guess is that the hybrid satellites must use something like this. I don't think that these really produce "pure" circular polarization, but I might be wrong. Perhaps they should be called eliptical. In any event, you can switch between R and L by varying the H and V polarity of a linear feed. You can receive linear with a CP feed, and you can receive CP with a linear feed, but the problem is that you can't separate the polarities, so you get interferrence.
Re the comment above about a CP lnbf working at any angle.... in theory, yes, but I'm not positive that this is completely true. It may be my setup, but I have seen differences with respect to how well one of my CP lnbfs work depending on what orientation I place it at, however I haven't really experimented much. My guess is that it depends upon what technique is used within the lnbf to receive the CP, and how pure the original signal is. There are some feeds that receive CP by placing teflon slabs inside a linear feed. I don't consider this to be pure CP reception, and in my opinion, this type of CP feed would probably be dependent on it's orientation. I see them as just a way of increasing the signal from one polarity over the other polarity. However again, I've never really experimented with one of these, so I'm probably wrong. But right or wrong, I find this whole subject interesting, and some day, I intend to experiment a bit. And perhaps take apart one of those CP lnbfs to see what is inside.