Circular polarization

[tkrandall]

I'm trying to understand polarity, specifically circular polarization as implemented by DBS providers like DirecTV.

My question is this: Does circular polarization, as opposed to linear polarization, allow for dish tilt/skew to not affect signal strength? For example, here in the Atlanta market we have quite a bit of skew on a Slimline dish. This means the 101 Ku dual LNB is at a significant angle off of vertical/horizontal. At the same location, however, a simple 18 inch dish would have zero tilt. I would think either the tilt compromises reception, or perhaps circular polarization is a method to render tilt irrelevant in this regard.

I have read that with linear polarization systems, fine tuning the LNB to offset the effect of tilt is part of the process.

[Mertzen]

I don't know if that is the specific design element of that.. But it also allows for perfect signal on masts that are not plumb
Linear polarity on a poorly mounted mast is a nightmare compared to circular.

[tkrandall]

Thanks.

I also just found this, which seems to confirm that circular polarization schemes are more robust in this respect.

http://www.frccorp.com/pdf/Why%20Circular%20Polorized%20Antenna.pdf

and more:
http://www.air-stream.org.au/Polarization

Circular polarization is most often used in satellite communications. This is particularly desired since the polarization of a linear polarized radio wave may be rotated as the signal passes through any anomalies (such as Faraday rotation) in the ionosphere.

Furthermore, due to the position of the Earth with respect to the satellite, geometric differences may vary especially if the satellite appears to move with respect to the fixed Earth bound station. Circular polarization will keep the signal constant regardless of these anomalies.

[carl6]

The skew/tilt setting on the multi-satellite dishes is used to get the right geometry of the dish with regard to aiming at multiple satellite locations, rather than to align with the polarization (as is done with linear polarization).

Even numbered transponders use left hand circular polarization and odd numbered transponders use right hand circular polarization. The LNB is switched between the two modes based on the DC voltage sent to it (18 V for left hand, 13 V for right hand).

[bobnielsen]

With a single-satellite dish, tilt isn't a factor with circular polarization, but would be if linear polarization were used. With the multisatellite dishes, tilt aligns the LNBs such that they are pointed correctly at all of the applicable satellites.

[william8004]

Back the the Air Force days I used to work on Air Traffic Control Radar. ATC radar uses both linear and circular polarization. Circular was used to reduce the effects of weather and rain. Does D* use it for the same reason?

[Tom Robertson]

Back the the Air Force days I used to work on Air Traffic Control Radar. ATC radar uses both linear and circular polarization. Circular was used to reduce the effects of weather and rain. Does D* use it for the same reason?
Based on the linked document, I think that is a big part of it. And not having to tweak the LNB alignment.

Thanks for the info about ATC radar. I learn something new everyday. :)

Cheers,
Tom