Thread: Az El vs Polar Mount

Does anyone have any figures on G/T penalty for dual feed skew on Az El offset feeds as opposed to polar mount or dish rotation? Since my skew is 23 degrees it appears that in addition to gain compromise the temperature might be also be increased for the lower feed in that its side lobes might see more earth temperature than the upper feed. I notice that Dish and Direct appear to rotate the dish rather than skewing the feeds. It wouldn't be easy to rotate the 90cm dish, but since I am considering a post mount it might not be too difficult to set the post at a polar angle rather than vertical if the G/T increase was worth the effort. Or my house is so close to a cardinal alignment I could almost pull the vertical post back to a polar angle and gain substantially on the angular support of the bipod attachments as well.

HUH????
That sounds way more technical than the Glorystar Forum is designed to be. If you want answers for that I would truly suggest you post the same question in the FTA discussion group. I am sure Anole, linuxman, ACRadio and others would know more of what you are talking about.... I am a extra class ham, and I remember some of what you are saying, but any technical answer I would try to give would be over my head. Hope you understand the Spirit of my answer.
POP

Any dual lnbf system is a compromise for sure. With out a doubt a skewed horizontal "elliptical" dish will do better feeding a dual lnbf setup (as Dish Network and DirecTV do).

However, I believe that an offset vertical "oval" dish is designed to "see" the satellite as a circle. If that is the case, then skewing an "oval" dish won't help.

I believe that you are correct that the temperature that is picked up would increase with the lnbf being moved off axis and over illuminating the edge of the dish thus degrading the Gain/Temperature ratio of the setup. For best performance I would mount two dishes rather than just one.

A polar mount would make it easier to swing one dish to different satellites but will not give you better G/T than just skewing the lnbf.
Bob

Originally Posted by wescopc

Any dual lnbf system is a compromise for sure. With out a doubt a skewed horizontal "elliptical" dish will do better feeding a dual lnbf setup (as Dish Network and DirecTV do).

However, I believe that an offset vertical "oval" dish is designed to "see" the satellite as a circle. If that is the case, then skewing an "oval" dish won't help.

I believe that you are correct that the temperature that is picked up would increase with the lnbf being moved off axis and over illuminating the edge of the dish thus degrading the Gain/Temperature ratio of the setup. For best performance I would mount two dishes rather than just one.

A polar mount would make it easier to swing one dish to different satellites but will not give you better G/T than just skewing the lnbf.
Bob

I would agree with your thoughts if we were talking about a prime focus feed rather than offset, but skewing an offset feed would have to increase the compromise especially for larger skew angles (though actual numbers might not support any action).

Originally Posted by elder

I would agree with your thoughts if we were talking about a prime focus feed rather than offset, but skewing an offset feed would have to increase the compromise especially for larger skew angles (though actual numbers might not support any action).

If the scaler of the lnbf is symmetrical (most are) then skewing the feed (lnbf) won't make a difference. The illumination of the dish will be the same - hence the Gain/Temperature remains the same. Now I would agree with you, if the feedhorn/scaler is matched to the dish in such a way that the G/T would be affected by skewing the lnbf. Dishes with such a setup probably skew the whole dish as a matter of design rather then just the lnbf. IMHO
Bob

I think I get your point. If the reflector is a true paraboloid of revolution rather than being specifically shaped for an offset feed then the skew will just paint an arc around the dish sweet spot and the compromise will remain constant. I was assuming it was designed for the offset in which case the any skew would cause both feeds to miss the spot with the lower one seeing more of the earth. The three feed Direct dishes appear to be shaped, can't tell if they are toroidal. Come spring I may play around with a penlight to see where the illumination hits the dish, but alas my hopes for a better G/T appear to be just a pipe dream!

Elder
I think we have mis-communicated a bit. I agree with you that skewing a dual lnbf setup compromises the illumination of the dish in relation to the amount of offset of the lnbf.

When I talked about skewing the lnbf vs the dish I was talking about a single lnbf mounted at the focus of the parabola.

Having said that, I believe multi-lnbfs on a standard "oval" dish are a compromise that can't be overcome by skewing the dish.
Cheers,
Bob

Bob, I greatly appreciate your input. I got the idea driving into my neighborhood noticing that the Direct logo on a dish was skewed. Then came my armchair thinking, next came reality.

This thread got booted from the GloryStar Forum so I'll provide a bit more info on the dish in question. It is a GeoSat Pro 90cm dish with a dual lnbf offset feed. I picked up on another thread that the dish has a 26 degree offset angle, so that may or may not lend credibility to my original question.

Technical Specifications

Mechanical

Effective Aperture 90 cm / 36"

Diameter 93.5cm x 85 cm