Az El vs Polar Mount

[elder]

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.

 

[rv1pop]

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

 

[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

 

[elder]

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).

 

[Wescopc]

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

 

[elder]

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!

 

[Wescopc]

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

 

[elder]

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.

 

[elder]

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.
[FONT=arial,helvetica,sans-serif]
Technical Specifications[/FONT]

[FONT=arial,helvetica,sans-serif]Mechanical[/FONT] [FONT=arial,helvetica,sans-serif]

Effective Aperture[/FONT] [FONT=arial,helvetica,sans-serif]90 cm / 36"[/FONT] [FONT=arial,helvetica,sans-serif]

Diameter[/FONT] [FONT=arial,helvetica,sans-serif]93.5cm x 85 cm

[/FONT]

 

[Anole]

lizard opinion:

The elliptical Dish and DirecTV dishes are skewed because:
- they want to pull in multiple nearby birds (that's why they're elliptical)
- an elliptical dish loses gain if aimed far east or west of direct south
- building the dish with fixed LNBs and skewed dish mount, makes for easier field alignment
- remember, Dish 'n Direct are circular polarity, so their LNBs don't really need to be skewed.
It's all a matter of convenience, I believe.

Now, as to the matter of c/n or t/n or whatever...
Are you experiencing unsatisfactory reception?
If so, there are two good solutions:
- get a bigger dish
- get two dishes and put each LNB at the actual focus
- I vote for: bigger dish -

Of course if this is just a theoretical discussion, please ignore me.
I just like to solve problems, and if there is none, ... well ... never mind. -

 

[elder]

The dual lnbf feed is for nearby birds (97 and 101) and I think the skew is for the arc as each lnbf could be rotated if need be to fine tune polarization. I guess my question is theoretical in context, but with a practical application if I am correct. I have some channels reading less than the recommended 50 and was looking for a possible simple solution rather than constructing an antenna farm.

 

[SatPhreak]

It may have a little bit of overillumination on part of the feed since it has dual lnbfs. If it was only a single lnbf in the main focal point it would see the dish as round with no overillumination at all, irregardless of the skew. An elliptical dish should get no overillumination, but there really should be no problem getting a good signal on the dish you have if it is aimed correctly.

You could try an elliptical Starchoice 75E(75cm) dish (it has a monoblock lnbf for 3.8 degree satellite seperation), but the slightly smaller dish size probably wouldn't make a difference in quality compared to the very slight overillumination you currently get. I have used an elliptical starchoice 60E(60cm) dish on 97/101 in the past and all channels came in but with lower quality, due to the undersized dish for most FTA sats, then I could get with my 90cm, single lnbf, motorized dish aimed at 97 or 101.

 

[elder]

Even without over illumination concerns if you do patterns on any feed you will find that it has side lobes as do all antennas. That's a secondary benefit of a Cassegrain reflector as the feed side lobes see cold space as opposed to warm earth. An offset reflector would offer similar benefit for high elevation angles however I live in the mountains so even though tilted up my feed side lobes still see trees and hillsides along with sky. An offset reflector should represent a parabolic section, maybe not even containing the center of curvature. I remain convinced that a polar mount would be better than skew, I just don't know if it's worth the effort. A side benefit would be that I could take a peak at G18 with little effort.

 

[B.J.]

.. An offset reflector should represent a parabolic section, .....

Actually, all satellite dishes should ideally be elliptical rather than parabolic. Basically, a parabolic surface will focus things infinately far away, but an elliptical surface will focus things a finite distance away. I know that the sats are essentially infinately far away as far as we're concerned, but technically the best surface would be a very extreme elliptical surface, which basically approaches being a parabolic surface.

{Just throwing out interesting [to me] aspects of this. }

 

[SatPhreak]

The problem with using elliptical dishes would be needing many dishes for each provider. The farther east or west of your true south that you are aimed at would require a slightly different dish as you would be looking at a sat farther away from you in distance on either end of the arc as you would if it was at your true south, and would need a slightly different focal lengh to get correct coverage. Elliptical dishes also require a feedhorn that is taller and narrow in width than one designed for an offset parabolic (which is round). Most DBS sats are high powered enough to make up for the slight loss in using a round feedhorn, but companies like starchoice (which uses FSS sats) have squashed feedhorns to get optimal signal, along with a dish that skews.

I have used my 60E on many sats (picking up 2 sats that are 4 degrees apart) and all though the signal is always less than my motorized dish, the quality seems to be linear compared to my larger dish. Farthest skew I have used in on was 123/127 with about a skew of 23-24 degrees.

An offset parabolic with 2 lnbfs and a large skew probably gets more overillumination than it would if the skew is near zero. Skewing the dish and setting the lnbfs at zero might be able to correct this.

 

[elder]

An offset parabolic with 2 lnbfs and a large skew probably gets more overillumination than it would if the skew is near zero. Skewing the dish and setting the lnbfs at zero might be able to correct this.

Food for thought, if I didn't want to take a peak at G18 (123) I could just set my feed mount skew to zero and rotate my mounting plate to a 23 degree skew and see if there is noticeable improvement. It would be easy to recover from that config.

In considering a polar mount I wasn't thinking motor, but just pulling my J post back to the proper angle and shimming the mounting plate so the J post is pointing north (it is close as my house has a near cardinal orientation). My bipod support arms are too flat to the J post for adequate support and the polar angle would also result in a more rigid mount. The only time I would move the dish would be for a peek at G18, if I liked what I saw there I might then consider a dedicated dish for G18.