Rotate offset dish?

TheToad

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Hi All,
Our 1.5m prime focus dish has taken too much of a battering this year and it's now just too difficult to maintain a decent reception.
Therefore we bought a 1.1m oval dish which is in much better condition, but we've found it's still quite tricky to get a signal on the channels we need because they are horizontally polarised which is of course using the narrow part of the dish.

If I were to rotate the dish 90 degrees, of course understanding that at the receiver I would need to swap V/H polarisation, would this work? Should I expect any particular problems with setup, besides the obvious mechanical problems of the mounting bracket?

many thanks in advance
Tony
 

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Hm, never heard of rotating the dish bringing more signal from a particular polarity.... the surface area collecting the signal doesn't change. I could be wrong though.

If you're right, then you can try rotating the dish along with the LNB then you don't have to change the polarity on your receiver.

Just so we know - what satellite/channels are you trying to receive and what you mean by battered dish? Warped, kicked around or? If it's warped, then you can try fixing that - prime focus dishes are ideal. Just put it face down on the floor with a little weight on top and after a few days it should regain its original shape.

I believe VS did this with his 1.8m dish.
 

TheToad

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The prime focus is not in a good way. Not just warped (fixed it many times and used strings to check if it's true) but the edges are also pretty badly dented and it was rather susceptible to twisting in the wind. It is a very very old dish. We only paid 15quid for it almost 3 years ago.

The particular birds we need to watch are Amos 2 10722,10759,10806 and Amos 3 11389,
The Amos 2 channels we could receive most with no problems, the Amos 3 channels almost impossible.
We're based just South of Loughborough.

My theory with rotating the dish is that in its normal orientation the V channels would be stronger because the dish is larger in that direction and the H not so because it is narrower, and therefore will not be getting as much of the horizontal signal.

Regarding the rotating of the LNB, I'm not sure that would work on an oval dish because the LNB is designed to illuminate the dish in the same orientation as the oval shaped of the dish isn't it? I did try our C120 but that wasn't happy.
 

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Can't think of a good analogy but try this, imagine every wave coming from the satellite as a small round beam of light regardless of polarity. Doesn't matter which way you orientate the dish it will still pick up the same amount of ight beams and focus them onto the LNB. Actually if you do spin it 90° it will be slightly less signal as the boom and LNB will obscure the dish.
 

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If you look at the dish from the point of view of the LNB then the dish will appear more circular. Even if its not exactly circular the LNB will, by design of the horn, only "look" at a circular section.

So rotatong the dish will have little or no effect.
 

TheToad

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Aha, yes, I see what you mean now, because the LNB is at an angle to the dish it is in effect still a circular dish. I guess the analogy is like the word 'slow' written on the road for drivers; when you stand next to it, it looks long and thin, but approaching it from a driving position the letters look 'normal'.

Thank you, this makes perfect sense.
 

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Amos 3, and 11389H in particular, is a pain in the backside. I only ever have blips of signal from this on my Fibo. Amos 2 is generally OK, though. I take it you're after Ukrainian and Russian channels? There are a bunch on 5E but Amos 3 does have a good selection.

There's always Viasat Ukraine on 5E, which is a very easy catch in UK, if you are prepared to paid for the subscription. I can get it for around $350 (yes, USD) plus postage from Ukraine for two years, which includes the receiver... but that's by-the-by.

Amos 3 would need a bigger dish than 1m. A good 1.5m should pull these TPs in... and if you can get 11389H, you should be able to get Xtra TV as well (again, be prepared to pay a sub).
 

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TheToad said:
If I were to rotate the dish 90 degrees, of course understanding that at the receiver I would need to swap V/H polarisation, would this work? Should I expect any particular problems with setup, besides the obvious mechanical problems of the mounting bracket?
You are half right in your reasoning, but for the wrong reasons. Elliptical antennas are used in many networks where the wider part of the reflector is optimised in the plane towards the satellite by 'twisting' it either side of vertical (using a third set of bolts between the elevation and declination ones). You may see them on SNG trucks, but are more common round here on the top of police vehicles stationed at known speed trap areas, and on the backs of post offices and gambling dens in the high street.

This is however used for improving sidelobe performance on a receive/transmit antenna but they only work with an elliptical feed that is matched to the design. On a receive only dish with a standard LNBF, or matched feed that is circular in shape, you are unlikely to see much of an improvement.

But there is no reason not to try, is there.
 

TheToad

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Thank you for the advice. I'm sure when I get around to studying for my advance radio exam I might lean a bit more in general, but time is in rather short supply these days...


Channel Hopper said:
But there is no reason not to try, is there.
Only one - the wife!
 

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TheToad said:
Thank you for the advice. I'm sure when I get around to studying for my advance radio exam I might lean a bit more in general, but time is in rather short supply these days...



Only one - the wife!
With knowledge of a choice comes opportunity
 
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