Is skewing adjustment really needed?

[7mdish]

Hello, after many years of satellite reception, in my experience I always thought that a well done motorised system needs some kind of polarity adjustment.
So, in my new system hoping to install before next winter I planned a motorised feed, see here:

Upside down offset dish

I noted that some forum member installed offset dishes upside down. Could anyone explain what is the reason? Which advantage with such installation? I really have no ideas. Thanks.

www.satellites.co.uk

But, at the same time, I'm asking why most users don't use any skew adjustment, also in big motorised dishes.
Well, I believe that no polar mount or H-H motor have such precision movements to decline the dish exactly in the right position to reach a perfect skew on all satellites (particularly on extreme birds), so I suspect that most users are OK when they receive the signal, and if not they think that the dish is too small or similar.
I would like to share your opinion about this problem. I'm convinced the skew adjustment is absolutely needed, so I just ordered a motorised feed for my new Raven 2.4m dish, no doubts, but what is your thought? Thanks.

 

[John]

Hello, after many years of satellite reception, in my experience I always thought that a well done motorised system needs some kind of polarity adjustment.
So, in my new system hoping to install before next winter I planned a motorised feed, see here:

Upside down offset dish

I noted that some forum member installed offset dishes upside down. Could anyone explain what is the reason? Which advantage with such installation? I really have no ideas. Thanks.

www.satellites.co.uk

But, at the same time, I'm asking why most users don't use any skew adjustment, also in big motorised dishes.
Well, I believe that no polar mount or H-H motor have such precision movements to decline the dish exactly in the right position to reach a perfect skew on all satellites (particularly on extreme birds), so I suspect that most users are OK when they receive the signal, and if not they think that the dish is too small or similar.
I would like to share your opinion about this problem. I'm convinced the skew adjustment is absolutely needed, so I just ordered a motorised feed for my new Raven 2.4m dish, no doubts, but what is your thought? Thanks.

Good question.
My short answer, which is relevant to my C-band installation, is yes.
You can get away with no skew control most of the time if: - you have a very large dish, 3mtr plus, are only interested in strong downlink program signals that have a big link margin so that any slight skew adjustment is not needed.
If you are not operating a Cband antenna NOT of the size mentioned above, like me-only a 2.2PF, and are searching for very low level signals and sometimes with an awkward FEC setting, then the ability to have incline tracking as well as skew control on the dish is an advantage to screw the last bit of performance out of the dish **Accurate dish assembly and feed components are just as important as well if your working on smaller Cband dishes like mine - it certainly is for me anyway. A 'one skew setting fits all' when you are covering a large East-West arc, when searching for adhoc very low rx signal levels does not work, as i have found out through practice.
Good luck with the install - just remind me again what the motorised feed looks like that you are ordering

 

[s-band]

I've found that skew adjustment can make a big difference at times. This is really noticeable on satellites like 0.8W where there are co-channel signals on the opposite polarisation in many cases. There is also the Eutelsat 3 degree offset relative to most other satellites in this region.

If the signal is good and there is nothing co-channel, on the satellite viewed or close ones, then skew is much less critical.

 

[Terryl]

Skew would be normally used on a multi LNB fixed dish,(or in some cases a single LN on a "C" band polar mount the skew should not affect the signal as the mount takes care of that, (if everything is adjusted right, and the mount is solid) the same with an H to H motorized Ku band setup.

Most "C" band LNB's I have installed use 13 and 18 volt polarity switching, my combo "C/Ku" LNB also uses 22 kHz to go between the two bands, I have no problems.

 

[Channel Hopper]

Alluding to the answer from John, I would suggest you are unlikely to need a skew adjustment for C band unless you are looking for linear transmissions, but circular depolarising does come with its own perculiarities with modern feeds, dielectric sorcery and pipe style LNBs.

For Ku band, on most reflectors 1.5m or above I have found skew alignment is only really important on many of SCPC channels where there is a second country/continental beam with similar strengths bleeding into the UK. I do not search out for obscure high bandwidth content however and so there may be a requirement to gather more clean signal for certain feeds (including high speed internet links if you are into that sort of stuff)

Some of the newer satellites also have similar/same frequency bouquets on opposite polarities, (which are not 90 degrees apart owing to the slant angle that they are being received on), which will play havoc with the more common voltage controlled polarity LNBs.

Start off by adjusting by hand with an analyser rather than pay top ££££ for something you may never really appreciate. The quality of the groundstand, the polarmount (and any inclination system) is going to have a far greater impact on the quality of reception of a long established and trusted 2.4m reflector.

I forgot to ask in the other thread, have you secured a Channel Master/Raven 2.4m system with the later Ka band specification ?

 

[Satfaca]

Good question. For max performance definitely needed so for a real Sat-freak no compromise...
One cannot believe what can be achieved sometimes only until he tries on his own. Rules for both C and Ku band (and probably Ka even more)...

 

[s-band]

Attached is an extract from the Eutelsat document showing the satellites with the skew offset.

Another reason that skew control is useful is to null interference from an off-bore satellite. That will have a skew offset from the one that is on-boresight. If you're after every last bit (which, I am) then skew is essential.

If you happen to be in the Pacific region then you'll need 45 degree offset for Optus satellites. (info courtesy @moonbase)

 

[deeptho]

Some of the newer satellites also have similar/same frequency bouquets on opposite polarities, (which are not 90 degrees apart owing to the slant angle that they are being received on), which will play havoc with the more common voltage controlled polarity LNBs.

Just curious, but why would the two polarisations no longer be orthogonal when received under a slant angle?

The direction of polarisation will definitely shift (that is why the polar mount causes the dish to slant at extreme angles), but
the two polarisations should remain orthogonal, shouldn't they? One polarisation could get more attenuated than the other,
but that still does not change the orthogonality, I presume.

 

[7mdish]

Thank you for your replies, very interesting. I forgot to mention that I was speaking of Ku band only. I have no C band yet, had some years ago with an old Corotor, then I dropped it and I don't think I will reinstall it on my new dish, at least not in my first setup, maybe in the future. Also, not sure if we have many signal in Ka band here in Italy, I don't think so. I heard about few local feeds some years ago, but I believe that Ka band is used mainly for data only, not TV broadcasting. So I speak of Ku band only.
My main goal is to get feeds and weak signals, and this is the reason for which I believe the skew is so important (the same reason for which I want a so big dish!). Yes I'm looking for the last bit too! Obviously skew is not needed if you want to get Hotbird or Astra satellite, but this is not my goal, not with this dish at least.
In my new setup I will test a Viking feed designed for offset dishes, I will tell you the performances but I must wait some week to get the dish, so not before next October I'm afraid. I will post the results here in the forum. Thanks.