Different positions for different polarities?

[Captain Jack]

I found a weird thing with C band on my dish. When I peak on one polarity, I lose signal on the opposite one. I have to do a few clicks of the motor to get the best signal between them. This happens on both linear and circular (Titanium LN....

Why would this be? Bent pins inside the LNB? Feedhorn wonky? This also happened on my 1,5m Gibby and now on the Laminas as well. The LNB is at the centre of focus being on the LNB rotator...

This does not happen in Ku at all. If I peak on one polarity, it will peaked on another.

Any ideas?

 

[hvdh]

First thought, adjacent satellites disturbing only one polarity, casing the "sweet spot" to move away a little.

 

[Captain Jack]

Hm possibly but this seems to happen on all satellites.

 

[John]

First thought, adjacent satellites disturbing only one polarity, casing the "sweet spot" to move away a little.

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+1 with hvdh.

I can get these symptoms with my 1.8 CM on Cband also @CJ . When you say "loose signal" , do you mean actual received signal strength as indicated on the receiver or do you mean a reduced "Quality or C/N" figure as reported by the receiver or analyser. For example, i can tune for max received signal strength on my TM receiver and get a "quality" reading of say 50. Nudging the dish one way or the other a click or two maximum will increase the quality reading but just drop the actual received strength down a fraction but can enable you to get a lock on a weak signal. Apart from that buddy, i 'aint got a clue

 

[Captain Jack]

Sorry, should have clarified. It's the quality that changes - I rarely pay attention to 'strength' as it doesn't tell me a great deal anyway.

 

[scopus]

I get the same on 40W on the 3805 transponder, the LHCP has a sweet spot a few clicks away from where the RHCP is. Wonder if this is an offset dish phenomenon?

 

[pride21]

hi CJ the only thing i get is a difference on certain transponders if i peak the dish east or west to get the best quality then other transponders suffer, vice versa with transponders an example is 49E 2640L if i peak that transponder then 3743L goes is this what you mean.

 

[John]

... I can vaguely remember something similar from a few years back when using my 2.2 PF looking at Cband feeds from 58West, differences between H & V sigs having to be tweaked separately ..... i was fairly convinced that i was looking at downlinks from two satellites co-located as near as damn it to each other at 58W.
@scopus .... yes , the slight differnce in polarity illumination properties with the non symetrical shape of an off-set ??.

Btw Chris .... that thing went moments after i drew your attention to it.

 

[scopus]

@John, you are getting far to technical for this time of day! lol....

Yes, was snapped up by somebody, wasn't me I'm afraid..

 

[Vipersan]

I get the same on 40W on the 3805 transponder, the LHCP has a sweet spot a few clicks away from where the RHCP is. Wonder if this is an offset dish phenomenon?

seen this phenomena myself both at 40W and 55W..
Almost as though there are 2 sats up there seperated by about 0.5 of a degree

 

[John]

@John, you are getting far to technical for this time of day! lol....

Yes, was snapped up by somebody, wasn't me I'm afraid..

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@John, you are getting far to technical for this time of day! lol....

Yes, was snapped up by somebody, wasn't me I'm afraid..

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Nay mind lad

 

[nelson_b]

Yes i,v experienced this too .. from time to time on some sats ... just though it was probably down to using offset l.n.b,s on an offset dish ... but maybe not ... just got round it by re aligning slightly . cant say i really noticed it on the 3m prodelin though.

 

[Vipersan]

Yes i,v experienced this too .. from time to time on some sats ... just though it was probably down to using offset l.n.b,s on an offset dish ... but maybe not ... just got round it by re aligning slightly . cant say i really noticed it on the 3m prodelin though.

TBH Nelson ..I saw it on Christina ..with a non offset feed ...I put it down to having a petalised structure to the PF at the time ...but am now convinced there is more to it than that..
A mystery I never solved ..just put up with it ..

I distinctly remember that Bolivia on 55W peaked 10 clicks away from the Jamaican mux when it peaked ..
rgds

 

[Trust]

Maybe it has to do with the different antenna positions in the lnb .
The focal lenght is also depending by the frequency .
At german board there are members with very complex adjustable C lnb constructions for focal lenght , scalaring adjustment and skew .

.

The Zinwelll has both antennas at the same distance