How much loss per degree ...

[Vipersan]

How much signal loss per degree should I expect on my Gibby 1.25 when an lnb is placed off focus ?
I'm currently trying to fit 2x ka and 1x Ku lnbs on a gibby x3 multi lnb bar..
Obviously they can't all occupy the sweet spot ..so this will be a compromise ..
This I already know ...but close grouping at the focus should give me optimum results if set up correctly.
Even then I suspect at least 3 degrees of seperation minimum..
Some idea of what to expect would be useful ..
..and keep in mind all 3 lnbs will be on a motorised dish ..so there will be some degradation except at the focus ..
the arc/H180 travel will be compromised except at the focus of course.
rgds
VS

 

[Vipersan]

Ok ..here is the compromise ..
Working well atm ..but then it is a clear rain free day ..
I'd estimate roughly 1db of loss per shift from focus of 3 degrees..but I can still lock Ocko today at 5.4db on the SL3
Ka lnb band D at the focus and the ku to the right as viewed from the dish.
The thick rimmed specs (cover) of the SL3 fee copper feed has been removed ..and am contemplating trimming down the diameter of the cone to allow closer placement of the feed to the focus without casting a shadow over the Ka centrallly placed lnb.
I estimate I can get about 1degree closer to focus without any serious degradation of signal capture by a reduced diameter cone.
In Ka every 0.2db counts...!!
rgds
VS

 

[Huevos]

I don't understand why the LNBs are so far apart. On a 1.25m dish you can get conventional LNBs 1º apart, which means you can receive from satellites separated by 2º without moving the dish.

 

[Vipersan]

The simple answer Huevos ..they are not conventional LNBs
If you look at the centre Ka lnb and the Ku lnb ..they are as close as they can be ..
The SL3s bulk doesn't allow it to get much closer to the centre lnb ..
it's simply not possible to close this gap much more ..another 0.5 to 1 degree perhaps.
rgds
VS

 

[Huevos]

Yes, but that limitation is inflicted by a bad choice of bracket design, not by what is possible. For close LNBs I use the bracket in the image below. LNBs can be as close as the feedhorns allow.

 

[Vipersan]

Absolutely not true regarding the R9215 Ka lnb and the SL3 ..
the physical mass of the SL3 in particular makes it impossible to close the gap in the same way 2 KUs on a well designed bracket would permit.
Remember the DirecTV SL3 is a triple broad bodied lnb ..and worse ..I have no choice to orientate this at 45 degrees to the norm..which effectively makes it appear wider.
rds
VS

 

[Huevos]

Absolutely not true regarding the R9215 Ka lnb and the SL3 ..

Right now it looks like the bracket is the first limitation.

TBH, I think something that moved the LNBs in and out of the focal point as required would be the most lossless solution.

 

[Vipersan]

A long long time ago in the days of analogue ..I think such a thing did exist ..
A fixed dish with a worm drive at the business end to move the lnbs into and out of focus ..
Or did I imagine it

If memory serves it was used to allow the user to change from BskyB circular polarised lnbs to linear..
possibly when astra 19E was used for uk satellite ..
But I could have dreamt it.

 

[Vipersan]

No ..didn't imagine it ..
Though my facts are a bit askew..
the original multisat ..on a fixed dish..
Not really to shift lnbs ..but position one lnb at various offset positions.
I suspect it could be adapted to do the job though.
Here it is being discussed ..
http://www.satellites.co.uk/forums/threads/satellite-dish-with-motorised-lnb-arm.110710/
Dont suppose anyone still has one ?

 

[Captain Jack]

The two non-feedhorned LNBs could probably be closer a little bit if the bracket was different but probably not worth the hassle changing it.

One concern is regarding the copper feedhorn covering a little bit of the centre LNB from the right side of the dish (looking in front of it) - or is that just an angle of the picture?

Do you have inclination control on Neil?

 

[Vipersan]

The two non-feedhorned LNBs could probably be closer a little bit if the bracket was different but probably not worth the hassle changing it.

One concern is regarding the copper feedhorn covering a little bit of the centre LNB from the right side of the dish (looking in front of it) - or is that just an angle of the picture?

Do you have inclination control on Neil?

No inclination control CJ ..
..and yes it is the angle that's deceptive..
Getting excellent results from the centre LNB .
I belive I can get the SL3 a little closer to focus ..but would mean reducing the diameter of the copper feed cone a little with minimal effect on reception..and it's on my to-do list.
rgds
VS