Offset lnb on a PF ??

[Vipersan]

Not the prettiest of projects ..and wasn't expecting a great deal ..
but am being pleasantly surprised atm..
Late post I know ..but it took hours to cast and shape a clamp to fit Christinas 15mm copper plumbing tripod ..
Cast in liquid plastic shaped sliced and drilled ..and is now on the tripod allowing a second cband lnb to be fitted as close to focus as is practical.
This _may_ have a detrimental affect on the signal arriving at the prime lnb as it _will_ cast a shadow ..
Nonetheless I can only try ..
Currently have my old Pauxis as the second lnb and getting better than expected circular results.
The idea is ..assuming all goes well...to have a linear lnb at the focus ..and a circular lnb offset.
The reason for doing it this way around is that polarisation and skew become irrelevant to the offset lnb....but critical for the linear at the focus.
No adjustments have been attempted ..

It's dark and it's late...but early results look promising.
Just scanned 40 West with the Pauxis RHCP ..
3600 to 4200
Number of transponders locked with the TBS card =25
No easy task mounting a second lnb on a PF ...but I'm pleased with the results..
A quick photo from my bathroom window ..(you can just about make them out)
Better pics tomorrow..
the lnb is mounted to the west of the Prime LNB ..
Since theres's only really Linear sats at the western end of the arc ...this seemed advantageous as it will also extend my reach Eastward if all goes well and give me back 62 and 64 East ..maybe...?
Easily 12 degrees offset ..LHCP 40 west results to follow ...and I'm using my crappiest lnbf and just thrown up for now..
rgds
VS

 

[Vipersan]

Not so hot the top 50% of the band in LHCP ...
only 17 transponders ..but I'm hoping this can be improved upon with a better lnbf ..correctly positioned and set up.
enough for tonight ..I'm knackered.
;)
rgds
VS

 

[pride21]

not bad results being 12 degress offset and the Pauxis lnb not being the best,could not make much sense of the picture lol

 

[Vipersan]

These pics might help clarify Pride..
My aim this morning is to try and lock at least BBC Africa on 1West ..
Not looking good so far.

As expected ..this may well be due to pointing the offset lnb at the dish centre which effectively loses the signal from half the dish ..
I may try aiming off centre ..but really an offset lnb on a PF is a nightmare ...since at the 'sweet-spot' ...to quote Highlander ..
'There can be only one'.
I can also try removing the concentric feed entirely thus getting the offset closer to centre ..a trade off maybe ?
this is all new territory for me ..
We shall see.
Of course I will always have the prime lnb at the focus ..so nothing to lose IF the second lnb doesn't cast too big a shadow ..which of course will be less without a feed.
rgds
VS

 

[Vipersan]

OK ..the shadow effect is definitely there ..again ..as expected..
I chose TPC at 1W to test this..
Without the offset fitted I usually get around 7.2 db
With the offset fitted this has dropped to around 6.7 db. (some of this may be weather related as it's seriously overcast atm)
I guess this is a fair tradeoff ..to have a second lnb on Christina.
Removing entirely or in part the concentric feed of the offset lnb might bring some of this back ..and also allow it to be closer to the sweet spot ..thus upping it's signal ?
All to play for and little to lose..
;)

 

[Huevos]

I've got the circular polarized LNB central (because that is the one that gets attenuated the most). Linear LNB is on the east side of the dish as most linear sats are out west. No difference in signal with or without the offset LNB, and the offset LNB gets exactly the same SNR as the central one. But there is some strange interaction between the offset LNB and the Ku LNB on some sats. Offset LNB receives from 6.5º off centre so must be offset by 3.25º. Maybe this is a compromise but it is 100 times better than taking the plate in and out.

 

[Huevos]

Without the offset fitted I usually get around 7.2 db
With the offset fitted this has dropped to around 6.7 db.

I find this hard to believe as the feed is less than 1% of the dish capture area.

TPC, 7.0dB at the moment.
Africa, 5.5dB.

 

[Vipersan]

It could just be that the signal was low this morning Huevos ..
I'll do a proper test on this when both lnbs are re-connected.
Good info and pics btw..

 

[Vipersan]

Hmm ..I simply cannot lock TPC with the offset lnb ..
I got it a little closer using a cut down feed ...
but still it refuses to even find TPC ..
Only getting 3db from 3723R (Fox) ...
Both lnbs are the same distance from the dish centre 32.5 cm
C1 PLL currently the offsert lnb btw.
I'll keep trying ..
Perhaps the problem is that it's a relatively deep dish ?
rgds
VS

 

[Huevos]

Only getting 3db from 3723R (Fox) ...

Nothing from that. Maybe the parameters on Lyngsat are wrong. What are you using.

 

[Huevos]

Why is the LNB so low?

 

[Vipersan]

The lnb is mounted a bit low ..but shouldn't be an issue since I can set position and elevation per satellite..
With the C1PLL as the offset looking for circular ...skew won't be an issue.

Fox is just a data stream so cannot be locked with a standard receiver..
I'm using the TBS card to reference it
rgds
VS

 

[Vipersan]

Just locked BBC Africa feed at a paltry 3.6db ..
By pointing the lnb slightly higher than dead centre ..
..a small increase in elevation was then required ...
Like I imagined ..this problem may be due to the fact that Christina is a deep dish ..so maybe using upper of lower segments gives a better representation of an offset dish ???
In my minds eye I still fail to see how an lnb offset from the focus but looking at the dish centre can behave like an offset dish ..
the signal arriving at the offset focus would be elongated ..into an oval
ie anything but sharp.
Please correct me if my minds eye view is totally wrong..
rgds
VS

 

[Captain Jack]

FWIW, I am barely getting a lock on TPC but BBC Africa feed is healthy at 5.9dB (Vu+ Uno figures).

 

[Huevos]

@VS, I don't know what you are doing changing elevation. Start by getting best reception on the central LNB then go a few clicks over and work on your offset LNB. After all the purpose is both LNBs track the arc, and if you are changing elevation that is obviously not the case. And forget theory for the time being. Just play with that LNB and see what works best, then think about why.

@CJ, TPC was always a problem for me on the Gibertini 1.5m. I haven't really picked up any more beams on any satellite with the 1.8m but reception of the ones I was receiving is a lot more solid.

 

[Vipersan]

@VS, I don't know what you are doing changing elevation. Start by getting best reception on the central LNB then go a few clicks over and work on your offset LNB. After all the purpose is both LNBs track the arc, and if you are changing elevation that is obviously not the case. And forget theory for the time being. Just play with that LNB and see what works best, then think about why.

@CJ, TPC was always a problem for me on the Gibertini 1.5m. I haven't really picked up any more beams on any satellite with the 1.8m but reception of the ones I was receiving is a lot more solid.

We are only talking about perhaps half a degree Huevos ..
The way the tripod is constructed makes mounting exactly where I'd like the 2nd lnb a bit tricky..
Custom built for the Alcoa not having access to an original.
I'd much prefer not to change the elevation but since this was an option on my system it made sense to try it and max out what was available.
cheers
VS

 

[Vipersan]

FWIW, I am barely getting a lock on TPC but BBC Africa feed is healthy at 5.9dB (Vu+ Uno figures).

Yep I too get healthy signals at the focus ..
over 7db TPC and 8+ db BBC Africa (figures from yesterday night).
..but the drop-off in signal is rapid once away from the sweet spot.

 

[Vipersan]

Tilted the look angle of LNB2 up a bit more now looking approx. 6 inches above dish centre ..
BBC Africa holding steady at 4.2db now..

 

[Costactc]

Not meaning to hijack VS but I slapped a ku lnbf next to my main esx241 c band lnbf a couple of days ago on my 2.4m PF dish. Signal quality hasn't been affected at all on c band and ku is scanning in well as well but both polarities are scanning in with the reverse being stronger then the actual one....I'm a bit puzzled.

 

[Vipersan]

Not quite sure what you mean by 'the reverse being stronger than the actual one' ?
Interesting though that you too appear to have your look angle (ku lnb) aimed off centre of the dish..
..thus using only a portion of the dish ..as though it were an offset ...which of course IS a section of a PF in reality.