How critcal is skew on a pf dish ?

[Vipersan]

Hi friends ..
Since I use an electrically switched LNB on my Prime focus dish ...the only aspect of reception I have never attempted to tweak is the skew ..
There has been no need ..but as everyone will know I am concentrating my efforts on fringe reception so I have a few basic questions ..
Just how critcical is skew adjustment on a pf dish ..?
Firstly ..since 90% of cband birds receivable in the uk are Right or Left polarized ..
Ia skew a factor ?
Am I right in assuming that the positioning of the dielectric plate in the lnbf throat is all that matters ...
In other words ..assuming the plate is correctly positioned at 45 degrees to the 2 aerials/lnbs within the throat that no amount of reasonasble error introduced by rotating the whole lnbf/plate assembly a few degrees will make any difference to reception signal strength or quality ?
Assuming this is true...
This would not effect any skew compenstation intoduced by rotating the lnbf minus the plate of course ...for Horizontal/Vertical transponders on those extreme birds such as 55W and 58 West.
So ..my question remains ...does a prime focus dish automatically position the lnbf/skew at the extremes of dish travel in the same way an offset dish does ?
Is orientation compensated for during prime focus dish travel throughout its passage of the arc ?
What I'm wondering is whether it would be advantageous to set the skew (if neccessary) of the lnbf at the extreme west of my dish position ...and that this tweak of maybe a few degrees would have little or no effect on L/R polarised transmissions from the other 80% of birds ..when the dielectric plate is inserted for exampl at 0.8West ..
I hope this all makes sense ..
I haven't tried any of this yet ..as I'm not sure if skew is such an important facror ...and don't want to mess up my system unneccessarily..
Any input most welcome.
rgds all
VS

 

[futumsch]

Hi Vipersan,
I have some experience with this quite regularly. Circular polarized LNBs do not need rotating for circular polarised signals. Rotating a whole circular polarised LNB body has no affect on signal strength on a circular polarised signal.

If your 1.8m dish is on a polar mount, you just need to set the LNB vertical at your due south, and the linear polarised signals of the rest of the linear polarised birds (bar the Astra and Hot Bird satellites) will match your LNB's skew. If your 1.8m was on a Az/El mount, then the skew for a linear polarised LNB would need to be adjusted as you move your dish across the satellite arc, otherwise the LNB would still be in the vertical plane.

I'm personaly not familiar with fitting a dielectric plate in LNB's for circular polarised reception (though I've read about it), but I would assume the dielectric plate doesn't care about the incoming signal, it needs to be set in relation to the tiny little antennas sticking up in the back of the throat of the lnb to seperate the left/right polarised signals.

So once you have set your LNB skew for linear satellites, your assumption is right that the positioning of the dielectric plate in the LNB throat is all that matters.

Just for completeness, the polarisation response of a linear LNB is slightly flat within a few degrees of it's optimal skew, and then drops off sharply to a 30dB+ null as it is rotated though 90degrees. This does not happen with a circular polarised LNB, your relying on the dielectric plate to give you the best cross polarisation isolation between LH and RH polarised signals.

Hope I'm not teaching your granny to suck eggs, but its an interesting thread.

 

[Vipersan]

Thanks for the reply Futumsh...
You have confirmed what I already believed ....
and yes ..my 1.8m baby is equipped with a ring polar mount ..
As shown in the assembly manual part 7 of this thread ...which I think you have already seen ;)
_http://www.satellites.co.uk/forums/picture-members-setups-general-satellite-installations/184816-unknown-dish-re-assembly-anyone-recognise.html

So since the lnbf was indeed set absolutely upright/zero @true south ...
I'm assuming thee would be nothing to gain by any skew setting at 58W ( my absolute extreme limit)
and in light of the fact there is a more or less flat response for a few skew degrees with a Cband lnb ..Then I couldn't hope to tweak a couple more dbs from the tricky old birds at 55 and 58W..
Looks like the only way I can grab anything extra now is in increase in dish size ..as mine is now practically 'optimised'..
cest la vie..
I'f only I could find a set of those elusive 'add on petal extensions' for my 'baby'...
thanks again
rgds
VS
;)

 

[nelson_b]

hi -
this year -- until recently i have been using a chaparral co rotor for C band on the 1.8 offset motorised dish
and i found that the skew control sometimes needed to be altered a little between different satellites when viewing circular transponders to get the best signal strength

don,t know if the same would apply on a prime focus though --

as i,m sure you know the dielectric plate is "fixed " in the co rotor - and the skew is altered by mechanical movement of the probe in relation to the plate

cheers nelson.b

 

[Vipersan]

Indeed Nelson ..
I do acually have an old Chaparral corotor ...
I robbed the scalar ring horn assembly as you have seen ..
Sadly I no longer have a receiver connected to my very complex system which is capable of driving the corotor motor ...so have permanently moved to the H/V switching type LNBF ..
I may try and manually rotate the lnbf to see if I can get anything else at these extremes ..
but will require some half decent weather as I'll have to get a receiver outside near the dish..
cheers m8
VS

 

[futumsch]

Hi Vipersan,
I'm going to go a bit hypothetical now and contradict myself. I've had my head in JD Kraus 'Antennas' which reminded me of my good ole days of a ground station calibration engineer spending many cold days up on a hillside with a linear dish pointed at a transmitting circular dish to measure 'axial ratio'. What it boils down to, is that you might have a perfect parabolic dish, but a cross polarised signal (the opposite pol to the one you want) can interfere with the co-polarised signal (the one you want) and the polarisation appears elliptical. (and lot of mathematics with squiggly lines in that would take me a month to understand).
I was calibrating 8-10 m dishes but the principle is the same, so I wouldn't discount tweaking your circular LNB for max signal. Its probably might make a gnats chuff of difference but squeezing every last ounce of signal at the LNB is going to mean better C/No through the rest of your system.
Do I make sense? Its late.. I'll get me coat.

 

[nelson_b]

Indeed Nelson ..
I do acually have an old Chaparral corotor ...
I robbed the scalar ring horn assembly as you have seen ..
Sadly I no longer have a receiver connected to my very complex system which is capable of driving the corotor motor ...so have permanently moved to the H/V switching type LNBF ..
I may try and manually rotate the lnbf to see if I can get anything else at these extremes ..
but will require some half decent weather as I'll have to get a receiver outside near the dish..
cheers m8
VS

hi --
just to report some findings regarding skew of the dielectric plate on circular transponders ,i,v been testing out the C band performance on some sats using a (small dish) 1.2 channel master on a fixed mount -- with pauxis l.n.b.and conical feed

i was looking at 40.5 west tonight and found that i could make quite large increases in signal on this sat by setting the "doctorofsat" dielectric plate slightly off centre from its normal position within the grooves -- as well as sitting it high up in the l.n.b throat -- ( a normal dielctric plate barely gave any signal)
on tv brazil- and on other transponders i almost doubled the quality reading by doing this --
and on cristovision it made the difference from no signal at all - to getting a watchable picture with no breakup --
this kind of skewing of the plate did not apply on 1 west when i set it up last week -- although only 1 transponder could be received from that sat with this dish

i must add that skewing the whole l.n.b. seems to make no difference to the signal of circular transponders on this fixed dish setup

i will be trying other sats for C band with the 1.2 as/when i get the time

cheers nelson.b.

 

[Vipersan]

Hi Nelson..
Nice results m8 ..
Basically a reversed corrotor ..
The plate rotating relative to the probe rather than the probe shifting relative to a fixed plate..
I await further results with interest ..whilst I resume efforts on the H/V birds at 55 and 58 W
btw ..I briefly pulled IMC Jamaica ...but i couldn't get it stable enough for pics ..
...and it _was _ a particularly clear day ..
Oh ...for a bit more surface area on my dish..
;)
rgds
VS

 

[wium]

Normally the LNB has a 0 or arrow at the back of the LNB throat and i set that mark to true NORTH/SOUTH where i am.There must be a reason for that marking.My LNB has a grove in the throat in which the Teflon plate fits and i always keep it in.I am aware that some LNB,s dont have a grove and i am not sure how one decide, where actually the plate goes.(perhaps in line with the 0/arrow)

 

[Channel Hopper]

In most feeds the dielectric plate is installed at 45 degrees from the rectangular transfer flange to the LNB. A combination of scalar feed optimising and plate design should have more of an effect than resetting the alignment of the plate, but in practice there is always something that gives an improvement.