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Polorization Setting

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Nov 9th, 2007 at 8:56pm  
Happy Day
Can any one help me more (in depth with real photo please) about how i set polorization of LNB. Thank you very much for your helping
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Reply #1 - Nov 9th, 2007 at 9:22pm  
No simple response can be provided unless/until we have some kind of idea of what kind of equipment you're attempting to work with.

//greg//
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Eric Johnston
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Reply #2 - Nov 10th, 2007 at 10:33am  
Wesam,

I received your direct email.   The image at http://www.satsig.net/pointing/polarisation-horizontal-adjustment-minus-46-deg.j... was incorrect.

I have amended it like so:

...

I hope this one makes sense.

The polarisation starting position, in this example, is nominal Horizontal receive polarisation, with the LNB at the top.  The LNB has its broad waveguide faces on either side and the pin inside the LNB is horizontal.

Then, while facing towards the satellite the feed is rotated 45 deg anticlockwise (negative).

If you are north of the equator, the satellite is towards the south east.

Best regards, Eric.
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« Last Edit: Nov 10th, 2007 at 12:35pm by Eric Johnston »  
 
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Reply #3 - Nov 12th, 2007 at 6:22pm  
dear Mr. Eric ;
thanks to all the help you provide for any one here in the forum and GOD belss you .
this is a very important question , Polerization setting or position .
i mostly work with internet system on different sats , as i informed you by direct email that i was using Promax satfinder and now get a satlook digital NIT for hunting satellites for the internet .
so i am now trying to understand the case from another site of view .
when i need to adjust antenna toward a sat , i ask the provider what is the LNB position ? . this is the first question to ask while we faced many problems
with the adjustment processes and the line-up with the NOC .
here i would like to ask you to explaine this whole process with the  theory and concept of the Polerization
i will ask you some simple questions .

1.how can i know the location of the LNB refering to Horizontal and the location refering to vertical ? , as you wrote in this article that the pin inside
the LNB determine this .is it correct ? if so which one is correct if the LNB position was on 3 o'clock , the pin should be upward or downward ? does it make
difference ? .

2. where is the zero Angle ? some refer it to 12 o'clock and some to 3 o'clock .

3.i am using a program named SMW link , which you provide it your GPS and the sat dgree and it calculate the lnb dgree for you , this not fit all internet
providers because as i see each one use a different angle .also as i am trying it right now for all satellits from 22 dgree west - 110 dgree east the LNB
angle varied from (54.5 deg) - (-54.6 deg) respectevily while i see many times LNb deg out this range used by ISP .

4. is the LNB position related with the sat ? , frequency ? transponder ?  or what ? .
i think no and that make the case complex to me because on the the same sat and with the same service , if i use NJR LNB i should put it in a dgree and when using Invacom LNB i will put it normal to that direction ??? .

5. let's assume that you will say that putting the LNB at 3 o'clock refere to vertical , what is the difference between putting it on 3 o'clock or putting it on 9 o'clock?

6. Patriot Dish and Visiosat dish have types come with small reflector , in the case of dish with small refelctor how i could count the LNB dgree ? .

some times when i am training on the satlook , and let say i am on sat with 60 dgree east , when i change the LNB to another direction the device read like
19.2 dgree east ????

i will wait your reply eager so i could ask anohter questions which it will help to lead all to  know the process of choosing the correct angle for the LNB .
also i will send this by e-mail to you directly with two photos which i couldn't put them here
with regards
Ahmed
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Eric Johnston
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Reply #4 - Nov 12th, 2007 at 7:17pm  
1 How can I know the location of the LNB referring to Horizontal and the location referring to vertical ?, as you wrote in this article that the pin inside  the LNB determine this. Is it correct ? if so which one is correct if the LNB position was on 3 o'clock , the pin should be upward or downward ? Does it make  difference ?.

Ans: The pin inside the LNB defines the receive polarisation direction.
...
If the pin is vertical then the polarisation is vertical.  In the case of vertical polarisation, it does not matter if the pin goes up from the bottom or down from the top.  The broad faces of the rectangular waveguide face in the polarisation direction.

2. where is the zero Angle ? some refer it to 12 o'clock and some to 3 o'clock.

Ans: The zero angle is your chosen starting position, vertical or horizontal.  Ignore any numbers on the scales.  Just use the top or side marks, typically marked 0 or 90 or -90.

3. I am using a program named SMW link, which you provide it your GPS and the sat degree and it calculate the lnb degree for you, this not fit all internet  providers because as I see each one use a different angle. Also as I am trying it right now for all satellites from 22 degree west to 110 degree east the LNB  angle varied from (54.5 deg) to (-54.6 deg) respectively while I see many times LNB deg out this range used by ISP.

Ans: I guess you are in the northern hemisphere.  If so, a polarisation angle 54.5 for south west satellite makes sensa, as does -54.6 deg for satellites to the south east.  

If you are in the northern hemisphere a satellite which is due south of you has zero polarisation adjustment angle.  If the satellite is to the south east then the polarisation adjustment angle is negative, anticlockwise, facing towards the satellite.  If the satellite is to the south west then the polarisation adjustment angle is positive clockwise.

If you are in the southern hemisphere.  Satellite due north of you has zero polarisation adjustment angle.  If it is north west then the polarisation adjustment angle is negative, anticlockwise.  If it is north east then the polarisation adjustment angle is positive clockwise.


4. is the LNB position related with the sat ? , frequency ? transponder ?  or what ? .
i think no and that make the case complex to me because on the the same sat and with the same service , if i use NJR LNB i should put it in a degree and when using Invacom LNB i will put it normal to that direction  .

Ans: Most satellites have their planes of polarisation parallel to the earth's axis and the equator.  Exceptions I know of are Eutelsat (+3.5 deg clockwise) and Astra (+7 deg clockwise).  Satellites have transponders on both polarisations.  That is why 1 deg accuracy is important so you don't mess up operations on the other polarisation or get interfered with from the other polarisation.  Discrete VSAT LNBs with a rectangular input waveguide are single polarisation devices and you have to turn it physically to make it the opposite.   (Satellite TV LNBs normally have dual polariation capability, switched internally.  The very latest VSAT Ku band TRIAs will have single polarisation transmit and single, but switchable alternative, polarisation on receive.)

5. let's assume that you will say that putting the LNB at 3 o'clock gives vertical polarisation, what is the difference between putting it on 3 o'clock or putting it on 9 o'clock?

Ans:  Same.  Both are alternative starting positions for vertical polarisation.  Use whichever does not involve hitting metal when the adjustment angle is later applied.  Sometimes the adjustment may approach + /-  90 deg if you are near the equator !
 
6. Patriot Dish and Visiosat dish have antenna types that come with small sub-reflector,  in the case of dish with small sub-reflector how I could count the LNB degree ? .

Ans: It doesn't make any difference (to linear polarisation) there being a subreflector.  Always set up the starting position to vertical or horizontal, and then apply the adjustment (clockwise+ or anticlockwise-) while facing towards the satellite.  

If there is no polarisation scale or it is too small to read to 1 deg accuracy then put an inclinometer sideways across the LNB or BUC surface.  Alternatively try wrapping a paper strip round the throat and marking it for one circumferance.  Then unwind and measure total circumference length in mm.   Adjustment required = total length x adjustment angle / 360 mm.  Mark that, reapply the paper and adjust according to your paper mark.

...
The picture above shows all possible starting positions for horizontal and vertical, plus the effect of applying a -45 anticlockwise adjustment.  Note that the view is with you behind the dish facing forwards towards the satellite.

...
The picture above shows how the cross-pol isolation varies as you adjust the polarisation angle.  The unwanted signal is -30 dB down or 1/1000 th of the wanted one at the exact centre.

Best regards, Eric.
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« Last Edit: Nov 12th, 2007 at 10:43pm by Eric Johnston »  
 
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Reply #5 - Nov 12th, 2007 at 9:37pm  
Just to give you a sense of perspective Ahmed, that "pin" to which you refer is the actual antenna. There's one associated with the transmitter, another with the LNB. I know it's counter-intuitive, but the dish is really only a reflector - not an antenna.

If the transmit antenna (pin) is straight up and down, the signals coming off it radiate with vertical polarization. If the pin is sideways, the signals coming off it radiate with horizontal polarization. On the LNB, a straight up and down antenna (pin) receives vertically polarized signals. A sideways antenna receives horizontally polarized signals.

//greg//
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Reply #6 - Nov 13th, 2007 at 10:57pm  
Hello sir Eric Johnston
I would like to thank you very very much for your good helping and I wish you happiness and more knowledge, because by you I can did to install my internet dish, I do not forget you.

Wesam Bhaya Iraq
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Reply #7 - Nov 19th, 2007 at 10:51pm  
Dear Eric ;
thanks very much for your help and that will make me increase the work on the Dish to get the best signal .
now i am reading many articles in the forum and i am sure it will lead to new questions.
for now , would you mind to explain futher more about the x-pol and the isolation .
during line up process , the hub measure on the level of the transmit i am sending and they receive , is there a way to measure the transmit quality which i am sending to the HUB ? .
with best regards .
Ahmed .
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