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Difference in cross-pol and co-pol feeds

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Oct 26th, 2005 at 3:50pm  
Having trouble in Iraq when we switch satellites from one bird to a new bird.We are on crosspol on one and switch to copol on the other.On one the down stream SNR's are lower that the other plus VOIP quality has dropped to unuseable.On the other bird the downstream SNR's are higher and the VOIP is useable?Could someone tell what the difference is?
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« Last Edit: Oct 7th, 2006 at 7:30pm by Admin1 »  
 
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Eric Johnston
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Reply #1 - Oct 26th, 2005 at 8:15pm  
The downlink power and uplink sensitivity of each of the two satellites may be different.  One satellite may need a larger dish to achieve the same service.

The reason you have to change from one polarisation to the other polarisation is just that the two services operate on different polarisations.   That is not unusual.  Even on one satellite there will often be services operating simultaneously on the different polarisations.  In such cases, you get interference from the opposite polarisation if your polarisation is not properly aligned.

You repoint the azimuth and elevation for the new satellite but remember you need to adjust and optimise the polarisation rotation angle as well.  Simply turning  the feed by 90 deg will not be accurate enough.   You need to set the polarisation rotation into a very sharp narrow null, to an accuracy of about +/-2 deg.   If you don't have the help of the hub (on the phone) making cross-polar null measurements while you transmit a clean carrier and very slowly turn the feed try this: Turn the feed up to 45 deg wrong each way and exactly find and mark two exactly equal level very degraded qualities.  Then halve the distance by wrapping a bit of paper round and marking the two marks and the exact centre null position.

Best regards, Eric.
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« Last Edit: Oct 27th, 2005 at 10:25am by Eric Johnston »  
 
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Muhammad Kumail Asadi
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Reply #2 - Dec 5th, 2005 at 5:54am  
sir kindly send me detail information
about feed , BUC and LNB
i wil b greatful for this help and favour
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Reply #3 - Oct 7th, 2006 at 4:15pm  
Can a cross pol linear feed be converted to co pol? If so how,if not why?
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Eric Johnston
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Reply #4 - Oct 7th, 2006 at 6:43pm  
Here is what each type of feed looks like and how they work:
...
This is the common cross-pol, dual port, variety.  Immediately after the feed horn and circular waveguide is an otho mode transducer OMT which separates the two orthogonal linear polarisations which are at right angles to each other.  In this case there is a flat vane across inside the circular waveguide to prevent the horizontal signals going to the right so they are forced upwards at the junction.  Horizontal signals are in upper waveguide and vertical polarisation signals in the waveguide to the right.   At this point there are two waveguide ports, which may be connected to LNBs or BUC transmitters.   In the case illustrated a receive  LNB is connected to one port (at the top) and a BUC to the other port (on the right).   To protect the LNB a transmit reject / receive bandpass filter has been inserted before the LNB.  

...
This co-pol feed is for one polarisation only.  At the back of the circular horn and circular waveguide only the horizontal polarisation signals are allowed to pass.  These are applied to two different filters.   One filter is receive bandpass and transmit bandstop. like the normal transmit reject filter, the other filter is transmit bandpass and receive bandstop.  This is connected to the BUC. Read the markings near the waveguide port flanges determine which is which.

A 4 port feed, as often used in large VSAT hub antennas,  comprises the two port OMT of the cross pol type plus a pair of co-pol dual filter assemblies. (4 filters in total).  This provides four ports, so you may transmit and receive on both polarisations using 2 LNBs and 2 BUCs.  If you do this, make sure the OMT used is of a broadband type intended to pass the full 3.7-6.4 GHz or 10.7-14.5 GHz via both horizontal and vertical polarisation ports.

So if you have a cross-pol feed (with a broadband OMT) you could, in theory, convert it to a co-pol feed by attaching a dual filter assembly from a co-pol feed and a dummy load (waveguide termination) to the unused polarisation port.   If you are talking about VSAT operations I recommend you simply buy the correct complete feed type.  If you are talking about a large hub dish consult the dish manufacturer about new parts.    

See here for larger images http://www.satsig.net/co-pol-xpol-feed-assemblies.htm
wxw
Best regards, Eric.


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« Last Edit: Oct 8th, 2006 at 12:00pm by Eric Johnston »  
 
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Reply #5 - Oct 9th, 2006 at 9:46am  
Thanks for the great illustration. Is co pol sometimes refered to as parallel pol?
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Reply #6 - Jul 24th, 2008 at 12:17pm  
Just noticed this post and although its way old thought I might be able to help somebody.
Basically I found out that a company called Invacom in the UK make a CO Pol OMT. It appears that this is a very special beast  and certainly isnt cheap. Comes with an adapter.

You can contact them at sales@invacom.com
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Reply #7 - Sep 11th, 2009 at 6:12am  
Hi,

I am using iDirect service using IS 12. Since IS 12 is co-pol satellite we use co-pol antennas to above service.

Now we need to migrate service from IS 12. We got solutions from 03 satellites. Let me know that they are co-pol or cross-pol.
1. NSS 6
2. ABS1
3.Intelsat 906.

Do I need to change antenna and feeder if go through them.

Pls. advice.

Regards,

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Eric Johnston
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Reply #8 - Sep 11th, 2009 at 10:37am  
Co-pol operation is unusual and needs special VSAT antenna hardware.

If you want to move your service to a different satellite ask the new satellite operator what capacity is available and what polarisation combinations are available.

If you operate exclusively in your own coverage area then you have one uplink beam and one downlink beam.

If you operate to a distant teleport then it is likely that you have 4 beams, two uplink beams and two downlink beams, cross connected.

Ideally you would want co-pol capacity (in your VSAT coverage area) on the new satellite.   If this is not available then you will need to change the diplexer filter assembly on your existing co-pol antennas to an ortho-mode transducer (OMT) + transmit reject filter(TRF).

Check dish size, BUC power and LNB LO frequency for compatibility with the new satellite.

See pictures of Ku band co-pol and cross-pol feeds.

Large teleport hub antennas often have 4-port feeds and can operate either co-pol or x-pol, or both simultaneously.

Best regards, Eric.
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Reply #9 - Sep 15th, 2009 at 10:42am  
Hi,

I have small doubt about co-pol and cross-pol.

I did some testing with Intelsat 906 and my HUB operator provide me details that Intelsat 906 is a co-pol satellite. So I used co-pol feeder and downlink SNR is very low. So I have turned the feeder assembly left and right, but got very poor values. Even HUB cannot see my tx carrier. Later I have changed it to cross-pol feeder and got good downlink SNR. And my router came online in the network.

So I'm asking that the reason for above. Can work with co-pol satellite by using cross-pol feeder.

Pls. advice.

Regards,

Was

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Eric Johnston
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Reply #10 - Sep 15th, 2009 at 7:46pm  
Regarding IS 906 and co-pol operation.

Please say if this is C band or Ku band ?

What name polarisation have you been told to receive:  
Vertical, Horizontal, Left Hand circular  or Right Hand circular ?

How many reflectors do you have on the antenna ? One or two ?

Where are you ?

You ask: Can I work with co-pol satellite by using cross-pol feeder.

If you have co-pol system when you need cross-pol system you will be able to receive a perfect signal but will then transmit on the wrong polarisation - so not acceptable.

These images below may help. Imagine the feed systems are lying flat on a table and you are looking directly downwards at them. Upwards, vertical, is directly towards you from the centre of the image.

Note that there is a feed horn on the left, then a polariser tube, then a diplexer joint or OMT, then the LNB and BUC.
If the polariser is removed, or oriented so the line of pins are uppermost, the feed becomes a linear polarisation feed.

...
The first image above shows the LNB and the BUC with their rectangular waveguides the same way round, both with the broad faces uppermost.  This is a co-pol arrangement.  Both receive and transmit are vertical linear polarisation when they merge into the diplexer.

When the polariser is attached, the vertical polarisation will change to a circular polarisation at the feed horn.

If you are told to use co-pol operation try the above arrangement.  If you can receive the outlink carrier you have everything correct. You will transmit and receive on the same circular polarisation.   If you cannot receive the outlink then rotate the polariser 90 deg to its other position.  The polariser goes to +45 deg or -45 deg either way from the line of pins in centre uppermost position.  Now you will see the outlink on the other circular polarisation and transmit on the same.

...
The second image, above, shows the LNB and the BUC with their rectangular waveguides at right angles, so the LNB is vertical and the BUC horizontal.  This is a cross-pol arrangement, the same as most Ku band linear systems.  The receive is vertical and the transmit horizontal polarisation when they merge in the OMT.

When the polariser is attached, the vertical and horizontal polarisations will change to two opposite circular polarisations at the feed horn.

Best regards, Eric.
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