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Set Polarization Prodelin 2.4m Circular Feed

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Mar 3rd, 2008 at 8:16pm  
Hi, I'm new to circular polarization.

I need help in adjusting the Polarization setting for Prodelin Circular Feed 2.4m Offset Antenna.

I'm using IS-906 West Hemi Beam (LHCP Uplink, RHCP Downlink) to link sites in Africa with Fuchsstadt Teleport.

Should I align the Polarization Mark on Cross Pol OMT with LHCP or RHCP Mark on the Circular Polarizer.

Some remarks in the Prodelin Feed Assembly manual that if the feed is LHCP the reflector will reverse it to RHCP and you will have a RHCP transmit system.

Is it because of the reference, e.g. if the wave is  Clockwise to me, it will be anti clockwise to someone viewing to opposite direction.

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« Last Edit: Mar 5th, 2008 at 2:20pm by Admin1 »  
 
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Eric Johnston
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Reply #1 - Mar 3rd, 2008 at 10:25pm  
See here for an image of a feed configured for C band circular polarisation in Intelsat hemi beam.

Please provide feedback here in due course saying if the information is correct or not.

At least with circular polarisation it either works perfectly or not at all.  There is no adjustment required. Just make sure the LNB is 45 deg one way or the other from the polarising device/elements/pins/slots.  If you get it in line it turns back into a linear polarised system!

Related forum discussion and images of co-pol and cross-pol C band circular polarisation.

Best regards, Eric.
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« Last Edit: Mar 5th, 2008 at 2:21pm by Admin1 »  
 
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Reply #2 - Mar 5th, 2008 at 6:13am  
The information provided is correct. I aligned the feed just as for  IS-904 Level 421 and it works i.e. LHCP transmit and RHCP receive. Thanks a lot for the information; those pictures were really helpful.

Can you explain the orientation of the signal's circular rotation (Left/ Right) with respect to Feed, Reflector, Satellite and then the reverse.

Thanks!
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Eric Johnston
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Reply #3 - Mar 5th, 2008 at 2:11pm  
The direction of circular polarisation is always defined as if you were transmitting, even if you are concerned with a receiver. Think of a simple helical wire antenna.

If a transmitter sends RHCP you need an identical RHCP  device at the receive end, assuming no reflections in between.  Think of two helixes directly screwing into one other.

If there is a single reflector in the path, it changes RHCP to LHCP and vice-versa.

When the label marked on the C band OMT lines up with the receive LNB this assumes the feed is being used to receive with a single main reflector, and not receiving either directly from a satellite or being used to receive in an antenna with both a sub-reflector and main reflector.

The RHCP and LHCP may be swapped over by putting the linear polarisation at the opposite 45 deg position on the other side of the line of the polariser pins/slots/vane.

This pfd file may help explain http://www.tscm.com/polariza.pdf

You say you now have a correct configuration.  Is there any chance you can send some pictures, including close ups of the labels ?  eric@satsig.net

Best regards, Eric.
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« Last Edit: Mar 5th, 2008 at 11:15pm by Eric Johnston »  
 
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Reply #4 - Mar 5th, 2008 at 7:20pm  
The pdf was indeed very helpful. It is unfortunate that I don't  have pictures; I may be able to send some from next installation.

Sincere regards.
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