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Assembling Feed C-band Circular Polarisation

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Ex Member
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Mar 23rd, 2009 at 10:12am  
Hi All,

I am having problems assembling my feed on C-band circular polarisation for Tx-RHCP and Rx-LHCP.

I have the following:

TXFCC-240CSCN      2.4M C-Band  Antenna      Patriot
NJT5669      5W BUC      NJR
NJS8476S      PLL LNB      NJR

Unfortunately there are no markings (RHCP or LHCP) on the hardware.

Any ideas or documentation will be very helpful.

Thanks.

Ime

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« Last Edit: Mar 23rd, 2009 at 2:41pm by Admin1 »  
 
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Eric Johnston
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Reply #1 - Mar 23rd, 2009 at 11:25am  
For circular polarisation there are two ways the feed  can be assembled.  One way for RHCP receive and the opposite way for LHCP receive.

What matters is the joint between the OMT and the polariser.   The OMT (ortho mode transducer) is a fancy name for the joint where the transmit and receive waveguides join and merge into a square or round tube that goes towards the feed horn.  The polariser is a long square tube, with two lines of slots/cavities along opposite sides.

The lines of slots/cavities need to be at +/-45 deg to the waveguides of the OMT.  There are two ways, so you will have to find which is right for you.   One way it will work perfectly, the other not at all.  You have 50% chance of being right first time, unless someone comes here and sends a picture of how they have it set and says what way it does. I've sent a message to Patriot asking for instructions.

Note that rotating the feed assembly as whole will have no effect on the choice of polarisation.  

Best regards, Eric.
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« Last Edit: Mar 23rd, 2009 at 2:39pm by Eric Johnston »  
 
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Eric Johnston
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Reply #2 - Mar 23rd, 2009 at 1:22pm  
Just received an explanation by phone and a diagram from Patriot.  I have amended the text below to match the diagram:

Your objective is that your antenna does: Tx-RHCP and Rx-LHCP.

Note you have single reflector, so the feed itself must do the opposite, i.e. feed transmits LHCP

1.  Have the OMT near to you and the polariser and horn away from you. This makes sure your notion of anticlockwise and clockwise are the same as mine. Think of the feed assembly as transmitting directly away from you.  This matches the picture I've been sent.

2.  Polariser:
Rotate initially so that the seam is at the top, then turn it 45 deg anti-clockwise.  The seam will now be at the 10:30 clock position, top left, and the smooth cover with the work "PRODELIN" will be at 2:30 clock position, top right.

3. OMT:
For feed LHCP transmit turn the OMT so the LNB is directly sideways, on the right hand side.

The feed now transmits LHCP which, when it hits the reflector becomes antenna RHCP towards the satellite.

Once you have your feed working please send me a picture to eric@satsig.net, so we know for certain which way it should be.

Best regards, Eric.
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« Last Edit: Mar 23rd, 2009 at 2:56pm by Admin1 »  
 
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Ex Member
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Reply #3 - Mar 23rd, 2009 at 4:39pm  
Thank you Eric. I will get back to you.
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Eric Johnston
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Reply #4 - Jun 17th, 2009 at 4:27pm  
Please send picture of your feed so that I can verify that all above is good advice and that this page Circular polarisation set up is correct.

Pictures from anyone else of your circular polarisation C band feed plus a statement of which way your whole antenna is transmitting would be appreciated.

Best regards, Eric.
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