SatSig topic:

Circular to linear polarisation

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May 31^st, 2011 at 10:38pm

Hello,

We have 11m C-band antenna and feed assembly which I am going to perform x-pol testing with AMC-7 for xmit and receive. I just informed that AMC-7 requires linear polarisation.

There is not any data sheet or installation manual available for antenna and feed assembly.

Looking at https://imageshack.us/photo/my-images/821/casi1p.jpg/
https://imageshack.us/photo/my-images/856/casi2.jpg/

can one determine that, Is the feed circular or linear pol.?

or if it is circular pol., then how to convert it to linear?
https://imageshack.us/photo/my-images/853/ant1l.jpg/
https://imageshack.us/photo/my-images/638/ant2c.jpg/

Please advice.

Thanks:
AEE

Jun 1^st, 2011 at 7:53am

The centre hub picture shows 2 receive waveguides with amplifiers attached, also 2 transmit waveguides with black tape covering the ends.

Look closely for any lables on the waveguides or amplifiers and indoors at the ends of the cables. Does it say RHCP/LHCP anywhere ?

I would point the dish at AMC-7 (linear C band satellite) and examine the spectrums at both receive waveguides. Note your actual polarisation adjustment angle and consider this relative to the calculated polarisation adjustment angle for AMC-7. Make sure the difference is not near 45 deg and adjust the polarisation if necessary.

If the signals are the same regardless of rotation of the feed system then you have a circular polarisation system.

If the signals are different then you have a linear system. Confirm this by turning the feed till you null out the cross-pol. "Turning the feed" looks extremely difficult in the picture so my guess is that your system is circular and the feed rotation intended to remain fixed. I can't tell which way is exactly up in the picture. A photo ftom exactly behind, with the camera level would help. If the system is linear there should be some means for readily turning the feed and adjusting the polarisation angle.

To see by inspection if the system is circular or linear you need to look behind the panel to find out what is there. That looks like being quite a big job. There may be an access panel round the front of the dish, sideways into the feed support tube.

Are you intending to use the antenna as receive only ? At one specific frequency ? Do you have LNAs or PLL LNBs with external 10 MHz reference ? If so, you might contemplate making your own external C band or L band combiner to get two circular polarisations back to linear. Not sure if it would work...

Best regards, Eric.

Jun 1^st, 2011 at 10:22pm

Hello Eric,

1. I have uploaded new pics.
2. I am intending to use Antenna both for Transmit and receive.
3. I have CA 140103 LNB, Norsat 3220 C-band PLL LNBs, Norsat 8520 C-band Digital DRO LNB(On dish). NJRC NJT5669F 5W C-Band BUC(on dish).
4. Found nothing on panel. Besides it's make & Model. "SA 8231 11m feed". I'll look behind panel using bucket truck.
5. Dish is pointed to AMC-7. and I have lock on CH. but not on CV.
6. "Turning the feed". I Can rotate feed from behind.
7. Will do some more testing and update Post.

Thanks much:
AEE

Jun 1^st, 2011 at 11:38pm

Assuming it is linear polariation and the red line points directly upwards.

You are currently receiving horizontal polarisation, tilted 15 deg clockwise. e.g. From Nunavut perhaps ?
or
You are receiving vertical polarisation, tilted 75 deg anticlockwise. From where ?

Where are you ?

Best regards, Eric.