SatSig topic:

Feed Horn polarisation

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Mar 22^nd, 2010 at 6:55am

Dear Sir ,

By viewing the feed horn and the shapes how do we determine whether receiving signal in feed is horizontal or vertical . How do we determine the feed supporting horizontal or vertical transmission . Please explain us sir
Also in waveguide how can we determine whether that is passing horizontal or vertical signal
Thanks in advance
regards
techtest

Mar 22^nd, 2010 at 9:14am

Horizontal receive polarisation is when the dipole pin inside the LNB input waveguide is horizontal. It does not matter if the pin is on the left side or right side.

Horizontal receive polarisation is when the broad faces of the rectangular LNB waveguide are on either side.

Horizontal polarisation:

or

If the LNB is mounted on the end of a filter arm it is likely that the arm is now upwards or downwards, both giving Horizontal receive polarisation.

Having first set the nominal (named) polarisation, you now need to apply the polarisation adjustment angle or skew, applicable to the location and satellite orbit position. Use a dish pointing calculator like Dish pointing Iraq * and note the polarisation adjustment angle to be applied. Facing towards the satellite in the sky, a positive + adjustment is clockwise. In some locations and with certain satellite the polarisation adjustment angle may be large, approaching 90 deg. Once you are in communication with the hub they will test your transmit polarisation and may ask you to adjust slightly so that your interference to other services is minimised. The accuracy required is +/-1 deg feed rotation.

* This calculator also shows you an approximate picture (as viewed with you standing behind the dish and facing forwards towards the satellite in the sky) of how your LNB arm should be oriented.

Best regards, Eric.

Mar 22^nd, 2010 at 4:00pm

Dear Sir
Thank you very much for the information . Could u please explain me sir by seeing Ku band and C -band feed horns and waveguides how can we say that the transmitting is which polarization and receiving is which polarization .

Also in C- band by seeing the feed horn how can we say that is LHCP feed horn or RHCP feed horn .

Thank for very much sir .

Regards
Techtest

Mar 22^nd, 2010 at 7:37pm

For C band linear polarisation, the same applies as for Ku band.

For C band circular polarisation the situation is much more complex.

Provided the polariser pins/slots inside the polariser tube are at 45 deg to the linear polarisations at the OMT you will get circular polarisation - which way, clockwise or anticlockwise, is difficult to determine.

Try reading page How to set up circular polarisation. It may take a long time to understand the page and even then you and/or the hub may be misunderstanding what is actually required, so success is definitely not guaranteed. Expect 50% chance of getting it right first time.

Incidentally, never ever look into a feed horn. All contemplation of a feed system may be safely done from the rear.

Best regards, Eric.