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Service Providers >> Satellite Connection in Africa >> circular vs linear polarisation

Message started by fotkamalex on Nov 10th, 2007 at 1:09pm

Title: circular vs linear polarisation
Post by fotkamalex on Nov 10th, 2007 at 1:09pm
I have a C-band system I have been using on NSS7 and I will like to Use it on NSS10.

I realised that Nss7 is Circular Polarisation and NSS10 is dual linear polarisation.

What improvment is needed  for me to move from Nss7 to Nss10.


Title: Re: circular vs linear polarisation
Post by Eric Johnston on Nov 11th, 2007 at 12:18am
If you are operating now with circular polarisation you will have a polariser between the feed horn and the OMT.  The feed horn is the conical part nearest the dish.  The OMT (otho-mode transducer) is the joint where the circular or square waveguide to the horn transitions to two rectangular waveguides, normally one for transmit and one for receive.  The polariser is a long section of square or circular tube with two rows of pins or slots along opposite sides.

In the case of circular polarisation the polariser needs to be set so its lines of pins or slots are at 45 deg to the transmit and receive waveguides.  There are two alternative positions, at 90 deg to each other.  One gives left hand circular polarisation on receive and the other gives right hand circular polarisation on receive.

In the case of linear polarisation you can remove the polariser.  Then turn the feed for the correct polarisation starting position and then apply the adjustment angle, as for Ku band.  Alternatively you can achieve linear polarisation by keeping the polarariser and turning the OMT relative to the polariser so that the receive and transmit waveguides line up with the pins/slots.  The polariser effect, which is to slow down one of the planes becomes irrelevent.  The existence of the polarisaer has a minimal negative effect on signal quality.  At least you won't risk losing it while in storage.  You still need to set the polarisation starting position and adjustment, this time you need to rotate the entire assembly.

Best regards, Eric,

Title: Re: circular vs linear polarisation
Post by Caiphus on Dec 14th, 2007 at 7:51pm
Hi Guys,

Can you confirm about NSS 10...are you uing C Band? If it's C Band..are you not using RHCP or LHCP for Polirisation....I thought Liniear is mostly used on Ku Band.....can anyone clarify this?



Title: Re: circular vs linear polarisation
Post by Eric Johnston on Dec 14th, 2007 at 8:51pm
Dual linear polarisation for NSS10, see here:

Circular polarisation is normal for C band operation since it is immune to ionospheric Faraday rotation which occurs in equitorial/tropical regions.  

Dual linear polarisation at C band is OK away from the equator, outside the tropical region.  In the tropics you will suffer severe cross pol interference and reduced levels due to the entire signal being rotated left or right by many degrees in the ionosphere.  The problem becomes even worse at lower frequencies like L band (Inmarsat) and at UHF I believe it can go round in circles.

Higher frequencies, like Ku band, are not rotated by the ionosphere, so linear polarisation is fine.

Linear polarisation saves money at the earth stations since a polariser is not required, but much skill is needed to set the polarisation angle correctly.

Circular polarisation is expensive at C band since the polariser is large and complex.  The skill required is just to assemble it correctly - it will either work perfectly or not at all for your wanted signal.

At Ka band the polariser for circular polarisation is small and can readily be incorporated in the feed design.  If the two alternatives RHCP and LHCP can be selected without too much difficulty then it makes life easier for Wildblue and Tooway installers as they don't need to learn and implement the skill to accurately set a polarisation angle.

Best regards, Eric.

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