LNB for circular polarized signal

If you are trying to receive a circular polarisation you need to change it to a linear polarisation before inputting the signal to the LNB.

To do this you need a polariser. This is a short or long, straight waveguide tube, typically with a square or circular cross section with some delay structure along the inside, oriented at 45 deg. The structure is typically two bands of slots or screws along opposite sides or a dielectric flat plate or vane set at 45 deg.

At one end of the polariser you have right hand circular polarisation(RHCP) and left hand circular polarisation (LHCP); at the other end you have vertical and horizontal linear polarisation. Whether RHCP converts to horizontal or vertical depends on the orientation of the 45 deg polariser. At the linear polarisation end you need a normal OMT to separate the two linear polarisations into separate rectangular waveguides.

This page may help explain: https://www.satsig.net/pointing/circular-polarisation-set-up.htm

Note that there is so much confusion between RHCP and LHCP that is quite normal to have to try both until you get it right - on 50% of occasions ! If your antenna has one reflector the polarisation changes during the reflection, so your feed needs to be the opposite to what comes from the satellite.

Circular polarisation is essential at low frequencies, like L band and C band, particularly in the tropics as linear polarisation gets rotated by varying amounts in the ionosphere. Ku band works fine with linear polarisation, the feed system is simple and cheap but the installer needs significant skill and patience to set the polarisation angle accurately to avoid interference to other services. At Ka band circular polarisation is being adopted, despite the increased cost of the polariser, as it de-skills the installers job, which I hope will help encourage more people to use systems like Wildblue, Hylas and KA-SAT. Installers either get it right or it does not work at all and there is less scope for them causing interference.
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Best regards, Eric.

Thanks sir eric, another question we have a 9m dish and we are using it to receive an RHCP signal from Intelsat and I did'nt see a polariser in it. The LNB is connected at the side of its feed. How is it possible?

Sir greg we are receiving C-Band video receive only on Intelsat 701.

Thanks,

ken

Sir is there an LNB that can receive a circular signal even it was on a linear feed?

Sir,

Where can I buy a polariser? Do I have to be specific on C- and Ku- Band before purchasing?

Thanks,

ken

"LNBF" refers to an LNB with Feed horn attached.  If it is specified to work with circular polaristion there will be a polariser device internally.

Find out how to change from LHCP to RHCP and check your indoor receiver is compatible.

Best regards, Eric.

The feed part of the LNBF needs to be suitable for the F/D ratio of the dish.
For example:

The feed on the left has a very wide radiation pattern to suit a circular, axial-symmetric, dish with the LNBF in the middle on equal length support legs and relatively close to the dish center. The feed on the right (conical horn) has a narrow beam and is suitable for an offset design with relatively long focal length to diameter ratio.

Best regards, Eric.

By "fiber glass", I presume that you are referring to a dielectric polariser plate, oriented at 45 deg.

What frequency and symbol rate does your receiver show for the carrier that you are locked to ?

What signal level, signal quality or bit error rate is displayed ?

Best regards, Eric.