C band, and lower frequencies like L band
maritime, operates normally with circular polarisation. Lower frequencies are affected by ionospheric Faraday rotation, which is worse nearer the equator. The rule is to use circular in the tropics at C band. Linear C band may be used at higher latitudes, but will suffer cross-pol interference towards the tropics.
A polariser tube is inserted between the feed horn and the OMT or diplexer. The function of the polariser is to change linear polarisation (at the waveguides for rx and tx) to circular at the feed horn throat and vice versa. The polarising device (septum vane, rows of pins or slots) is at +/-45 deg to the waveguides. One way makes right hand circular polarisation RHPC, the other makes left hand circular polarisation LHCP. Don't ask me which way is which!.
See pictures here for explanation and set up of circular polarisation
. When circular polarisation reflects off a dish surface or sub-reflector it reverses. Ku band
is normally operated linear. For transmit/receive VSATs there are two feed alternatives, co-pol and cross-pol operation.
Cross-pol needs an ortho-mode transducer (OMT), which gives you two waveguides at right angles, one tx the other rx.
The other alternative is co-pol feed. This is single polarisation and you need a frequency diplexer (two filters) to seperate the tx and rx frequency band and give you two waveguides, tx and rx.
See here for co-pol and cross-pol, linear Ku band feed
in USA/Canada (Wildblue) uses circular cross-pol polarisation. Ka band in Europe (Tooway) uses Linear cross-pol polarisation.
Circular polarisation has the advantage that it can be assembled one way or the other and it then either works perfectly or not at all. No skill needed by installer, except to choose the correct hardware.
Linear has to be manually adjusted and the skill to this and the need for co-operation of the spectrum analyser at the hub to make the adjustment to +/-1 deg accuracy is a big problem and the cause of much cross-pol interference and poor operational service. Great skill required.
Best regards, Eric.