Theory for use of polarisation controlElectromagnetic radiation is polarised. - Or is it not ? This issue is far more complicated than one might think at a first glance. When we receive "unpolarised" galactic noise, our antennas create polarisation because we use linear receivers, and they have full sensitivity for a wave with one particular polarisation - and zero sensitivity for a wave with the orthogonal polarisation. If you are interested in polarisation, and how it relates to detection of weak signals in noise, look here What is Polarisation ? Signals vs Noise For a rather long, but fairly non- mathematical discussion.
It is not very complicated to use polarisation control look here for an explanation how it works How to Select Polarisation for EME for aurora and sporadic E it is much easier Polarisation Control for Aurora gives some background, and my experiences so far.
Cross yagis are often used for circular polarisation only. This is nearly always a bad idea, look here why: Circular vs linear polarisation.
Simple Practical Polarisation ControlThe obvious and most simple solution to polarisation control is to use two antennas of orthogonal polarisation and switch between them by use of a relay. This is probably a perfect solution on 1296MHz for EME, where circular polarisation is used. On lower bands linear polarisation is used by most stations, and then switching between a pair of orthogonal, linearly polarised antennas will cause a 3dB loss now and then. Still simple switching systems are very useful. As compared to a single polarisation, they literally open a new dimension to the EME'er. There are different solutions, look here for details: Simple Polarisation Switching By the addition of a second pre amplifier and a second transmit receive relay, a lot more flexibility is possible. The transmit options are still limited, but new possibilities arise on the receive side just by adding simple, low level components. Separate Polarisation Switching for Tx and Rx
Advanced Polarisation ControlIn order to change the transmit polarisation in finer steps one can use a "switchless combiner". This is a device used by broadcast engineers, and it allows continuos switching over from one antenna to another. When it is used to continuously switch power from a horizontal to a vertical antenna, all possible linear polarisations are obtained. This method was described at the 3rd International 432MHz+ EME Conference 1994 by SM2BYA, Gudmund Wannberg. Look here for details: The Switchless Combiner
The switchless combiner may be used for transmit only, for receive only, or both transmit and receive. There are several possible configurations, and I do not think it is self evident which solution to select. Look here for some hints: Using The Switchless Combiner for Polarisation Control
To use the switchless combiner for polarisation control, a phase shifter is needed. For the transmitter, at high power levels, relays or mechanically variable length cables have to be used (as far as I know). For receive, phase shifters are easily made as electronically tuneable filters. Look here for details: An electronic Phase Shifter for the Receiver
Schottky diode ring mixers can be used as combiners to provide electronic polarisation control for the receiver. Here are practical details Polarisation Control for the Receiver With Diode Mixers
The ultimate performance on the receive side is obtained if a two channel receiver is used. Just using your head for the combination of the signals from two orthogonal antennas using the fact that you have two ears provides surprisingly good results. Even better is to use a computer to process the two signals in such a way that the ears receive a signal that maximises S/N by properly combining the two signals. For details, look at Adaptive Polarisation