The cband receiver has always been a native
linear receiver. back in jun05 ganesh built a polarization switch
box that would change from linear to circular using a hybrid.
We are now discussing putting a hybrid in the new 4-8 ghz receiver to allow the vlbi users to generate the circular polarization that they need. NRAO uses a hybrid in the dewar before the lna. Another option is to put a hybrid after the dewar.
Rumor has it that ganesh's attempt to put a hybrid after the dewar (back in 2005) did not work very well. I'm now trying to resurrect those measurements.
Documents found on maestro: