Phase jumps in a QHQ phase shifter — some consequences

Abstract

A phase shifter consisting of a rotatable halfwave plate placed between two identical quarterwave plates, when used at wavelengths far away from the intended wavelength, can act as an equally good phase shifter with phase shifts of the opposite sign. Such behaviour is the opposite of achromaticity, usually associated with such devices, and has its origin in “SU(2) phase jumps”. A radio antenna array equipped with such phase-shifters can “look” in different directions at different wavelengths at the same time.