Polarization of Radio Telescopes Larry D'Addario 2008 Mar 14

Polarization "Purity" Every antenna produces perfectly "pure" polarization. If we deliver a sinusoidal signal to an antenna's terminals, the polarization that it radiates in a given direction is well defined. It can be represented by a point on the Poincare Sphere. By reciprocity, in the receiving direction it is sensitive to that same polarization and is completely insensitive to the orthogonal polarization (the opposite point on the Poincare sphere, across a diameter). We can build two co-located antennas with different polarizations (e.g., a reflector with a dual-polarization feed), and we usually do.

Polarization Specifications To measure unknown polarizations, dual-polarization antennas are needed. The actual polarizations of the antenna are generally unimportant, but it is best if the two are close to orthogonal. The actual polarization at a given frequency and direction can be determined by calibration. A difficulty is that it may not be the same when the frequency or direction is varied. Therefore important requirements are –Variation of polarization with direction across the usable portion of the beam, perhaps out to the first null –Variation of polarization with frequency across the full bandwidth of the antenna

Polarization Matching and Cross Polarization In communications (but not in radio astronomy), the polarization of a receiving antenna must be matched to that of the transmitting antenna on the other end of the link. –If not, SNR is reduced –And non-zero sensitivity to the orthogonal polarization may lead to interference from another user (polarization diversity) This leads to the concept of "cross polarization" as a specification, often quoted and measured by antenna manufacturers. It's important in this context, but not in radio astronomy. "Cross polarization" is conventionally measured with respect to a nominal polarization that is purely linear. This is an arbitrary choice, and results should be interpreted accordingly.