Five-Level Waveguide Correlation Unit for Astrophysical Polarimetric Measurements

Recent astrophysical measurements require an accurate evaluation of the correlation product between very weakly correlated signals. Waveguide correlation units represent a very appealing solution for these applications because of their stability and low-loss features. A new waveguide correlation unit architecture is discussed in this paper. It extends over five levels in the E-plane and integrates hybrid couplers, filters, a phase shifter, matched loads and various matched transitions. All the aspects concerning the analysis, design, and manufacturing procedures are described, focusing on the reduction of the spurious correlation terms (systematic errors). An aluminum Ka-band prototype has been designed and experimentally tested. The device exhibits spurious correlation parameters of the order of 30 dB, in a 10% frequency band, in good agreement with the predicted data. Thanks to this feature, the presented architecture is suitable for high-performance correlation polarimeters, where very high sensitivity to the relevant signal, as well as very high rejection of both the unpolarized component of the radiation under test and the instrument noise, are required.