Double Square Waveguide Directional Coupler for Polarimeter Calibration

A novel full-band square waveguide coupler design based on directional couplers which couple the TE10 and TE01 orthogonal modes in a square waveguide is presented. This waveguide coupler is aimed at the calibration of polarization receivers. This is composed of a pair of rectangular waveguide directional couplers, which are rotated 90° between them and both are coupled to the main square waveguide through each one of the square section walls. The coupler covers the full frequency band from 10 to 18.9 GHz. It has inherent low cross-polarization, which allows obtaining any known elliptically polarized wave at a square waveguide when a signal is applied to the couplers. The fabricated prototype of this coupler exhibits 31 dB of coupling, with flatness of ±3.8 dB and excellent cross polarization better than 50 dB over the whole band.