210–365 GHz 90° Differential Phase Shifter for Wideband Circular Polarizer

In this article, a wideband 90° differential phase shifter in the 210–365 GHz band with a fractional bandwidth of 54% was designed, fabricated, and measured to develop a wideband circular polarizer for millimeter/submillimeter wavelengths. To achieve a wide bandwidth, we designed a 90° differential phase shifter with a flat phase-difference characteristic by combining three different types of 90° differential phase shifters to cancel out the frequency dependence of the phase difference between the two polarizations. The developed phase shifter has a phase difference of 90 ± 6° and a return loss better than 17 dB for both polarizations over the frequencies 210–365 GHz, in addition to waveguide transitions necessary for
S
-parameter measurements. The developed phase shifter can be used as an alternative for achieving simultaneous observations of dual-polarization signals at 230 and 345 GHz, as proposed by the next-generation event horizon telescope, which aims to observe black holes.