Tests of a planar L -band orthomode transducer in circular waveguide

This note describes tests made of a planar L-band orthomode transducer consisting of four probes symmetrically arranged in a circular waveguide and two wideband baluns, made from back-to-back slotline-to-microstrip transitions. We designed this device to model at large scale (>50:1) the performance of a compact millimeter wave transducer for use in an astronomical receiver where probes and baluns are microfabricated with SiON thin-film dielectric deposited on a quartz substrate. In the L-band model, probes and baluns are printed on 0.010-in.-thick Duroid 6006, mounted on a 0.25-in.-thick piece of fiberglass and placed flush against the waveguide. Each pair of opposite probes is coupled through a balun to a 50 Ω microstrip output. Measurements show that, from 1.2 to 1.9 GHz, the orthomode transducer has an output return loss less than −18 dB and cross polarization less than −35 dB. It should be possible to scale this design to cover bands of high atmospheric transparency at 100, 230, or 345 GHz.