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

This note describes tests made in March 2000 of a planar L-band orthomode transducer. The transducer consists of 4 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 the performance of a millimeter wave transducer 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′′ thick Duroid 6006, mounted on a 0.25′′ thick piece fiberglass and placed flush against the waveguide. Each pair of opposite probes is coupled through a balun to a 50 ohm microstrip output. Measurements show that, from 1.2 – 1.9 GHz, the orthomode transducer has an output return loss less than –18 dB and crosspolarization less than –35 dB. It should be possible to scale this design to cover the 80-115 GHz or 210-270 GHz bands.