Development of a 385-500 GHz Orthomode Transducer (OMT)

Abstract —We report on the development of an orthom ode transducer (OMT) for ALMA Band 8 (385-500 GHz). The OMT is a scaled model of that of ALMA Band 4 (125-163 GHz), which has a Bφifot junction and a double ridge. The transmission loss of the OMT at 4 K was derived to be 0.4-0.5 dB from noise measurements with an SIS mixer. The polarization isolation was measured to be la rger t han 20 dB from quasioptical measurements. For electromagnetic d esign, effe cts of mechanical errors have been studied and then a robust design with allowable mechanical errors of 10 μm has been obtained. I. I NTRODUCTION An orthomode transducer (OMT) is a passive waveguide device that separates a received signal by a feed horn into its two orthogonal linearly polarized components. For submillimeter receivers, the conventional way to separate orthogonal polarizations is to use a wire grid, which is a quasioptical device that consists of free-standing wires. Optical systems of a dual polarization receiver with a wire grid or an OMT are shown in Fig.1. The merits and demerits of an OMT compared with a wire grid are the following: Merits 1) Optical system of a receiver can be quite simple and compact. An ellipsoidal mirror, a corrugated horn, and a wire grid can be removed from that with a wire grid. 2) There is no beam squint between two polarizations. 3) A problem of the life time of a wire grid can be solved. Demerits 1) A Fabrication is relatively difficult. 2) The Joule loss of the waveguide is added, although an ideal wire grid has almost no loss. These demerits can be solved if we can design mechanical robust OMT, and if waveguide is gold-plated. The transmission loss at 4 K is calculated as 0.5 dB/ 25 mm at 385 GHz when the OMT is cooled down to 4 K. We assume the conductivity of gold film at 4 K is 1.0 x 10