Tolerance Analysis of Octave Bandwidth Millimeter-Wave Planar Orthomode Transducer

TL;DR

This study analyzes the manufacturing tolerances of octave bandwidth planar orthomode transducers for millimeter-wave polarization measurements, using electromagnetic simulations to optimize design and understand performance sensitivities.

Contribution

It provides a detailed tolerance analysis of planar OMTs with two probe geometries, offering insights for improved design in CMB experiments.

Findings

Classic probe achieves ~93% co-polar coupling

Waveguide misalignment can reduce co-polar coupling by 10%

Cross-polar coupling remains below -50 dB

Abstract

Planar Orthomode Transducers (OMTs) are commonly used for polarization measurements at millimeter wavelengths. We present an optical coupling study of an octave bandwidth planar OMT in circular waveguide based on 3D electromagnetic simulations. We quantify results through metrics such as co- and cross- polar coupling, reflection, and waveguide leakage as a function of the OMT construction geometry. We evaluate the tolerance of these metrics to the waveguide backshort distance, probe impedance, waveguide gap size, and waveguide-to-probe misalignment. Two probe geometries are studied: the `classic' shape used in several previous experiments, and a new `wineglass' geometry. The bandwidth ratio of both optimized OMTs is 2.0:1, defined where co-polar coupling exceeds 80%. The average co-polar coupling, cross-polar coupling, reflection, and waveguide leakage of the classic probe is approximately 93%, $<$-50 dB, 5% and 2%, respectively and depends slightly on the exact frequency range. The wineglass probe co-polar coupling is $\sim$ 2% larger. Radial waveguide misalignment at the level of 4% of the waveguide radius can result in up to a 10% reduction in co-polar coupling and -20 dB cross-polar coupling in one polarization. These results may be used to guide the detector module designs of future Cosmic Microwave Background experiments and beyond