Double Square Waveguide Directional Coupler for Polarimeter Calibration

Beatriz Aja,Luisa De La Fuente,Enrique Villa,Eduardo Artal

doi:10.1109/tmtt.2019.2899853

Beatriz Aja, Luisa De La Fuente + Show 2 more

Open Access

https://doi.org/10.1109/tmtt.2019.2899853

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Abstract

A novel full-band square waveguide coupler design based on directional couplers which couple the TE10 and TE01 orthogonal modes in a square waveguide is presented. This waveguide coupler is aimed at the calibration of polarization receivers. This is composed of a pair of rectangular waveguide directional couplers, which are rotated 90° between them and both are coupled to the main square waveguide through each one of the square section walls. The coupler covers the full frequency band from 10 to 18.9 GHz. It has inherent low cross-polarization, which allows obtaining any known elliptically polarized wave at a square waveguide when a signal is applied to the couplers. The fabricated prototype of this coupler exhibits 31 dB of coupling, with flatness of ±3.8 dB and excellent cross polarization better than 50 dB over the whole band.

Highlights

DIFFERENT polarimetric radiometer architectures have been developed with the aim of measuring the polarization of sky observations or geophysical purposes [1]-[10]
EXPERIMENTAL RESULTS The S-parameters of the coupler are measured with an Agilent Technologies E8364B vector network analyzer (VNA) using a TRL calibration of the WR62 standard waveguide
All the measurements are compared to electromagnetic simulations done with the Finite Element Method-based software ANSYS® High Frequency Structure Simulation (HFSS) Electronics Desktop, using driven modal solution type and adaptive mesh analysis with convergence set to a maximum delta S of 0.005

Summary

INTRODUCTION

DIFFERENT polarimetric radiometer architectures have been developed with the aim of measuring the polarization of sky observations or geophysical purposes [1]-[10]. Other similar methods consist in observing rotatable dielectric sheets to generate a partially polarized signal, where a dielectric sheet acts as a beam splitter transmitting most of the sky radiation but reflecting a small polarized fraction of the radiation from an ambient load perpendicular to the beam [15], [16], or polarization selecting wire-grids mounted on a mechanical rotating ring [17] To those methods, a phase retardation plate can be added to produce an extra reference temperature with circular polarization and calibrate fully polarimetric radiometers [18][21]. With the purpose of calibrating a fully polarimetric receiver, in which at least three linearly polarized waves and one circularly polarized wave are needed, a solution based on waveguide couplers to inject test signals is proposed For this design, waveguide aperture directional couplers, which are based on Bethe’s theory about small aperture coupling [26], [27], have been designed.

SQUARE WAVEGUIDE COUPLER DESIGN

Coupler Design

Prototype fabrication

Stokes parameters of calibration signals

CONCLUSION