A 600 GHz Orthomode Transducer based on a Waveguide Integrated Wire Grid Polarizer
Paper in proceedings, 2012
Abstract — We present a new type of orthomode transducer (OMT), involving the integration of a quasi-optical
component, namely a wire grid polarizer (WGP), with an E-plane probe transition, in a waveguide topology. The goal has
been to create a lightweight and compact device for waveguide integration, capable of covering all frequency channels to
be used by the FIRE instrument¤
The design consists of a metallic split block, in which a quadratic waveguide supporting the two fundamental
modes of propagation (TE10 and TE01), is formed using regular CNC milling. Just as its quasi-optical counterpart, i.e. a
free standing WGP, a waveguide integrated WGP insert is used to separate the two orthogonal TE modes, which are
either reflected or transmitted (depending on polarization). A planar E-plane waveguide probe transition realized on a
GaAs membrane is used inside a square waveguide to extract the reflected mode without affecting the transmitted mode.
For a first proof of concept the OMT has been designed for the possibility of full characterization and therefore the
transmitted mode is guided through a stepped impedance transformer to a standard rectangular waveguide interface,
while a second E-plane probe transition to a rectangular waveguide, is used for the reflected mode.
Simulations carried out using Ansoft HFSS show that the OMT design can be made relatively broadband
(currently about ~14% with a return loss better than 15 dBs) and with cross-polarizations better than 30 dB. The probe
and WGP can be manufactured using high precision photolithography or E-beam lithography making it scalable to both
lower and higher frequency bands. Assembling an OMT of this type involves mounting of small membrane structures (< 1
mm) into waveguides and the design can thus be subject to misalignment errors which may degrade the performance.
Hence, a tolerance analysis identifying the most sensitive parameters will be presented together with the different design
trade-offs. Practical aspects regarding the assembly of the waveguide probe and WGP into the block will also be
presented along with S-parameters measurements. It is the belief of the authors that this new type of OMT concept will
allow for broadband operation with relative low losses at high frequencies (i.e. above 600 GHz), where regular WGPs so
far has been the most common way to separate two signals of orthogonal polarizations.