Title: "Preliminary test results of the turnstile junction waveguide
          orthomode transducer for the 1 mm band"

Authors: A. Navarrini, A. Bolatto, and R. L. Plambeck
Radio Astronomy Lab, University of California, Berkeley

Abstract:
We tested five prototype waveguide orthomode transducers (OMTs) designed for
the 200-270 GHz frequency band. These OMTs, based on a turnstile junction,
have identical designs (Navarrini et al. 2005), but were fabricated by
different manufacturers using different techniques and materials. The OMTs
were tested at NRAO, Charlottesville, using a Vector Network Analyzer (VNA)
in the frequency range 210-310 GHz.  Three of the OMTs have average room
temperature insertion loss of ~1 dB or better, average input and output
reflection of approx -18 dB, and cross-polarization and isolation of
order -30 dB over 210-270 GHz.  The integrity of the tuning stub in
the turnstile junction is the key for good performance: a gap between the
quadrants there causes additional losses up to several tenths of a dB.
Filling up the gap with indium gave, for the OMT with best performance,
a transmission loss better than -0.8 dB over the entire 210-290
GHz band. Input and output reflections are better than -12 dB, with
cross-polarization and isolation better  than -25 dB across the same
band.  Two of the OMTs had problems, showing narrow and deep transmission
resonances in the band of interest. Modeling at 1 mm and experimentation
with a K-band scale model of the OMT indicate that these resonances are
probably related to fabrication errors that cause a misalignment of one
(or more) of the four quarters of the OMT. These misalignments change the
electrical path length of the waveguide between the turnstile junction
and one of the power combiner inputs, causing a phase difference between
the sidearms of the turnstile junction that gives rise to reflections in
the power combiner.