A Novel Low-Loss Overmoded Waveguide System for mm-Wave Cosmology

Millimeter-wave signals need to be carried over distances of 1 meter or more with low loss forapplications in precision cosmology such as beam pattern measurements of antennas. We present a novel,particularly simple approach to convert from standard rectangular waveguide to an overmoded circularwaveguide and back again. We give a design for a transition from rectangular W-band (75-110 GHz)waveguide to overmoded circular waveguide and for long circular waveguides made of copper tubes. Themeasured loss of the transitions is below 0.25 dB and the loss in the circular waveguide is 0.7 dB over a 3m length, including circular waveguide joints at each end. This result is consistent with calculations