Non-Imaging Winston Cone Concentrators for Submillimeter-Wave, Overmoded Waveguide

We describe the design, simulation, and measured performance of concentrators designed to couple submillimeter wavelength radiation from free space into highly overmoded, rectangular, WR-10 waveguide. They consist of a combination of a Winston cone (also called a compound parabolic concentrator or CPC) with an adiabatic circular to rectangular transition. They are intended for use as adapters, between instruments using overmoded WR-10 waveguide as input or output and sources propagating through free space. Unlike conventional waveguide-coupled antennas, a geometric optics analysis is more appropriate than a mode-by-mode electromagnetic calculation of impedance and far-field pattern. Six separate designs were studied, with input diameters from 5 to 16 mm, and “throat” diameters (i.e., diameters at the circular interface between cone and transition sections) of 1 to 4 mm. Measurements at 394 μm wavelength (760 GHz) using a far-IR waveguide laser beam indicate efficiencies of 40%-55%. The angular response is primarily determined by the Winston cone, and is well predicted by geometric optics theory, i.e., approximately constant out to an angle determined by the ratio of input to throat diameters. The efficiencies are primarily determined by the transition section, and for all concentrators are consistent with an average reflectance of 94% from the gold-plated, electroformed, interior surfaces. For each individual concentrator, efficiency variations with polarization, angular orientation and beamsize are below the measurement uncertainty.