Helical Coupler from Rectangular-to-Circular Waveguide

The very-large-array (VLA) radio telescope utilizes a low-loss TE₀₁ circular waveguide transmission system. During the design of this system a coupler was developed which couples directly from a standard millimeter rectangular waveguide to the TE₀₁mode in highly over-moded circular waveguide. In contrast to previous couplers which used periodically spaced groups of coupling holes, this design wraps the rectangular waveguide in a helix around the circular waveguide to give a continuous array of coupling apertures for maximum coupling and a compact mechanical configuration. The helix angle is chosen to match the phase velocities of the rectangular and circular waveguide modes at a given frequency. In particular, couplers have been designed and fabricated which couple from WR-28 (26.540-GHz) and WR-22 (33-50-GHz) rectangular waveguides to the TE₀₁ mode in 20- and 60-mm-diam circular waveguide.