WT4 Millimeter Waveguide System: Waveguide Design and Fabrication

The WT4 transmission line is a 60 mm diameter string of circular waveguide sections composed of 9 m long dielectric-lined and 4.4 m long helix waveguide sections, with dielectric-lined waveguide comprising approximately 99 percent of the transmission line. The dielectric-lined waveguide is manufactured from steel tubing whose geometric distortions are controlled on a power spectral density basis. The steel tubing is processed into waveguide by the application of copper and dielectric layers along its inner bore and by the attachment of flanges at its ends. The materials system and processes were chosen for extremely low ohmic losses, minimum geometric distortions, and high long-term reliability. The helix waveguide is formed from a closely spaced array of helically wound wires cast into an epoxy system, which is in turn enclosed in a steel jacket. The key design features of this helix waveguide are (i) the specially prepared iron-loaded epoxy which provides ohmic dissipation to spurious mode energy and (ii) the precision temperature control applied to the curing of the epoxy to assure minimum geometry distortion.