Low loss flexible dielectric waveguide for millimeter-wave transmission and its application to devices

Dielectric properties have been measured at 94 GHz for thallium halide materials KRS-5 and KRS-6 using Fabry-Perot resonance transmission and short-waveguide reflection in dielectric-filled waveguide section. The dielectric constants are 31-32 for KRS-5 and 28-31 for KRS-6; the loss tangents are .02 for both. These values predict a waveguide loss of 8dB/cm for a closely-confined HE sub 11 mode in KRS-5 at 94 GHz. Difficulties in coupling to such high dielectric constant fibers have so far prevented direct measurement, but scale model studies at 10 GHz yield a successful coupling technique to the HE sub 11 mode through an intermediate dielectric. Propagation in a 0.5 mm diameter KRS-5 fiber was observed with 0.2 dB/cm loss, much lower than that predicted for HE sub 11; the as-yet unidentified mode has a phase velocity about c, but small external fields and less than 1.5 dB loss from a 90 deg, 20 cm radius bend. Propagation constants for KRS-5 waveguides were calculated and losses estimated for various designs. A loosely coupled HE sub 11 KRS-5 fiber with foamed-teflon cladding could exhibit 3 dB/meter loss, with 3 dB/meter additional loss from 60 cm radius bends. A simple theory of relative quiding properties for multilayer cylindrical waveguides is given.