Low-loss dielectric waveguides are promising for use instead of metal ones, but problems in transitions have to be overcome. A simple and effective structure made of a monocrystalline sapphire waveguide has been designed. Experimental results at 75-110 GHz indicate good matching with metal waveguides (VSWR /spl les/1.13) and low insertion loss (0.05-0.35 dB for 47 mm dielectric section).
Lioubtchenko, D., Dudorov, S., Mallat, J., Tuovinen, J., & Räisänen, A. (2001). Low-loss sapphire waveguides for 75-110 GHz frequency range. IEEE Microwave and Wireless Components Letters, 11(6), 252-254. https://doi.org/10.1109/7260.928929