Quasi-optical waveguide W-band spectrometer for precision dielectric measurement of absorbing materials

A quasi-optical waveguide bridge spectrometer is designed and constructed for the precision measurement of dielectric permittivity and loss tangent of medium and highly absorbing materials over an extended W-band frequency range. An electronically sweeping backward wave oscillator (BWO) is used as a source of tunable coherent radiation in the frequency range 70-118 GHz. The high output power of BWO (typically 50-100 mW at each frequency) and high sensitivity receiver system employing a liquid helium cooled InSb detector enable the accurate transmission measurement of highly absorbing materials. Data for very absorbing materials such as the low resistivity silicon and germanium are presented and compared.