Microwave measurement of the permittivity for high dielectric constant materials using an extra-cavity evanescent waveguide

This article is concerned to a TE01n mode resonant cavity coupled through a hole located in the center of the end wall to a cylindrical waveguide (equal in diameter to hole) supporting the evanescent TE01 mode. When the evanescent guide contains a dielectric sample, propagation of the TE01 wave will be permitted in the dielectric filled part. The air filled part in front of the sample is used to adjust the coupling level; the air filled part of the sufficient length behind the sample is used to form a matched reactance termination, otherwise a metal block is inserted to form a short-circuit termination or a reactance termination. It is shown that while using these arrangements quite a large change in the resonant length (or resonant frequency) and Q factor of the cavity resonator will be obtained, when the sample possessed suitable electric thickness is inserted into the evanescent guide. Therefore, it should be capable of yielding accurate values of the complex permittivity for high dielectric constant materials. Fundamental principles and theoretical error of measurements of the complex permittivity as a function of the electric thickness in the sample due to the uncertainty of measurements in the resonant length and Q factor are discussed. The measured results at some frequencies of the X band and Ka band on two ceramics are given. The technique is compared with the parallel-plate method, showing that the dielectric properties have comparable values in both methods.