An Improved Analytical Model for Fast Evaluating Absorbing Material Properties for 220-GHz FW BWO

Y-band devices, including folded waveguide, backward-wave oscillator, and traveling wave tube, are to be developed in our group, which use absorbing material in its attenuator to suppress self-oscillation or match the boundary conditions. This paper describes a developed analytical model of a rectangular waveguide system based on the voltage scattering matrix analysis to fast calculate the relative permittivity and electric loss angle tangent of a ceramic sample in consideration of waveguide wall loss. Compared to the high-quality factor cavity method and quasi-optical cavity method, this transmission/reflection measuring method in consideration of ohmic loss is of low cost and more convenient to obtain the material properties in a large-frequency range, by sacrificing some accuracy. 3-D numerical model is established to verify our method. The result shows that our method is accessible to give a fast message of material properties in a certain frequency range. The error analysis of this equivalent method is also discussed in this paper, which shows that this method is too error sensitive to be a practical tool in consideration of the controllable machining and assembly errors. Then, a simplified method is proposed to make the calculation result less sensitive to the parameter errors. The evaluation shows that the relative measuring error of the attenuating material to be tested can be confined to 25% around the frequency of 220 GHz. In advantage of its configuration, the absorber with this material to be used guarantees the small reflection ratio when the value of permittivity and loss tangent of the attenuating material varied by 25%.