HFSSTM Modelling Anomalies with THz Metal-Pipe Rectangular Waveguide Structures at Room Temperature

Abstract

Air-fllled metal-pipe rectangular waveguides (MPRWGs) represent one of the most important forms of guided-wave structure for terahertz applications. Well-known commercial electromagnetic modelling software packages currently employ over-simplifled intrinsic frequency dispersion models for the bulk conductivity of normal metals used in terahertz structures at room temperature. This paper has compared various conductivity modelling strategies for normal metals at room temperature and characterized rectangular waveguides and associated cavity resonators between 0.9 and 12THz. An expression for the geometrical factor of a rectangular cavity resonator has been derived for the general case of a metal characterized with r 6 1 and !? > 0. In addition, a method for determining the corresponding lossless frequency of oscillation has been given for the flrst time for such models. Using these techniques, a quantitative analysis for the application of difierent models used to describe the intrinsic frequency dispersion nature of bulk conductivity at room temperature has been undertaken. When compared to the use of the accurate relaxation-efiect model, it has been found that HFSS TM (Versions 10 and 11) gives a default error in the attenuation constant for MPRWGs of 108% at 12THz and 41% errors in both Q-factor and overall frequency detuning with a 7.3THz cavity resonator. With the former, measured transmission losses will be signiflcantly lower than those predicted using the current version of HFSS TM , which may lead to an underestimate of THz losses attributed to extrinsic efiects. With the latter error, in overall frequency detuning, the measured positions of return loss zeros, within a multi-pole fllter, will not be accurately predicted by the current version of HFSS TM . This paper has highlighted a signiflcant source of errors with the electromagnetic modeling of terahertz structures, operating at room temperatures, which can be rectifled by adopting the classical relaxation-efiect model to describe the frequency dispersive behavior of normal metals.