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

Air-filled 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-simplified 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 12 THz. An expression for the geometrical factor of a rectangular cavity resonator has been derived for the general case of a metal characterized with μr 6= 1 and ωτ > 0. In addition, a method for determining the corresponding lossless frequency of oscillation had been given for the first time for such models. Using these techniques, a quantitative analysis for the application of different 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-effect model, it has been found that HFSS (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 7.3 THz cavity resonator. With the former, measured transmission losses will be significantly lower than those predicted using the current version of HFSS, which may lead to an underestimate of THz losses attributed to extrinsic effects. With the latter error, in overall frequency detuning, the measured positions of return loss zeros, within a multi-pole filter, will not be accurately predicted by the current version of HFSS. This paper has highlighted a significant source of errors with the electromagnetic modeling of terahertz structures, operating at room temperatures, which can be rectified by adopting the classical relaxation-effect model to describe the frequency dispersive behavior of normal metals.