HFSS/spl trade/ modelling anomalies with electrically thin-walled metal-pipe rectangular waveguide simulations

The authors have found modelling anomalies with the simulation of electrically thin-walled metal-pipe rectangular waveguides (MPRWGs_~using Ansoft's commercial software package HFSS/spl trade/). This problem is evident depending on how the simulations are set up. A numbers of different modelling strategies were investigated, to examine the attenuation of a simple X-band copper-walled MPRWG at 10 GHz, With the normal copper-walled object drawn, but not using the 'solve inside' option, the simulations proved accurate to theoretical predictions for wall thicknesses greater than approximately /spl pi//spl delta/. When the 'solve inside' function is used, only simulations with a physical thickness at approximately /spl pi//spl delta/ were accurate, even if a radiation boundary was applied to the outer surface of the MPRWG. However, if a physically small air-gap of the order of millimetres is introduced between the outer surface of the MPRWG's walls and the radiation boundary, the simulations proved accurate to theoretical predictions for wall thicknesses less than approximately /spl pi//spl delta/. Finally, if the normal copper-walled object is replaced by a 'layered impedance boundary', having assigned theoretical wall thickness and bulk conductivity for copper, then it was found that the simulations accurately predict theory for all wall thicknesses. This discovery has profound implications for the accurate simulation of MPRWG structures and components.