Spectral-domain analysis of submillimetre-wave microstrip filters

We consider the cut-off frequencies of high-order modes in boxed submillimetre-wave microstrip filters. It is shown that many of the filters in use at the present time are not cut off in the way that the designers imagine. It is also shown that for ease of manufacture the height of the microstrip channel should be 0.7 times the width and the thickness of the quartz substrate should be 0.5 times the width. This optimum geometry suggests that the upper frequency limit of conventional waveguide components is 1THz.