High order mode planar resonators for mm-wave applications

Planar resonators with open boundaries have been widely used at microwave frequencies, in microstrip antennas and superconducting filters, for example. Most applications have used the lowest order resonant mode, such as the TM₁₀ mode in rectangular patch antennas, but at mm-wave frequencies, the lateral dimensions become too small, leading to fabrication, feeding and power handling problems. An alternative strategy would be to use higher order modes of the TM family in enlarged resonators, which also have the advantage of lower radiation losses. In this paper, both computer simulations and experimental results are presented that demonstrate the achievable performance of high order mode resonators for frequencies above 30 GHz. Particular attention is given to the problems of input and output coupling for two-port resonators operated in the TM₃₃ mode and the need to avoid coupling to adjacent modes