Polarisation Independent Submillimetre Wave Annular Slot Frequency Selective Surface

In this paper we use numerical predictions to show that a freestanding frequency selective surface (FSS) can be designed to separate the two channels 316.5- 325.5 GHz and 349.5 - 358.5 GHz with an insertion loss < 1 dB, a channel isolation > 20 dB and crosspolar levels < 21 dB in both the TE and TM planes at 45deg incidence. The structure consists of two closely spaced identical metal screens which are perforated with an array of nested shorted annular slots. The magnetic current mode which is supported by the periodic screens at resonance is determined by the diameter and the orientation of the asymmetrical slots. This design feature is used to tune independently the spectral responses of the FSS in the two planes of incidence. The periodic structures have been fabricated using a precision micromachining technique. Quasi-optical swept frequency measurements in the range 295 - 359 GHz yield results that are in close agreement with the computed copolar and crosspolar values.