Dual Polarised Sub-mm Wave Frequency Selective Beamsplitter

In this paper we describe the design of a Frequency Selective Surface (FSS) which gives a polarisation independent spectral response when the filter is orientated at 45° incidence. The structure consists of one or more freestanding screens each containing a closely packed array of nested short circuited annular slots. A computer model is used to design a two layer FSS which can separate the bands 316.5-325.5GHz and 349.5-358.5GHz with >20dB isolation and <1dB insertion loss in both the TE and TM incident planes. A single periodic array, which is the basic building block of the cascaded screen structure, has been fabricated using a precision micromachining technique. Quasi-optical swept frequency transmission coefficients which were measured over the range 305-345GHz are shown to be in good agreement with the numerical results. A concept for improving the robustness of the structure in order to provide space qualified beamsplitters for future polarimetric atmospheric sounders is presented.