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This paper reports the design, construction and electromagnetic performance of a new freestanding frequency selective surface (FSS) structure which generates coincident spectral responses for dual polarization excitation at oblique angles of incidence. The FSS is required to allow transmission of 316.5-325.5 GHz radiation with a loss les 0.6 dB and to achieve ges 30 dB rejection from 349.5-358.5 GHz. It should also exhibit crosspolarization levels below -25 dB, all criteria being satisfied simultaneously for TE and TM polarizations at 45 deg incidence. The filter consists of two identical, 30 mm diameter, 12.5 mum thick, optically flat, perforated metal screens separated by 450 mu m. Each of the ap 5000 unit cells contains two nested, short circuited, rectangular loop slots and a rectangular dipole slot. The nested elements provide a passband spectral response centered at 320 GHz in the TE and TM planes; the dipole slot increases the filter roll-off above resonance. The FSS was fabricated from silicon-on-insulator wafers using precision micromachining and plating processes including the use of deep reactive ion etching (DRIE) to pattern the individual slots and remove the substrate under the periodic arrays. Quasi-optical transmission measurements in the 250-360 GHz range yielded virtually identical copolarized spectral responses, with the performance meeting or exceeding the above specifications. Experimental results are in excellent agreement with numerical predictions.