A Polarisation Independent Bandpass FSS

A Frequency Selective Surface (FSS) may be viewed as a filter of electromagnetic waves in the frequency domain. A common class of FSS can be constructed by placing a periodic array of conducting elements on a dielectric substrate or slots in a conducting surface respectively. Multiple layers of such an FSS may be cascaded for greater effect. This application note describes a free standing nested annular slot type FSS which was recently fabricated for the European Space Agency by the Institute of Electronics, Communications and Information Technology at Queen's University Belfast in Northern Ireland. The design was carried out using CST MICROWAVE STUDIO® (CST MWS) [1,2].

The FSS was designed to operate at 321 GHz with a low insertion loss of < 1 dB, and an isolation of better than 20dB between the 316.5 – 325.5 GHz and 349.5 – 358.5 GHz frequency bands. It consists of two closely spaced identical metal screens containing a periodic array of nested annular slots. The slots in the second screen were rotated by 180 degrees relative to the first screen, to provide coincident rejection levels. CST MWS was successfully used in optimising both the FSS dimensions and the layer spacing for the criteria where both TE and TM oriented plane waves arrive at a 45 degree angle of incidence to the surfaces. An SEM image of some elements in one of the FSS layers is shown in Figure 1.

The structure was modelled as an infinite array in CST MWS by utilising CST's full Floquet mode implementation to simulate a single unit cell of the FSS. The simulation was performed with a tetrahedral mesh in the frequency domain; an adaptive mesh refinement in the passband ensured a high accuracy. The CST model and its final mesh are shown in Figure 2. The design model's geometry was fully parameterised to allow the FSS to be optimised.

The nested short circuited annular slot configuration in this FSS allows it to act as a filter for both TE- and TM-polarised incident plane waves. The effect of the structure on the electric field distribution at 321 GHz for both incident plane wave polarisations is shown in Figure 3. The outer slot provides a full-wavelength resonance for a TE-oriented E-field, while the inner slot provides a half-wavelength resonance for a TM-oriented E-field.

Two FSS configurations were constructed and measured. One had the slots in the two screens in the same orientation, while a second had the slots in opposite orientations, i.e. the slots were aligned but mirrored. The first FSS shows isolation of better then -19 dB isolation for a TE polarised incident wave, but better than -29 dB for the TM polarised wave [2]. Rotating the second screen by 180 degrees (while keeping the slots in the two screens aligned) improved the TE isolation but reduced the TM isolation, as illustrated by the results shown in Figure 4. However, both plane wave polarisations fulfilled the -20 dB isolation criterion for this screen configuration.

The effectiveness of the FSS is shown graphically in Figures 5 and 6. A plane wave with 45 degree angle of incidence in the passband at 321 GHz experiences less than 1 dB attenuation as it propagates through the FSS. In the stopband at 350 GHz, in contrast, the wave is mostly reflected and the measured forward propagating field experiences an attenuation of more than 20 dB.

CST would like to thank Dr. Raymond Dickie and Dr. Robert Cahill of the ECIT at Queen's University Belfast for providing us with the model, photographs, and measured results.

[1] Dickie, R., Cahill, R. Gamble, H., Fusco, V., Huggard, P., Moyna, B., Oldfield, M., Grant, N. and de Maagt, P., "Polarisation independent bandpass FSS," Electronics Letters, Vol. 43, pp. 1013-1015, 2007.
[2] Dickie, R., Cahill, R. Gamble, H., Fusco, V., Huggard, P., Moyna, B., Oldfield, M., Grant, N. and de Maagt, P., "Polarisation independent submillimetre wave annular slot frequency selective surface," Proc. European Conference on Antennas and Propagation, Nov. 2007.