Equivalent circuit model for linear to circular polarisation converters based on bandpass freestanding FSS screens

The electromagnetic performance of bandpass resonant Frequency Selective Surfaces (FSS) is dependent on the dimensions of the various unit cell parameters, namely periodicity, metal thickness and slot length and width. This paper presents an equivalent circuit model (ECM) for a linear to circular polarisation convertor based on a cross slot dipole array. The ECM which accounts for coupling between the orthogonal slots was used to relate the values of inductance and capacitance to the physical dimensions of several different structures that were each designed to work at a centre frequency of 10 GHz. These L and C values were obtained using full wave EM simulations in conjunction the Nelder-Mead method for curve fitting. The 3dB axial ratio (AR) bandwidth of the bandpass FSS screens was maximised by selecting the unit cell dimensions to give a large ratio of inductance to capacitance. Measured results obtained over the range 8 to 12 GHz for a manufactured structure, shows good agreement with the spectral responses calculated from the ECM and full wave simulations.