Sub-millimetre wave linear to circular polariser converter

The goal of this study is to design a frequency selective surface (FSS). The proposed structure consists of a freestanding screen comprised of a crossed slot. The element comprises two straight slots of different length which are perpendicular to each other. Linear to circular polarisation, CP, conversion is achieved by adjusting the ratio of the orthogonal slot lengths of the structure. The crossed slot is designed in order to minimise reflection and axial ratio when the screen is orientated at 45deg relative to an incident plane wave. We show that the slot width has a smaller effect on matching to free space than its length, therefore during parametric studies the widths of the slots are held constant. The design was modelled and simulated using CST Microwave Studio software. The transmission loss of the structure is -3.2 dB and a minimum axial ratio of <3 dB was found between 325 and 326,6 GHz i.e. 0.5%, bandwidth with a minimum value of axial ratio of 0.11 dB at 325.89 GHz.