Optimising the axial ratio bandwidth of resonant linear to circular polarisation converters

Abstract

By careful design of the unit cell, resonant FSS's are capable of converting a linearly polarised wave to a circularly polarised wave whilst simultaneously filtering an incident signal at 'out of band' frequencies. Printed FSS's fit for this purpose have been widely reported in technical literature, leaving room for improvement on research into slot based FSS topologies. This project aims to address this deficit by optimising the geometry of this class of FSS and improving on published work. The operating frequency of these polarisation converters is centred around 10GHz.

Previously published work reported a 3dB axial ratio bandwidth of 2.4% for a cross-slot topology. The impact of the geometrical parameters on BW was investigated in this thesis, and from this study a 3dB AR BW of 4.6% was engineered by use of suitable unit cell dimensions. Embedding the freestanding slot array between identical dielectric slabs introduces an additional freedom when designing the transmission mode polarisation converters. The performance is shown to be strongly dependant on the thickness and permittivity of the dielectric cladding used. For the low permittivity case investigated (εr=2.1), using slabs that are 5mm thick (λ/4) results in an increase in the 3dB AR BW at normal incidence to 7.9%. Increasing the permittivity of the dielectric cladding to 9.0 increases the prominence of a secondary peak that is exhibited in the spectral amplitude responses and which can be used to further expand the 3dB AR BW to 24.5%.

Replication of the embedded FSS performance in a thinner and lighter freestanding structure was also investigated. Separation of the slot into two orthogonal rectangular slots in each unit cell was found to increase the 3dB AR BW to 6.3%, while increasing the number of rectangular apertures to eleven achieved a 3dB AR BW of 9.2%.

Date of Award	Dec 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Robert Cahill (Supervisor), Dmitry Zelenchuk (Supervisor) & Vincent Fusco (Supervisor)