Design of compact microstrip branchline couplers for broadband circularly polarized DRAs

A systematic procedure for design of a broadband circularly polarized dielectric resonator antenna (CP DRA) is presented. An important stage of this process is the design of a microstrip branchline coupler that realizes a required excitation of a particular dual-input CP DRA over the frequency range of interest. This excitation is developed to improve the axial ratio, otherwise degraded due to coupling between the dual inputs of the DRA. The designed branchline couplers have smaller footprint than corresponding quadrature hybrids. At the final stage, simulation-based tuning is applied to the entire CP DRA-feed structure to control several figures of interest, e.g., broadside AR, reflection coefficient, and total radiated power over the frequencies, as well as to account for coupling and parasitic radiation. An example of a 2.2 GHz cylindrical CP DRA illustrates our approach which, to our understanding, is also suitable for different types of dual-input CP antennas.