A Circularly Polarized Quad-band Annular Ring Antenna with Asymmetric Ground Plane Using Theory of Characteristic Modes

Herein a theory of characteristic mode (TCM)-based design of circularly-polarized (CP) quad-band compact microstrip antenna is proposed. It involves one annular ring radiator having eight symmetrical slots along its boundary and three circular closed ring resonators (CRRs) on the bottom side of the substrate. Initially, CM analysis was carried out for the radiator without a feeding structure, to determine the modal currents and their corresponding modal fields (radiation patterns) of existing modes. This helped in recognizing the symmetric modes to be selectively excited by the feed to furnish CP and gave direction for selecting an asymmetric CPW-fed structure as the feed of choice. Evolution process of the antenna geometry shows that the eight symmetrical slots in tandem with the CRRs generate wide impedance bandwidth (IBW), while measured quad CP bands are obtained through uses of an asymmetric ground plane resonating at 5.63 GHz (120 MHz), a cross-shaped slit at 7.69 GHz (650 MHz), a rectangular open loop at 9.91 GHz (1200 MHz), and a tuning stub in the feeding structure at 12.09 GHz (160 MHz). Series of parasitic strips augmented CP radiation and eliminate ripples in the radiation pattern. Measured findings relate satisfactorily with simulations done using Ansys HFSS 2020 R1. A low cost FR-4 substrate is used to fabricate the antenna with an optimized dimension of 35 x 30 x 1.6 mm(3). The measured IBW ranges from 4.36-4.82 GHz, 5.50-5.78 GHz, and 5.95-beyond 14 GHz. The proposed antenna may find suitable applications in C band, X band, and 5 GHz WLAN devices.