Monostatic Copolarized Simultaneous Transmit and Receive (STAR) Antenna by Integrated Single-Layer Design

In this letter, a monostatic and cochannel simultaneous transmit and receive (STAR) antenna with identical circularly polarized radiations of transmitting (TX) and receiving (RX) channels is designed using a thin single-layer substrate. Four concentrically arranged microstrip radiating elements are properly excited by two sets of feeding networks: the feeding network of transmitting antenna (FNT) and the feeding network of receiving antenna (FNR). To fit FNT and FNR into the same plane, the FNR consists of a compact sequential-phase structure placed at the center of the antenna, while the FNT is aligned along the side with a planar crossover junction armed. Two vital techniques, orthogonal feeding points and leakage signal cancellation based on feeding networks, are utilized for satisfactory isolation level between the TX and RX ports. The experiment results demonstrate an isolation at least 41 dB and up to 54 dB, an axial ratio better than 1.7 dB, and a realized gain of 7.2 ~ 10.5 dBic in the 2.4 GHz WLAN band. Our design scheme may overcome the profile and cost limitations of bulky multilayered STAR antennas, and thus portends potential for the highly integrated inband duplex systems.