Broadband Full-Duplex Monostatic Circular-Antenna Arrays: Circular Arrays Reaching Simultaneous Transmit and Receive Operation

Broadband copolarized monostatic circular-antenna arrays based on mode orthogonality principles are presented for in-band full-duplex (FD) applications. The simultaneous excitation of circular phase modes in a circular array with four elements provides ideally infinite isolation. However, once the realistic beamformer network (BFN) is used, the isolation between the transmitter (Tx) and receiver (Rx) is reduced and becomes bound by the internal leakages. To mitigate the undesired leakages, two possible configurations are demonstrated: 1) a single-layer (SL) circular array excited simultaneously with mode 1 (M1) at the Tx and mixed modes at the Rx, and 2) a dual-layer (DL) circular array excited with M1 at the Tx and mode 0 (M0) at the Rx. The arrays are prototyped, in which each SL array is made of four broadband monocone antennas. A measured voltage standing wave ratio (VSWR) <;2 and average isolation of 45 dB for a SL array more than at least an octave bandwidth, and 35 dB from 0.8 to 1.3 GHz and 45 dB more than 1.3 GHz for the DL array are obtained. A simple isolation enhancement layer is integrated with the shared BFN to recover from the imbalances and asymmetric leakages with an isolation improvement of up to 50 dB more than a 2.8-MHz tuneable bandwidth. The envelope correlation coefficient (ECC) is used to quantify the similarity between circular Tx/Rx array modes, and the ECC >0.6 for M1 Tx/mixed-modes Rx and >0.75 for M1 Tx/M0 Rx are achieved.