Abstract

Simultaneous transmit-and-receive (STAR) antennas are important components of in-band full-duplex wireless front ends. However, transmit-to-receive leakage through the antenna limits the achievable antenna-domain isolation. This paper presents a dual-circularly polarized patch antenna incorporating a decoupling feeding network (DFN) to suppress the residual coupling between the transmit and receive ports. A stacked patch radiator with cross-aperture coupling generates right-hand and left-hand circularly polarized radiation from two separate ports. The DFN introduces an additional coupling path whose magnitude and electrical phase are adjusted to produce destructive interference near the target frequency. The fabricated prototype exhibits overlapping −10 dB impedance and 3 dB axial ratio bandwidths from 4.1 to 4.4 GHz, a minimum measured S21 of −50 dB, isolation higher than 30 dB from 4.25 to 4.28 GHz, and a peak realized gain of 7 dBic. The measured high-isolation range has an absolute bandwidth of 30 MHz and a fractional bandwidth of approximately 0.70% around 4.265 GHz. Therefore, the proposed DFN provides narrowband antenna-domain isolation enhancement around the designed frequency without requiring additional patterned decoupling elements on the radiating aperture. The proposed antenna is primarily intended for fixed-frequency or narrow-channel STAR front ends rather than broadband high-isolation operation.

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