An in-band full-duplex antenna for 2.4 GHz WLAN using differential-feed based neutral coupling for isolation enhancement

This article introduces a unique design approach for a compact in-band full-duplex (IBFD) antenna with good isolation in a wider bandwidth at the 2.4 GHz WLAN. The meander slot baluns are innovatively constructed in the feeding network, one balun on the top substrate layer and the other on the bottom substrate layer of the ground, to realize double differential feeding and polarization diversity. The metallic vias that connect one of the baluns in the feeding network with the co-radiating patch further reduce the antenna's overall size. The balun's neutral coupling characteristics of differential potentials, which reduce the leakage currents by creating null potential, and differential filtering, which suppresses the leakage currents by non-coupling fields, generate high isolation in the antenna. The designed balun offers a stable differential feed with a trivial magnitude and phase imbalances of (0.04–0.06) dB and $\pm 1^0$, respectively. The antenna offers a minimum isolation of 78 dB with a wider bandwidth of 130 MHz. The far-field measurement shows broadside radiation with low cross-polar levels of less than −32 dB. A prototype is fabricated on FR-4 substrate layers with an overall size of (0.8 × 0.8 × 0.012)${\lambda }_0$ (${\lambda }_0$ is the free space wavelength of the frequency $f_0=2.4$ GHz) to validate the proposed design performance. The measured results of the prototype properly correlate with the simulated results.