An Ultrawideband Quad-Mode SIW Cavity-Backed Slot Antenna for NextG Communication and Sensing Arrays

This article presents a novel substrate integrated waveguide (SIW) cavity-backed slot antenna (CBSA) to enable the next generation of communication and sensing infrastructure. The antenna achieves a bandwidth of 40%, with a low profile ( $\lt 0.1\lambda _{\text {c}}$ ), high front-to-back ratio, good polarization purity, high isolation, and scalability to large active arrays such as phased arrays and reconfigurable intelligent surfaces (RISs). To achieve wideband operation, four resonant characteristic modes are excited in a compact T-shaped cavity with a rectangular slot aperture. The interaction of these four modes is shown to create fictitious shorts along the length of the slot that results in multiple resonances. Characteristic mode analysis (CMA) and optimization are used to place four such fictitious short modes at the right frequencies in order to cover a fractional bandwidth of 40%, which is shown to be close to the theoretical bandwidth limit of a single slot with broadside radiation. The antenna is integrated into a $2 \times 1$ back-to-back array with a novel ultrawideband (UWB) out-of-phase power divider to achieve a symmetric beam pattern. This unit cell is used to demonstrate a high isolation, full-duplex, broadband antenna system. Next, the antenna is integrated into a $1 \times 6$ electronically scanned array (ESA) to demonstrate 1-D beam steering over the entire band. Finally, 2-D scalability is investigated by prototyping a $2 \times 2$ triangular lattice array. All the antennas presented are fabricated in a standard four-layer printed circuit board to cover the 20–30-GHz band.