A High‐Gain Planar Endfire Circularly Polarized Antenna With Small Aperture Based on Slotted Substrate Integrated Waveguide

ABSTRACT In this paper, we present the design of a planar endfire circularly polarized (CP) antenna utilizing a periodic structure. The CP antenna is constructed on substrate integrated waveguide (SIW) that incorporates a periodic double‐sided rectangular slot structure, culminating in a Vivaldi antenna at the end. The rectangular slot array generates vertically polarized (VP) electric field components for the far‐field, while the Vivaldi antenna contributes horizontally polarized (HP) electric field components. By adjusting the distance between these two components, we achieve a phase difference of 90° between the two polarization components. This results in a planar endfire CP antenna that operates within the frequency range of 23.8 to 25.8 GHz, with an axial ratio of less than 3 dB. The antenna achieves a maximum gain of 11.73 dBi. The combination of high gain at small aperture translates to superior aperture utilization and easy edge/board integration, supporting compact endfire arrays, terminal‐side AoA links and 24‐GHz ISM sensing. Compared with recent planar endfire CP designs, the proposed radiator delivers comparable bandwidth but higher gain with smaller width, using a simple, low‐loss, and manufacturing‐friendly single‐layer PCB implementation.