In this work, a leaky-wave antenna (LWA) with pinnate-shaped slots and substrate integrated waveguide (SIW) is presented. The suggested antenna design employs eleven periodic slots to increase its gain to 13.1 dB. The antenna exhibits a narrow beamwidth for all scanning frequencies, scanning from forward to backward for frequencies higher than the cut-off frequency. It radiates at − 76˚, − 30˚, 6˚, 15˚, 20˚, 28˚, 60˚, and 80˚ for frequencies 26.4 GHz, 32.4 GHz, 31.3 GHz, 29.2 GHz, 27.8 GHz, 26.2 GHz, 25 GHz, and 30 GHz, respectively. TE10 mode excitation occurs in the proposed SIW-based LWA, in which the antenna leaks the wave through periodic slots embedded in the uppermost layer of SIW. The EM-wave leakage and phase constant can be controlled by varying the slot orientation. The presented LWA is highly directive, with high radiation efficiency and a wide impedance bandwidth (9 GHz) ranging from 25 GHz to 34 GHz. The experimental results of the designed LWA match the simulation results.