In this letter, we propose an extremely high scanning-rate slot array antenna utilizing uniplanar-coupled waveguide cavities, allowing for easy fabrication and a lowprofile planar implementation. In contrast to current techniques employing alternating electric and magnetic coupling topologies, our novel approach relies on a pure magnetic coupling topology, serving as the key for uniplanar implementation. We design and fabricate both metallic waveguide and substrate-integrated waveguide implementations to showcase the effectiveness of the proposed technique. Experimental results demonstrate that the two uniplanar-coupled waveguide antennas can scan angle ranges of 65 • and 70 • , respectively, within a 40 MHz frequency band centered at 4 GHz (equivalent to a 1% bandwidth). Compared to state-of-the-art methods, these proposed frequency-scanning antennas exhibit exceptionally high scanning rates, ease of fabrication, and a low-profile implementation.