This work presents the design, optimization, fabrication, and measurement of a \(2 \times 2\) circularly polarized microstrip patch antenna array intended as an elementary radiator for 76–77 GHz MIMO radar prototype validation. Circular polarization (CP) is achieved using a sequential rotation phase technique with four linearly polarized patches arranged on a square grid and excited with \(90^\circ \) progressive phase shifts. Aperture-coupled feeding is employed to isolate the radiating patches from the feed network, enhancing pattern stability and impedance bandwidth. A series–parallel microstrip network is synthesized to deliver equal power to all elements with the required phase progression, using a \(32.5~\Omega \) quarter-wave transformer for \(50~\Omega \) input matching. The antenna is implemented on Rogers 3003 substrates with truncated-corner patches to improve axial ratio and matching. Measurements using a vector network analyzer (VNA) with mmWave frequency converters confirm a \(-10\) dB impedance bandwidth of 10.9 GHz, good matching at 76.5 GHz ( \(S_{11} \approx -23\) dB), peak realized gain of 10.9 dBi, and axial ratio below 3 dB over 74–77.5 GHz at boresight. Radiation patterns exhibit broadside coverage with \(\sim 40^\circ \) half-power beamwidth in both principal planes, suitable for imaging radar. Sensitivity analysis shows robust performance against feed-line width tolerances and \(\pm 0.1\) variations in substrate permittivity, with maintained gain and circular polarization across the target band. The results validate the proposed element as a compact, peak realized gain of 10.9 dBi at 76.5 GHz, as a compact CP radiator for MIMO antenna array prototype measurements and anechoic chamber validation.
