A Compact X-Band Radar Beamforming MIMO Antenna with a Modified 4x4 Butler Matrix

Abstract

Indonesia's vast maritime territory requires compact, mobile, smart radar antenna systems for effective security monitoring. This study evaluates 4x4 MIMO array antennas with conventional and modified Butler matrices, assessing crossover reduction's effect on S-parameters and radiation patterns. The conventional design achieves higher peak gains (11.9 dBi edge, 9.75-9.76 dBi middle) than the modified design (10.7-10.8 dBi), though the modified matrix offers more consistent return loss (-12.5 dB) versus -10.6 dB (middle) and -27.3 dB (edge) for the conventional design. Isolation performance differs by port location: the modified matrix improves edge-port isolation (-12.9 to -21.2 dB, S14/S41) relative to the conventional design (-6.3 dB), while its middle-port isolation (-7.2 dB, S23/S32) is weaker than the conventional design's (-12.8 to -15 dB), indicating a redistribution rather than a uniform improvement. Despite a flipped middle-phase response from the crossover omission, the modified matrix retains functional radiation patterns. These results show that the modified design achieves a more compact, balanced configuration with improved edge-port isolation and return loss, but at the cost of lower peak gain and larger tilt-angle deviation (±3° versus ±2°) compared to the conventional design. Beyond simulation, a fabricated 2x2 MIMO array with a 2x2 Butler matrix was measured via VNA, confirming beam-steering at 340.39° and 31.36° and validating the crossover-reduction approach in hardware.