Antennas with a cosecant-squared radiation pattern are employed in surveillance and search radars. One of the most critical features of this radiation pattern is its ability to maintain a constant received power from a target flying at a fixed altitude but varying distances from the radar. In this paper, a monopulse array antenna with a cosecant-squared fan-beam radiation pattern, wide bandwidth, low sidelobe level, and slant polarization is designed for high-precision target search and tracking. The monopulse array antenna consists of 2×8 microstrip radiating elements with a modified proximity-coupled feed. To reduce mutual coupling between the radiating elements, metallic walls are placed between them. Additionally, a second ground plane is introduced behind the radiating elements to enhance gain. For synthesizing the cosecant-squared radiation pattern, the amplitude and phase of each element are optimized using a genetic algorithm, accounting for mutual coupling effects. Asymmetric amplitude coefficients are employed to reduce sidelobe levels, and a Schiffman phase shifter is designed to achieve the desired cosecant-squared radiation pattern. A key aspect of the phase shifter design is the use of a uniform reference line. Finally, a rectangular 180° hybrid coupler is utilized to improve azimuth tracking, enabling the antenna to exhibit a monopulse radiation pattern in this direction. Simulation and measurement results show good agreement. The antenna achieves a 34% fractional bandwidth (3.8–5.4 GHz), a gain of 13.05 dBi, a sidelobe level of 14 dB, a null depth of 24 dB, and a beamwidth of 26°. The radiation pattern closely follows the cosecant-squared curve.