This paper discusses about the design of a specific radar antenna at ISM frequency band. The overall radar system will be utilized as a trajectory generator for a developed autonomous chaser boat prototype in which the radar antenna was designed to meet this requirement. Fresnel Zone free-space calculation revealed that the radar antenna should be placed at the lake side with a minimum height of 2.5 m. By considering the possible size and distance of the tracked boat, the required radar antenna vertical beamwidth for sufficient elevation coverage is at least 26°. Further design process in relation to desired azimuth tracking resolution revealed that the required horizontal beamwidth is less than 15°. A 4×8 microstrip array antenna was designed by simulations to meet these requirements. The simulation results revealed that the 3dB horizontal beamwidth was 12.2° and the 3dB vertical beamwidth was 160°. Two 4×8 microstrip array antennas were fabricated by utilizing the simulated design, one as transmitting antenna and the other as receiving antenna. Then, the overall performances were justified by experimental antenna measurements. The simulation and measurement results showed that the designed radar antenna can operate at ISM band with sufficient 3dB horizontal beamwidth of less than 15°.