The synthetic aperture radar (SAR) payload is made up of two major components, the sensor electronics (SE) subsystem and the SAR antenna. This technical note focuses on the RADARSAT-2 SAR antenna radio frequency (RF) and mechanical design and presents an overview of the antenna RF performance requirements and design trade-offs that led to the baseline design concept. Design of the key antenna subsystems is described, and some of the measured RF performance is shown.