Abstract: A phased array antenna structure capable of operation at millimeter-wave frequencies and having multiple ring slot radiator elements (10). The RF feed structure for each radiator element includes a feed via (28) extending part-way through a multi-layer structure (FIG. 3) on which the radiator elements (10) are formed and a strip line feed probe (30) extending from the via (28) toward the radiator element. A key feature facilitating high-frequency operation is the inclusion of multiple mode suppressors (32) surrounding the via (28) and providing a smooth transition from a coaxial mode of RF transmission to a strip line mode of RF transmission. The feed probe (30) is tailored to provide either a narrow-band or a wideband frequency characteristic.