One embodiment is a ring-slot radiator having: at least one radiating element having a characteristic of dual resonance, double-tuned in which an open ended strip and a cavity are structured to resonate at at least two different frequencies. Another embodiment may have: a ring-slot structure having at least one ring-slot opening on an infinite ground plane that forms a radiating element; a probe-fed strip structure that excites the slot; another ground plane underneath the strip; and a plurality of suppression elements around the ring slot to suppress parallel plate and surface wave modes.