A monofilar Archimedean spiral is adopted as a circularly polarized slot radiator which is electromagnetically coupled to a feed line. Numerical analysis of the coupling between the slot and the feed line reveals that, when the slot arm length is properly chosen, a traveling-wave magnetic current is distributed along the slot, leading to the radiation of a circularly polarized wave. On the basis of this radiation mechanism, a spiral slot printed antenna fed by a triplate transmission line is experimentally designed to obtain a unidirectional radiation beam. It is found that an axial ratio of 0.6 dB is achieved, and that the spiral slot is matched to the transmission line with a VSWR of less than 1.5. An array antenna composed of 32 printed spiral slots is constructed, attaining a 5 % bandwidth for a 3-dB axial ratio criterion. The formation of several tilted beams is also demonstrated.