A millimeter-wave radial line slot antenna fed by a rectangular waveguide through a ring slot

A radial line slot antenna (RLSA) is a high gain and high efficiency planar antenna proposed for DBS subscribers in the 12 GHz band. In recent years, several radio systems have been developed in the millimeter wave band. In conventional RLS, a radial waveguide has been fed by a coaxial feeder. In the millimeter wave band, fabrication of the feeder is considerably difficult, since the physical size is small. High accuracy in manufacturing is needed to give high antenna efficiency. We have proposed a planar feed circuit with a ring slot and a microstrip line (see Kigure, Y. et al., IEICE Natl. Conv. Rec., B-1-102, 1999). The feed circuit may be easily integrated with an RF planar circuit. In this structure, however, undesired radiation to the back of the antenna is significant and the antenna efficiency is degraded. In terms of efficiency in the millimeter wave band, a rectangular waveguide is more attractive for feeding a radial waveguide. A rectangular-to-radial waveguide transformer through a ring slot is proposed. This transformer is slightly bulky, however there is no leakage. It is suitable for integrating with existing RF modules that have an interface to the rectangular waveguide. The rectangular-to-radial waveguide transformer through a ring slot is designed and fabricated at the 12 GHz band and we integrate a RLSA with this feed circuit at the 38 GHz band.