A radial line slot antenna (RLSA) 400 mm in diameter is designed for a plasma source at 8.3 GHz as a candidate for a large area high density plasma source with a narrow process space. A RLSA with paired slots arrayed concentrically on its aperture is designed and applied to a plasma process system which does not need a magnetic field. First, electrical performance without plasma is predicted and measured. Reasonable voltage standing wave ratio (VSWR) as well as uniform field distribution on the antenna aperture is realized. As applied to the plasma test equipment, high electron density of more than 1?1012 cm-3 as well as fairly uniform (?2%) distribution at a low electron temperature is produced in the vicinity of the antenna aperture through a quartz glass window. Control of uniformity in ion flux profiles by blocking the antenna aperture is demonstrated.