A cavity-backed spiral antenna with ultra-wide band width is presented. By using a parabolic reflector, the radiation of the antenna becomes uni-directional, and the gain is promoted. The antenna is fed by a coaxial cut-away balun with fan-shaped patch which keeps a wideband impedance matching. The simulated bandwidth for VSWR < 2 covers 0.36 ~ 6.83GHz or bandwidth ratio of 19:1, and the gain a...