A new compact configuration of a printed CPW-fed semicircular slot antenna with a CPW-to-CPW step transition is presented. Its bandwidth is enhanced for ultra-wideband (UWB) operation by combining linear and stepped CPW-to-CPW transitions along the feed line. Its measured 10 dB return loss bandwidth is 3.1–13.8 GHz (126%). This, together with its nearly constant measured gain of 3.5 dBi ± 1 dB from 3 to 10 GHz, makes it suitable for UWB applications. Details of the proposed antenna are described, and both theoretical and experimental results are presented.