This paper proposes a miniaturized half-circular-slot UWB antenna. Using an analysis of the neld patterns, we show that the original circular-slot UWB antenna operates on a series of multi-pole radiation based on $TE_n$ modes, and a perfect magnetic wall exists along an axis of symmetry on the antenna. Using the perfect magnetic wall we designed and fabricated the miniaturized UWB antenna on RF-60A substrate with t=0.64mm, ${varepsilon}_r=6.15;and;tan;{delta}=0.0025$, which not only has the half size of the original but also maintains UWB characteristics. The measured gain of the miniaturized antenna is $-2.1{sim}4.3dBi$, which is comparable with the gain, $-2.7{sim}3.1dBi$, of the original circular-slot UWB antenna. The radiation pattern is also similar to that of the original antenna.