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Ultrawideband (UWB) antenna is one critical part of the impulse radar system. In this paper, a shallow Vee-cavity shielded planar dipole antenna working at the center frequency of 400 MHz for UWB vital sign detection radar is investigated numerically and experimentally. It consists of two half-ellipse-shape arms tuned with four semicircular slots and a shallow Vee-cavity. The voltage standing wave ratio (VSWR), input impedance, and radiation waveforms are all analyzed. The resistive and slot loading techniques for improving impulse radiation and reducing late-time ringing are adopted. Several important parameters of the designed antenna have been studied. The influence of a shallow rectangular cavity and the proposed Vee-cavity on the antenna characteristics is also under consideration. Finally, one prototype antenna is fabricated and applied to UWB vital sign detection radar system. The experimental results indicate that the proposed half-ellipse antenna with a Vee-cavity works satisfactorily in the radar system.