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A wide-band unidirectional cavity-backed bowtie antenna with stable radiation patterns is proposed in this paper. It is differentially fed by a parallel strip line via a transition from a microstrip line. The corners of the conventional triangular bowtie dipole are rounded to achieve a larger impedance bandwidth. A circular ring, acting as a special reflector, is placed within the cavity, between the ground plane and the bowtie dipole, for stabilizing the broadside radiation patterns over the whole operating band. Numerical simulations and parametric studies are performed to verify the effectiveness of the ring reflector for radiation pattern improvement when it is electrically close to the bowtie dipole. The final design is fabricated and measured. Measured results indicate that the proposed antenna features an impedance bandwidth of 91.4% for SWR les 2, a 7.3-9.5 dBi broadside gain, a total size of 1.56 times 0.325lambda0 2 (lambda0 is the free-space wavelength at the center frequency) and stable radiation patterns over the whole band.