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A broadband antenna with simultaneous flat gain and filtering characteristic is proposed in this paper. To achieve a flat gain response, rectangle slots and a bandpass filter are loaded on the antipodal Vivaldi antenna. The loaded rectangle slots adjust the phase of the surface currents to widen the operating bandwidth, as well as reduce the far-field radiation cancellation of the reverse currents in the end-fire direction, thus improving the gain. In addition, a bandpass filter is added to the antenna to further equalize the currents amplitude and suppress out-of-band signals. The measured results show that the proposed antenna achieves an impedance bandwidth of 101.9% (3.99–12.28 GHz) and an out-of-band suppression level of 15.5 dBi in the upper stopband. What is more, an extremely low gain fluctuation of ± 0.464 dBi is realized in the 4–12 GHz. Compared with conventional broadband antennas, the antenna features simple structure and better performance, which achieves broadband with simultaneous flat gain response and filtering characteristic. The proposed antenna may be applied in high-precision radar sensing system. [ABSTRACT FROM AUTHOR]