This letter presents a novel four elements ultra wideband (UWB) antipodal vivaldi antenna array (AVAA) design for radar and microwave imaging applications. Initially, the radiation flares of the antipodal vivaldi antenna (AVA) is designed based on an elliptical curve shape. Next, the substrate end is shaped as a triangular shape and a negative index metamaterial (NIM) has been incorporated into the AVA aperture to act as a director. A gain enhancement of 2 dB is obtained for the proposed element compared to conventional one. The proposed antenna element is fabricated and measured. There is a good consistency between the simulated and measured return loss. In addition, four UWB vivaldi antennas with substrate end shaping are placed along the z-axis in a linear antenna array configuration. The four NIM structures of the four antennas are placed together to form a sheet which is perpendicular to the four antenna substrates. A gain enhancement of about 6 dBi is obtained compared to the proposed single element through the whole UWB frequency range. The proposed antenna array bandwidth extends from 3 GHz to 16 GHz. The proposed antenna array is fabricated, measured, and good agreement is obtained between simulated and measured reflection coefficients