Ultra-wideband antennas covering 1-18 GHz are required for Direction Finding (DF) and phased array applications in electronic warfare and communication systems. Several antennas such as Archimedean spirals, Log periodics, Ridged horns have been extensively used for ESM-DF applications. In this paper an all metal Vivaldi antenna covering 3-18 GHz is designed using HFSS software, and hardware has been realized. A measured VSWR of less than 2.5 over 3-18 GHz is obtained. Radiation patterns are satisfactory both in simulations and measurements. There is fairly good agreement between the two. Further parametric studies are carried out on the single antenna with side and back walls, and this design is optimized for VSWR of less than 2.5 over the band. This antenna is used in a linear array of 8 elements. For this array in simulations, scanned patterns devoid of grating lobes are obtained from 3.0 GHz to 9.0 GHz, and results are presented.