This paper presents a modified structure of the Vivaldi antenna based on an FR4 dielectric substrate of 0.8 mm thickness and 110 mm × 96 mm dimensions for ultra-wideband (UWB) detection systems. The modified Vivaldi antenna has two major novelties. (1) Bottle-shaped slots are etched on both sides of the classic Vivaldi antenna to extend the surface current path length and concentrate electromagnetic energy. ( 2) Rectangular metal strips are implemented in front of the exponential slot to guide the UWB radiation waveform in the desired direction. The modified Vivaldi antenna is evaluated using the CST 2020 software and measured using the vector network analyzer ranging from 3 to 8 GHz. The obtained results show that a maximum peak gain of nearly 9 dBi at 4 GHz can be achieved, the radiation efficiency is improved to 80%, the main lobe level is increased, and the side lobe level is decreased. To further demonstrate the performance of the proposed Vivaldi antenna, a UWB imaging experiment is designed. The imaging results show that the imaging system using the proposed antennas can obtain images with good resolution, which indicates that the antenna has good radiation characteristics and can be used for UWB detection applications.