This work presents a flat-gain antipodal Vivaldi antenna (AVA) for ultra-wideband (UWB) microwave imaging applications. The proposed antenna demonstrates measured and simulated impedance bandwidth from 2.3 GHz to 20 GHz with a flat gain over the band, with minimum gain variation of 2.9 dBi. The antenna is tested initially for its practical utility by measuring its specific absorption rate (SAR) value. The SAR value is observed in the simulator by modeling a realistic homogeneous as well as a heterogeneous breast phantom. The SAR value obtained complies with both American and European standards when averaged over 1 g as well as 10 g of tissue. The designed antenna is further utilized to detect multiple tumors in a realistic homogeneous and heterogeneous breast phantoms developed in the laboratory environment. The widely popular delay and sum (DAS) algorithm is utilized to reconstruct the tumor images. The imaging is done outside the anechoic chamber with an in-house monostatic microwave imaging setup just to make it more convincing with real-world dynamic scenario. The homogeneous phantom with four embedded tumors each of radius 4 mm and the heterogeneous phantom with two tumors, one of radius 3.5 mm and another of radius 5 mm are imaged in this work. The imaging results demonstrate that tumors of different sizes can be detected accurately in the case of both homogeneous and heterogeneous breast phantoms. AVA with an oil paper layer for bandwidth enhancement as well for achieving flat-gain response is first of its kind to be reported in this work Also, the proposed work utilizes different phantoms for imaging several tumors of differing sizes, which has not before been described. © 2022 Wiley Periodicals LLC.