A novel printed monopole antenna for ultra-wideband (UWB) applications based on loop sleeve structure in the ground plane is presented. The proposed antenna consists of a rectangular radiating patch, a circular-shaped tapered feed line, and a pair of loop sleeves in ground plane which provides a wide usable fractional bandwidth of more than 100% (3.19-11.03 GHz). Two rectangular loops are inserted on the ground plane's corners, hence additional resonance is excited and much wider impedance bandwidth is produced. In order to demonstrate the usefulness of the proposed antenna for microwave imaging system, the SAR distribution using six elements in a circular array configuration is presented. The proposed antenna exhibits uniform SAR distribution, high efficiency and almost omnidirectional radiation patterns and provides an acceptable gain over whole band.