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In this letter, a novel ultrawideband (UWB) printed monopole antenna (PMA) for use in a circular cylindrical microwave imaging system is presented. The proposed antenna consists of a square radiating patch and a ground plane with a pair of E-shaped slots and a rectangular slot with a pair of horizontal T-shaped strips protruded inside the slot, which provides a wide usable fractional bandwidth of more than 120% (2.97-12.83 GHz). By cutting two modified E-shaped slots with variable dimensions on the ground plane corners and also by inserting two T-shaped strips inside the rectangular slot located in the ground plane center, additional resonances are excited, and hence much wider impedance bandwidth can be produced, especially at the higher band. The proposed antenna has an ordinary square radiating patch, therefore displays a good omnidirectional radiation pattern even at higher frequencies, and also its radiation efficiency is greater than 86% across the entire radiating band. The designed antenna has a small size of 12 ×18 mm2. Simulated and measured results are presented to validate the usefulness of the proposed antenna structure for circular cylindrical microwave imaging system.