In this paper, we present a new design of multi-resonance monopole antenna for use in microwave imaging systems is presented. The antenna configuration consists of an ordinary square radiating patch, a feed-line, and a modified ground plane with pairs of L-shaped slits and parasitic structures which provides a wide usable fractional bandwidth of more than 130% (2.95-14.27 GHz). In the presented design, by cutting a pair of L-shaped slits and also by embedding a pair of inverted L-shaped parasitic structures in the ground plane additional resonances at 9.5 GHz and 13.7 GHz can be achieved. By using these structures, the usable upper frequency of the antenna is extended from 10.3 GHz to 14.27 GHz. The proposed antenna has a symmetrical structure with ordinary square radiating patch, therefore displays a good omni-directional radiation patterns even at higher frequencies. The antenna has sufficient and acceptable gain level and also its radiation efficiency is greater than 86% across the entire radiating band. The designed antenna has a small dimension.
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