Omnidirectional Monopole Antenna for Use in Circular Cylindrical Microwave Imaging Systems

In this paper, a new design of multi-resonance ultra-wideband monopole antenna for use in circular-cylindrical microwave imaging systems (MIS) is presented. The proposed antenna consist of a square radiating patch and a ground plane with three modified H-shaped defected ground structure which provides a wide usable fractional bandwidth of more than 150 % (2.5–17.5 GHz). The 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 85 % across the entire radiating band. In the presented antenna structure, by cutting a pair of horizontal H-shaped slots in the ground plane, additional resonances at lower and higher frequencies (2.9 and 10.7 GHz) can be achieved. In addition by employing a vertical H-shaped slot in the center of ground plane the fifth and sixth resonances at 14.7 and 17 GHz are generated. By using these modified structures in the ground plane of the proposed design, the usable upper frequency of the antenna is extended from 10.3 to 17.5 GHz and the usable lower frequency of the antenna is decreased from 3.1 to 2.5 GHz, respectively. The designed antenna has a small dimension. Good return loss and radiation pattern characteristics are obtained in the frequency band of interest. Simulated and measured results are presented to validate the usefulness of the proposed antenna structure for MIS applications.