Epsilon-Negative Transmission-Line-Based Annular-Ring-Loaded Circular Microstrip Antenna With Improved Performances

A novel epsilon-negative transmission line (ENG TL)-based annular-ring-loaded (ARL) circular microstrip antenna (CMSA) is proposed in this letter. Compared to a conventional CMSA, the proposed structure offers improved impedance bandwidth by exciting the TM11 modes on both the driven CMSA and the coupled ENG TL-based annular ring. Due to the enlarged radiation aperture, the gain is correspondingly enhanced. Moreover, the proposed structure exhibits lower levels of radiation along the horizon and toward the backside as compared to conventional CMSA. The mutual coupling characteristics of the proposed antenna are also investigated, and the simulated results show that the E-plane mutual coupling between the proposed ENG TL-based ARL CMSAs decreases much more rapidly with spacing than that for conventional CMSAs. To validate the proposed concept, prototypes with a center operating frequency of 6.5 GHz are fabricated and tested. Good agreements are achieved between simulation and measurement.