Bandwidth enhancement for single-feed circularly polarised microstrip antenna with epsilon-negative transmission line-based annular ring

A novel low-profile single-feed circularly polarised (CP) microstrip antenna (MSA) loaded by a coplanar parasitic epsilon-negative (ENG) transmission line (TL)-based annular ring with enhanced bandwidth is proposed. By exciting the TM₁₁ modes on both the driven patch and the coupled ENG TL-based annular ring, two minimum axial-ratio (AR) points are generated. It is the combination of the two minimum AR points that result in the bandwidth enhancement. An antenna prototype operating at the WLAN 5.2 GHz band is exemplified to validate the proposed concept. The measured results show that the proposed antenna with a low profile of 0.035λ₀ obtains an overlapped bandwidth (return loss <;-10 dB and AR <; 3 dB) of 4%, which is 2.3 times wider than the conventional CP MSA. Moreover, the proposed antenna exhibits higher gain and lower backward radiation as compared with conventional CP MSAs.