A Dual-Band Circular Patch Antenna with a Small Frequency Ratio

A dual-band circular microstrip patch antenna, exciting the dominant TM
11
-mode and broadside higher-order TM
12
-mode with a frequency ratio of 1.17, is presented. The patch is loaded with two symmetric arc slits inspired by the surface current distribution of the TM12 mode. This will facilitate elongating the current path in the patch's periphery and reduce the frequency ratio. The unwanted sidelobes in the E-plane radiation pattern of the TM12 mode are consequently eliminated. In addition to the frequency-ratio reduction benefits, the antenna shows stable peak gain of ~8 dBi and a front-to-back ratio of at least ~18 dB in both bands. The results have been validated by prototyping and testing the antenna in practice.