A wideband monopole antenna with ring-shaped ground plane

A novel design for monopole antennas with wideband operation and stable radiation patterns is described. The proposed monopole antenna has a three-dimensional ring-shaped ground plane, and it can be treated as a monopole-loop composite antenna. Two pure resonant modes are simultaneously excited in the antenna structure, and their resonant frequencies are respectively at 2.45 and 5.4 GHz. By introducing a pair of shorting posts into the ground plane, the two resonant modes can be coupled together. The resultant impedance bandwidth (VSWR<2) is about 70% with respect to the center frequency 4.5 GHz. Moreover, a stable bi-directional radiation pattern with a peak gain of 5 dBi and a cross polarization level of less than -15 dB is obtained within the impedance bandwidth. For the proposed antenna, parametric analyses are performed and an antenna prototype is also constructed. Both measured and simulated results are given.