Compact circularly polarised antenna utilising the radiation of the ground plane based on the theory of characteristic modes

This research proposes a single-fed circularly polarised (CP) antenna operating at 2.4 GHz for wireless local area network applications by utilising ground radiation, which is particularly applicable to small-size printed circuit boards (PCBs) such as the Internet of Things (IoT) devices. To simultaneously excite two orthogonal degenerate ground modes for CP wave radiation, ground mode tuning (GMT) technique is further adopted to adjust the resonant frequencies of the two ground modes. The proposed small antenna element is located at an optimum position of the PCB based on the analysis of the theory of characteristic modes for simultaneously exciting two ground characteristic modes. Experiments verify good performances with the compact size of 0.28 ${\rm \lambda }_0$lambda 0 x 0.28 ${\rm \lambda }_0$lambda 0 x 0.008 ${\rm \lambda }_0$lambda 0 (including the ground plane and antenna element) at 2.45 GHz. The proposed antenna achieves 3 dB axial ratio bandwidth of 10.6% (2.36-2.62 GHz) and -6 dB impedance bandwidth of 7.4% (2.34-2.52 GHz). The measured efficiency of the proposed antenna is 55.3%, and the direction of the maximum radiation for LHCP and RHCP is + z and - z, respectively. The measured gain is 1 dBi at 2.45 GHz. The proposed antenna applies to small-size PCBs for CP communication, which is desirable for practical wireless products.