A Characteristic-Mode-Based Polarization-Reconfigurable Antenna and its Array

This paper reports a novel polarization-reconfigurable antenna based on the theory of characteristic mode (CM), which can electronically alter its polarization states between linear polarization (LP), left-hand circular polarization (LHCP), and right-hand circular polarization (RHCP). A square patch with an I-shaped slot is used as a main radiator, which has two LP orthogonal CMs with the same modal significance and 90° characteristic angle difference at 2.4 GHz. A simple feeding network is designed to excite the different combinations of the two modes. By properly choosing the state of the excitation, the LP, LHCP, and RHCP waves can be generated. A <inline-formula> <tex-math notation="LaTeX">$1 \times 4$ </tex-math></inline-formula> antenna array is designed for the demonstration of array application, and experiments are carried out to validate the proposed design. The measured overlapping impedance bandwidth (BW) (<inline-formula> <tex-math notation="LaTeX">$\vert \text{S}_{11}\vert \le -10$ </tex-math></inline-formula> dB) and 3-dB gain BW is 2.25–2.7 GHz in the LP state. The overlapping BWs in conjunction with the 3-dB axial ratio BW are 2.16–2.65 and 2.1–2.55 GHz in the LHCP and RHCP states, respectively. The measured maximum gains are 7.2 dBi, 9.78 dBic, and 9.8 dBic for the LP, LHCP, and RHCP states, respectively. The common BW for all three polarization states is 2.25–2.55 GHz or 12.5%. Experimental results show a good performance of the proposed antenna array.