Reducing Interference in MIMO Systems Using a Small Metamatrial Balun

In this paper, we propose a method to handle the isolation problem of a MIMO antenna for Wi-Fi access point(AP). Firstly, a quasi-Yagi antenna is designed with a transmission-line balun at the feed. Secondly, this conventional quasi-Yagi antenna is adopted for the two closely placed MIMO elements for 2.4 GHz-AP equipment, and the coupling between them is checked. The 180° delay-line of one element is long and next to the feed of the other element, which is an obstacle to the isolation improvement. Hence, the conventional balun is replaced by right and left-handed phase shifters, which take up a much smaller area of the feed and result in improved isolation. In addition, the MIMO radiating elements based on the metamaterial phase-shifters show less inclination of the main beam from the intended direction. The MIMO quasi-Yagi antennas without and with the metamaterial phase-shifter balun are designed, and their return-loss, beam-patterns and isolation curves are compared. Also, the full-wave simulation results are given with the measured ones.