28-GHz-Band Loop Antenna Arrays Loaded with Varactor Diodes for OAM Beam Steering

A circular loop antenna array loaded with varactor diodes has been developed for orbital angular momentum (OAM) beam steering. In the planned OAM beam steering system, the loop antenna arrays are placed close to each other and switched. Here, an interference issue occurred between loop antennas of the same radius. To prevent that, varactor diodes were implemented at the maximum current points of the loop antennas to vary the equivalent loop circumference. In addition, the varactor diode on-state was utilized as a resistive-loss component. In an evaluation of the short-range transfer experiment by using a 28-GHz-band loop antenna array with modes 1, 3, 5 and 7 to confirm the effect of the proposed antenna, transfer suppressions of 3.1, 13.4, 12.1 and 11 dB, respectively, were observed by switching the diode states. In the long-range transfer evaluation employing closely placed loop antenna arrays with modes 1 and 3, the assumed interference was successfully reduced by 7 dB through diode switching.