Microwave Orbital Angular Momentum Mode Generation and Multiplexing Using a Waveguide Butler Matrix

In this paper, we propose a convenient microwave orbital angular momentum (OAM) mode generation and multiplexing method operating in the 18 GHz frequency band, based on a uniform circular array and a Butler matrix. The three OAM modes −1, 0, and +1 were generated and verified using spatial S-parameter measurements; the measured back-to-back mode isolation was greater than 17 dB in the full 17 GHz to 19 GHz range. However, the radiated OAM beam centers were slightly dislocated and varied with both frequency and the mode index, because of the non-ideal characteristics of the Butler matrix. This resulted in mode isolation degradation and transmission distance limitations.