A Three-Mode OAM Antenna with a Butler Matrix Feeding Network

A $2\times2$ uniform circular array (UCA) consisting of four patches fed by a Butler matrix is proposed for simultaneously generating three OAM modes. Compared with other patched UCAs with feeding networks on the top layer, individual elements of the proposed structure are fed by a matrix on the backside. The feeding network designed on the lower layer is comprised of five 90° hybrid couplers, one cross couplers, and two 30° phase shifters. Both results of the simulation and the measurement illustrate that the proposed antenna can generate three OAM modes, which works at 7.5GHz and whose bandwidth of return loss below -10dB is 10.7%.