The multiple-input and multiple-output (MIMO) characteristic modes of the double ring circular array antenna were calculated, and it was confirmed that excitations equivalent to the orbital angular momentum (OAM) modes could be obtained. By calculating the array factors with the MIMO eigenvectors as the weighting coefficients, uniform excitation was obtained as the principal mode, and the mode in which the phase was rotated by an integer multiple of 360° in the circumferential direction was obtained as the higher-order modes. In addition, a non-null beam with a non-uniform distribution, which was not obtained in a single ring circular array, was observed. Non-null beams were observed in circular arrays with two or more rings even when the distance between transmission and reception, the number of elements, and the element arrangement were changed.
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