| Summary: | In this paper, an Archimedean spiral antenna has been developed to flexibly radiate orbital angular momentum (OAM) wave with tunable modes. The operation mechanism for generation of the OAM wave is elaborated in terms of phases of the currents flowing on the spiral arm of the Archimedean structure. Two pairs of feeding probes are introduced to excite the Archimedean spiral antenna. With the signals with different phase differences fed into the spiral antenna, the OAM waves with the modes of <inline-formula> <tex-math notation="LaTeX">$l=\pm 1$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$l=\pm 2$ </tex-math></inline-formula>, and their all mixed modes are radiated, respectively. Simulated and measured results are given to demonstrate that the proposed Archimedean spiral antenna radiates the tunable OAM waves from the sole mode to the mixed four modes in an overlapped band of 5~6 GHz.
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