For the application of electro-magnetic (EM) wave with orbital angular momentum (OAM), which is also called the vortex beam, it is essential to determine the real OAM mode of the transmit antenna, i.e., accurately measure the OAM mode of the manufactured antenna with systematic error. The traditional methods measure the OAM mode based on the OAM far-field approximation or the phase gradient in the transverse plane. The corresponding performance degrades when alignment error is not negligible or OAM modes increases. In this paper, an accurate OAM measurement of EM wave based on rotational antenna is proposed. Specifically, the EM beam with helical phase fronts can be well measured via frequency shift detection by rotating the OAM wave at the transmitter. The accuracy can be greatly improved compared with the traditional ways.
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