An orbital angular momentum (OAM) detection approach at microwave band is proposed. A transmittance function is exploited to model a transmissive metasurface. Then the metasurface is designed to convert an OAM wave to multiple waves, only one of which is gaussian. The radiation direction of the gaussian wave is distinguishable according to the order of incident OAM. Consequently, by locating the gaussian wave, the incident OAM can be conveniently determined. We use a simple field source to simulate the incident OAM wave in fullwave simulation. It largely simplifies the simulation process when an incident wave carrying OAM is needed. Both numerical and full-wave simulation results are provided to validate our design and they show a good agreement with each other. Then, the metasurface is optimized for high directivity. Our work can provide an efficient and effective way for OAM detection in radio communications.
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