  The topological charge of an electromagnetic vortex beam depends on its
wavefront helicity. For mixed vortex beams composed of several different
coaxial vortices, the topological charge spectrum can be obtained by Fourier
transform. However, the vortex beam is generally divergent and imperfect. It
makes it significant to investigate the local topological charges, especially
in radio frequency regime. Fourier transform based methods are restrained by
the uncertainty principle and cannot achieve high angular resolution and mode
resolution simultaneously. In this letter, an analysis method for local
topological charges of vortex beams is presented based on the empirical mode
decomposition (EMD). From EMD, the intrinsic mode functions (IMFs) can be
obtained to construct the bases of the electromagnetic wave, and each local
topological charge can be respectively defined. With this method the local
value achieves both high resolution of azimuth angle and topological charge,
meanwhile the amplitudes of each OAM modes are presented as well. The
simulation and experimental results confirm the validity of the EMD based
method.
