  Vortex beams carrying orbital angular momentum are extensively studied owing
to its potential to expand channel capacity of microwave and optical
communication. By utilizing the Pancharatnam-Berry phase concept, an
ultra-wideband single layer metasurface is proposed to realize the conversion
from incident plane waves to reflected vortex beams covering a considerable
bandwidth from 6.75 to 21.85 GHz . An equivalent circuit model combined with
broadband phase shift network is developed to effectively design the meta-atoms
in metasurface. It is the first time to design wideband metasurfaces with the
phase-based characteristics. To verify the proposed model, some deformed square
loop meta-atoms are proposed to construct the metasurfaces with broadband OAM
characteristic. Moreover, the vortex beams with the integer (l = -3),
fractional (l = -1.5), and high-order (l = -10) OAM mode are generated
respectively. Based on an OAM spectral analysis, the mode purity of the
generated vortex waves is discussed in detail. The experimental results achieve
a good agreement with those obtained from the simulation, thus proving the
effectiveness and practicability of the proposed method.
