Metal lens for collimation of orbital angular momentum radio modes

We investigate the feasibility of using a metal lens for collimating Orbital Angular Momentum (OAM) radio modes. In the microwave regime, metal lenses are an attractive alternative to dielectric lenses because of their low weight and low cost. However, it is not immediately clear whether an OAM mode would successfully propagate through a metal lens due to the presence of parallel metal plates that block the transfer of energy in the plane transverse to propagation. We explore by simulation the coupling of an OAM mode from a uniform circular dipole array of diameter 180mm to a planoconvex lens of plate spacing 50 mm, focal length 1100mm and maximum thickness 800 mm, and then its re-radiation into free-space. A multi-level fast multiple method was used for the calculation. Our results indicate metal lenses can be used with OAM modes with minimal loss of mode purity.