Far-Field to Near-Field Transformations for Antenna, Free-Space Waveguide, and Scatterer

The energy-viewpoint-based methods for calculating the energy-decoupled modes (DMs) of antenna, free-space waveguide, and scatterer are simply reviewed. Some far-field to near-field transformation methods for antenna, free-space waveguide, and scatterer are proposed by using the DM-based modal expansions. The DM-expansion-based far-field to near-field transformation methods have the following two main advantages over the conventional multipole-expansion-based far-field to near-field transformation method: 1) the former can calculate the fields inside the circumscribing spheres of antenna and scatterer, but the latter cannot; 2) the former does not have any restriction on the shape of the input port of free-space waveguide, but the latter is only applicable to the free-space waveguide whose input port is a spherical surface.