We can relate the concept of equivalent currents to physical optics. In this case we generate an artificial surface that covers a source of radiation. The incident fields generate surface electric and magnetic current whose radiation cancels the internal fields and generates the external pattern. We use these at the apertures of antennas such as horns. By using the dyadic Green’s functions we can calculate the near-field patterns and the coupling between antennas when the assumption is made that the presence of a second antenna does not alter the aperture fields. Given the outward normal ˆn, we calculate the equivalent currents by
 (1)We must use both electric and magnetic current densities on the surface to replace the internal fields. If the ratio of the electric field to the magnetic field equals the impedance of free space (376.7 ), the combination of the two currents produces the radiation of the Huygens aperture source when used with the dyadic Green’s function. We use equivalent currents for a variety of analyses over flat apertures such as horns and paraboloidal reflectors, but they can also be used with curved structures or apertures.
We can, for example, use equivalent currents for calculation of the effects of radomes. Locally, we assume that the incident waves are plane waves and use boundary conditions to calculate reflected and transmitted waves. It is necessary to separate the incident wave into parallel and perpendicular polarizations. These polarizations have differing reflection and transmission coefficients. We generate one surface on the inside of the radome and another on the outside. We use locally free-space waves for the reflected and transmitted waves lying outside the radome. Both these waves can be replaced with equivalent currents. The equivalent currents produce null fields inside the radome when combined with the incident wave radiation. Including these equivalent currents in a PO analysis, we add the effect of the radome.
Equivalent currents can also be used with lenses. We use the incident waves combined with the idea of locally plane waves to calculate reflected and transmitted waves at each surface and replace them with equivalent currents. We include the dielectric constant of the lens in the dyadic Green’s functions for the internal radiation of the lens to calculate the fields at the second surface. We apply locally plane waves at the second surface to determine the transmitted and reflected rays and then replace them with equivalent currents. Because the lens has internal reflections, we need to apply an iterative PO analysis to calculate the multiple reflections between the two surfaces. The method converges rapidly because the internal reflections are small. |
