A system for unobtrusive measurement of surface currents

A backpropagation technique is investigated that uses the plane wave spectrum to reconstruct the subwavelength features of the current distribution on flat surfaces without damaging the surface under measurement. The technique employs spectral and spatial filtering of the measured signal to capture the propagating spectrum as well as a portion of the evanescent spectrum. Special consideration is given to avoiding exponential amplification of measurement noise during backpropagation of the evanescent spectrum. This approach uses equipment commonly available in antenna measurement laboratories, for example, a network analyzer, a horn antenna, a magnetic field probe, and an automated positioning apparatus for the probe. The approach is first studied using computer simulations. The results from the simulations are then used as a guide to develop an actual measurement system. The measurement system is constructed and tested by measuring the surface current on a slotted plate. The measured current distributions are compared with results from finite-difference time-domain (FDTD) computations to demonstrate the feasibility of the technique.