Determination of surface currents by backpropagation of field measurements

Knowledge of surface currents is useful for evaluating antenna performance and tailoring the scattering characteristics of objects. For surfaces where these currents cannot be easily calculated, it is useful to have a technique for surface current measurement that does not require modification of the object. Previously, we described a measurement system for this purpose and simulated its performance using the finite-difference time-domain (FDTD) method. In this system, the magnetic field is measured on a planar surface a distance s in front of the object supporting the currents. These measurements are back propagated using a plane wave spectrum approach to obtain an estimate of the current. Parametric studies were performed to determine the effects of spacing s, sampling density, scan area, probe size, and signal to noise ratio (SNR) on the performance of the system. Only preliminary measurements were available at that time. Since then, more extensive measurements have been performed using an improved probe design, namely, a shielded loop with two feed points. This paper presents the results of the new measurements as well as details of a novel, local backpropagation approach.