E-Pulse Diagnostics of a Coated Conductor Using a Nonplanar Interrogation Field

When a coated planar conductor is illuminated by a transient plane wave, the late-time scattered field may be written as a series of natural oscillations with frequencies dependent on both the material properties of the coating and the incidence angle of the illuminating wave. Thus, changes in the material properties of the coating may be diagnosed using the E-pulse technique. Using the E-pulse technique with a nonplanar interrogation field is more complicated. Since the incident wave may be viewed in terms of a plane-wave decomposition, with each component having a different angle of incidence, a natural mode series representation of the reflected field may only be approximately valid. However, it is anticipated that as the standoff distance of the source is increased and the incident wavefront becomes more planar, the E-pulse technique will provide adequate capacity to diagnose changes in typical coatings. To investigate this effect, the canonical problem of a coated conductor illuminated by a line source is used to determine the effect of standoff distance on the performance of the technique. It is shown that E pulses created using the natural resonance frequencies found for plane wave illumination may be used in the diagnosis, and that the performance of the technique improves as the standoff distance is increased.