An improved single waveguide probe technique is presented to nondestructively determine the permittivity and permeability of sheet materials. The impetus for the technique is to address the unreliable results yielded by the existing single probe two-layer method while preserving its physical ease of measurement. Included in this article is the theoretical development of the method and its experimental validation. The theoretical development uses Love's equivalence theorem to derive a magnetic field integral equation that is subsequently solved for the theoretical reflection coefficient using the method of moments. Experimental permittivity and permeability results for two magnetic shielding materials are presented and compared to results obtained using established techniques to validate the proposed method. Profile plots of the electric and magnetic fields in the material under test region of the measurement geometry are provided and analyzed to yield further physical insight.

A two-iris waveguide-probe technique is introduced for measuring the electromagnetic properties of a lossy conductor-backed material layer. A flanged open-ended rectangular waveguide is applied to the material under test, and the reflected signal is measured under two conditions. The reflection is first measured when the aperture of the waveguide is unobstructed; then, the reflection is measured with an iris placed in the aperture of the guide. These two measurements allow the extraction of both the permittivity and permeability of the material. The theoretical reflection coefficient necessary to perform the extraction is obtained using a rigorous full-wave approach combining a modal expansion in the waveguide and iris regions with a magnetic-field integral equation formed using equivalent currents at the waveguide aperture. The optimum iris size is determined by minimizing the propagated error due to instrumentation uncertainty and by comparing the extracted parameters to those found using a two-thickness method. Measurements of a commercially available magnetic radar-absorbing material demonstrate the feasibility of the two-iris approach.

Waveguide probes are useful for extracting the electric and magnetic properties of material layers, but the accuracy of the results is limited by the accuracy of the theoretical model. Using more modes in the expansion of the waveguide fields produces better results, but the computational cost increases with the number of modes squared. This letter analyzes the dependence of solution accuracy on the number of modes used and, based on this, introduces an extrapolation technique that allows the number of modes to be significantly reduced with little loss of accuracy.

A two-layer nondestructive method for characterizing the electric and magnetic properties of lossy conductor-backed magnetic materials using a ∞anged rectangular-waveguide probe is examined. The two re∞ection measurements necessary to determine both permittivity and permeability are made by flrst applying the probe to the material under test and then applying the probe to a known-material layer placed on top of the material under test. The theoretical re∞ection coe-cient is obtained using a rigorous full-wave solution, and an extrapolation scheme is used to minimize the error due to truncating the modal expansion of the waveguide flelds. An error analysis is performed to compare the performance of the technique to the two-thickness method, which utilizes two difierent thicknesses of the material under test. The properties of the known material layer that result in the least error due to network analyzer uncertainty are determined. The sensitivity of the two-layer method is also explored and discussed.

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.

A full-wave solution for material parameter extraction which uses two independent measurements is presented. When the addition of random noise on the measurements is considered, the estimates of the propagation of those errors greatly reduces the confidence in the two-layer results. Future work shall investigate the two-layer method using different thicknesses and types of material for the top layer.