Sub-Wavelength Measurement of Electromagnetic Inhomogeneities in Materials

Microwave microscopes that measure surface impedance or roughness have been demonstrated with fine spatial resolutions of less than a micron. These microwave probes are practical only for samples less than a few inches in size. However, composite materials, in applications such as multi-layer radomes, embedded frequency-selective surfaces, or integrated EMI shielding, have larger-length-scale features embedded within a multilayer laminate. Diagnosing larger-scale, subsurface features, such as joints/seams, periodic elements, imperfections, or damage, is driving a need for methods to characterize embedded electromagnetic properties at mm- or cm-length scales. In this research, finite-difference time-domain (FDTD) simulations and experimental measurements were used to investigate a probe technique for measuring sub-wavelength-sized features embedded within a dielectric composite. For these applications, the probe interacted with the sample material via both evanescent and radiating fields. A dielectrically loaded, reduced-size, X-band waveguide probe was designed in a resonant configuration for improved sensitivity. Experimental measurements demonstrated that the probe could characterize small gaps in ground planes embedded within a dielectric laminate. Simulations also demonstrated the possibility of detecting more-subtle imperfections, such as air voids.