High Resolution and Polarization Independent Microwave Near-Field Imaging Using Planar Resonator Probes

Abstract

Various near-field microwave imaging probes were developed previously for nondestructive testing of material structures. Sensitivity and resolution are key parameters for quantifying the efficacy of a given imaging probe. For polarized targets like cracks, the sensitivity of conventional aperture probes depends on the relative orientation between the probe's electric-field vector and the target. In practice, target orientation is typically unknown. Furthermore, the imaging resolution of conventional aperture probes is limited to aperture dimensions. Obtaining imaging resolution in the order of few millimeters using conventional aperture probes requires operation at high frequencies, (above 30 GHz). In contrast, planar probes consisting of loops loaded with spiral resonators could provide resolution in the order of few millimeters while working at relatively low frequencies (below 1 GHz). Unlike conventional aperture probes, the radiated near-fields of these probes are characterized by two significant components oriented along orthogonal axes. Therefore, the probe intrinsically provides a polarization-independent response. In this paper, the near-field imaging resolution of the planar resonator probe is further enhanced, and its polarizationindependent response is demonstrated for the first time. The response of three planar probe prototypes of different dimensions is analyzed based on numerical simulation and experimental results. The performance of the probes is extensively evaluated by acquiring line-scans and 2D-images of metal and dielectric samples. The near-field images of the metal sample obtained using the planar probes working below 1 GHz are compared to the images obtained using aperture probes working at 24 GHz, 33 GHz, and 70 GHz. Furthermore, the probe imaging performance for a polarized target (i.e., narrow cuts) is compared to dualpolarized circular probe working at 24 GHz and open-ended coaxial probe working at 10 GHz. Overall, the proposed probe yielded more than twofold increase in the image signal-to-noise ratio (SNR) compared to all the probes utilized herein. INDEX TERMS non-destructive testing, microwave imaging, planar probe, near-field, polarization, spiral resonator This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.

Detection of Surface Cracks in Metals Using Microwave and Millimeter-Wave Nondestructive Testing Techniques—A Review

“…At zero liftoff, the waveguide is in contact with metal surface (short circuit) and the presence of the crack, once passed through the aperture area, is sensed through the change in the phase of the reflection coefficient [33], [47]. Assuming no losses, the [77] magnitude of the reflection coefficient remains unity in this case [47]. However, at nonzero lift-off and/or when losses are considered, the magnitude and phase both change due to the presence of the crack [47].…”

“…The scanning/imaging lateral resolution depends on the size of the footprint where the probe concentrates the fields (typically in the order of the diffraction limit for the near-field aperture probes). With open-ended waveguide probes, the frequency (i.e., wavelength) dictates the waveguide aperture dimensions [101], which in turn dictates the lateral imaging resolution [53], [58], [77]. It is established that the near-field lateral resolution for aperture probes is half the largest aperture dimension [53], [65].…”

“…This is primarily due to the fact that, in the vicinity of the aperture, the fields are focused within a footprint which is comparable to the aperture size. Table I shows the comparison between the spatial resolution (lateral) obtained using some of the reported near-field aperture probes [77]. To enhance the sensitivity and/or resolution of the near-field waveguide probes, tapering the aperture and loading it dielectrics/resonators were successfully demonstrated as well [56], [57], [58], [59], [60], [61], [62], [63].…”

“…Open-resonators employ inefficient EM radiating elements designed to concentrate the field in the proximity of the main radiating structure to increase the sensitivity and resolution [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82]. These methods were successfully demonstrated to detect 8-and 2-µm-wide fatigue cracks in steel in 1973 [67] and 1975 [68].…”

Crack Detection and Corrosion Mapping Using Loaded-Aperture Microwave Probe

“…Although a resolution of 2 mm was reported in [36] at a relatively lower frequency than the proposed probe, the results were obtained using an array of split ring resonators operating at small standoff distance of 0.6 mm. It is noted here that only a few previously developed probes were designed and optimized for crack detection while no imaging results were presented (except in [24]) to elucidate the sensitivity/practicability of the probe towards any other defect types like corrosion beneath a layer of paint.…”