Near-Field Probes for Subsurface Detection Using Split-Ring Resonators

Most of the previous microwave near-field probes and imaging techniques focused on surface imaging, providing ultrahigh lateral resolution. Few microwave near-field probes were developed for subsurface detection, offering both high lateral and depth resolution with varying degrees of effectiveness. In this work, a novel microwave near-field probe using a single split-ring resonator is introduced with the primary focus of subsurface detection. The design is simple, compact, inexpensive, and easy to fabricate using printed circuit board technology. Fourier spatial analysis of the field of the new probe reveals a substantial enhancement of the evanescent field, thus making a significant difference in subsurface detection. Experimental results illustrate that a small 3.24-mm aluminum block immersed in 1% sodium chloride (NaCl) solution and positioned 4 mm away from the surface was successfully detected using a probe operating at 1.218 GHz. For this particular experiment, where the size of the object was λ/74 , the detection ability of the new probe was tested using 2% and 3% NaCl solution as well. The phase changes due to the depth of the object demonstrate that the new probe is able to sense the presence of the same object in very lossy medium (3% NaCl whose loss tangent is approximately unity) with depth of 1-2 mm in spite of a standoff distance of 1-mm air and a container thickness of 6.35 mm.