Subwavelength defect detection in reflection and transmission using an electromagnetic jet lens

This paper presents a near-field microwave sensor based on an electromagnetic jet lens, designed to achieve subwavelength detection and to assess spatial resolution in both reflection and transmission. The lens operates at 30 GHz and generates a localized plane-wave field with a full width at half maximum close to the diffraction limit. A benchmark methodology using dedicated test plates is introduced to experimentally quantify the detection and resolution capabilities of the sensor, on both high-contrast metallic features at the surface and low-contrast air voids buried inside a 9 mm dielectric stack. Objective criteria based on amplitude and phase thresholds, spot width, and Rayleigh-like contrast are applied to evaluate its performance. On the metallic targets, isolated features are detected down to 0.3λ in every intensity channel, and down to 0.2λ in the reflected phase, a gain that follows from the cumulative nature of the phase perturbation; on the low-contrast buried voids the limits are coarser and the intensity channel becomes the more sensitive one, so that the phase advantage is shown to depend on the nature of the defect rather than being universal. Two neighbouring features of subwavelength size are separated down to λ/2, in agreement with the measured focal spot, whereas a λ/4 gap between two extended features is still detected, a limit that is set by contrast and not by diffraction. The sensor operates without contact at a standoff of 0.5λ and gives access to both S11 and S21 within a single fully calibrated two-port acquisition, a combination that the resonant near-field probes reported so far do not provide. The compact, two-port measurement configuration provides a robust and low-loss architecture compatible with standard vector network analysers, making it suitable for localized electromagnetic sensing and near-field microwave imaging.