The efficiency and effectiveness of airport security can be improved through fast and high-fidelity detection of concealed threat objects. If detected objects can be accurately characterized, the number of false alarms will be reduced, whereas terrorist threats can be more quickly noticed and apprehended. This article aims to accurately characterize the physical properties of weak dielectric objects (such as explosive threats) covering a conductive plane (such as the human body) using a single-frequency millimeter-wave (mm-wave) radar system with a Fresnel aperture. While the conventional synthetic aperture radar (SAR) method requires data over a broad bandwidth, our proposed method works even at a single frequency and is particularly suitable for focused-beam CW mm-wave sensing systems. This method is validated with experimental data.