We describe a step-frequency microwave radar imaging system that is suitable for nondestructive evaluation (NDE) and ground-penetrating radar (GPR) applications. The system includes a computer-automated microwave measurement apparatus along with nonlinear inverse scattering imaging algorithms. Through the use of an inverse Fourier transform, the SFR data is transformed into a synthetic time-domain pulse, and imaging algorithms are applied to the time-domain data. A calibration procedure involving the use of calibration targets is described in order to remove pulse distortions due to the effective aperture and dispersive nature of the antennas, as well as those distortions due to connectors, transmission lines, directional couplers and amplifiers. Reconstructions of various metallic and dielectric scattering objects including metallic rods, glass rods and plastic PVC pipes from real measurement data collected in our laboratory are shown.