Techniques for imaging a search region to detect a dielectric target include placing multiple electrodes outside the search region. At least two electrodes are activated independently of each other. Emitter circuits are connected to corresponding emitter electrodes. Each emitter circuit is configured for using its emitter electrode to produce an electric field with wavelength longer than about one hundred meters. Receiver circuits are connected to corresponding receiver electrodes. Each receiver circuit is configured for using its receiver electrode to measure a property of an electric field produced, at least in part, by an emitter electrode. A processor determines a property of a dielectric target inside the search region based on measurements from the receiver circuits using a subset of the emitter circuits. Among other uses, these techniques allow humans to be detected inside building-size regions, even when hidden by visually opaque blocking material of small dielectric constant.