Abstract: Apparatus, primarily for the short-range detection of metallic objects, includes a sensing head formed by a pair of strip transmission lines located in close proximity to one another and in association with a ground plane. Pulse generating means is arranged to apply pulses to one end of one strip-line, and pulse receiving means is connected to the corresponding end of the other strip-line for detecting an output pulse induced in the line. The strip-lines are terminated by their characteristic impedances, and output pulses from the said other line are characterized by any change in coupling between the lines due to the interposition between them of a material having a dielectric constant different from that of the medium surrounding the metallic objects.