Appropriate impedance detectors placed within an antenna impedance-matching unit can facilitate rapid automatic matching. However, the operation of the detectors normally requires a substantial r.f. signal to be passed through the matching unit and this can cause undesirable radiation from the antenna. A novel impedance detection technique is described which operates with such low r.f. signals that the resulting transmission cannot be detected beyond about one wavelength from the antenna. The method determines whether or not the impedance under test lies within a specified region of the complex impedance plane; the boundary of this region is set by the modulation levels applied to the injected signals and it can be made to delineate the resistance, conductance, modulus, v.s.w.r. and phase thresholds shown to be required.