Abstract: An apparatus is described for detecting metal objects in an interrogated environment. An alternating magnetic signal is transmitted, and received by a receiving coil. The signal is amplified, and is then synchronously demodulated based on a transmitted frequency. The synchronously demodulated signals are low-pass filtered, and then changes in the amplitude of these low-pass filtered signals are detected. The outputs are cross-correlated and interrograted, and are used to control a controlled phase shifter. The output of the controlled phase shifter is used as a reference input to a first one of the synchronous demodulators, and a shifted transmitted signal is used as a reference to the second synchronous demodulator.

Abstract: A sensing head for a metal detector including means for magnetic transmission and detection of a retransmitted signal from a target by means of at least one coil, the head being characterized in that at least some of the conductor material forming the coil and providing for passage of current through the coil is such as to provide for such passage wherein the arrangement is such or the selection of conductor material is such as to restrict the magnitude of eddy currents in the conducting material. Coils wound from wires in parallel, and in particular, Litz wires, are disclosed.