Frequency dependent pulsed gain modulated metallic object detector

Abstract

A metal detector apparatus particularly adapted as a locator of small, usually obscure, isolated objects such as treasure items, antipersonnel mines, weapons, shrapnel, and other such items. The detector is of the portable transmitter-receiver type having a dual balanced receiving loop, together with a transmitter loop co-balanced relative with the receiving loop arrangement, whereby the unique loop sensing setup, together with a dual channel receiver, provides a separate 'right' and 'left' bearing indication to the operator of the obscure object's whereabouts. The imbalance between the normally balanced loop arrangement electromagnetic fields, brought about by the nearby presence of a metallic object acts to produce a relative indicative difference in each receiving channel which provides the bearing, while the relative combined indication magnitude gives an indication of proximate distance between the detector and the sought object. The detector provides an automatic, digitally advanced, sequential step-like change in the overall receiver sensitivity through a range of discrete values of responsiveness. Each discrete response value recurs at a related, finite repetition rate which results in an overall characteristic stepped swept gain effect, thereby producing instantaneous variegated values of signal level which are detected at finite threshold levels and thereby usually adapted into a pair of audio frequency tones which, when coupled to an operator's ears, serve to give a stereotonic effect with the average pitch indicating distance, while the pitch difference indicates bearing.