A directional concealed object locator, say metal detector, employing a beat frequency oscillator and at least two search oscillators. The search oscillators each include a loop antenna which in part serves to determine the search frequency signal rate of each oscillator and functionally serves to produce...http://www.google.de/patents/US4255710?utm_source=gb-gplus-sharePatent US4255710 - Plural search frequency directional metal detector apparatus having enhanced sensitivity
