transistors 32 provide periodic grounds to the interrogation coils 30 so that current flow is produced through the coils to cause resonance after the ground is removed the interrogation coils 30 thus produce a magnetic field between the doorways which define the field of detection the magnetic field produced is described in greater detail below detector system 1 is designed to detect the presence of an activated marker within the field of the detector coil such a marker is further described in u s pat application ser no 943 529 to robert h richardson filed sept 18 1978 and now u s pat no 4 222 517 basically when the magnetic marker is in the active state the magnetic field produced by the interrogation coils 30 causes the marker to produce harmonic signals which can be detected by the receiving circuits of system 10 the harmonic signals generated by the marker are received by the detector coils 28 one of which is located within each of the doorways of system 10 the received signals from the doorways is transmitted as channel number one signal through line 60 from a first doorway and channel number two signal through line 62 from a second doorway the received signals for the two channels are transmitted to an amplifier and filter 64 which bandpass filters the signals at the desired harmonic frequency and amplifies the signals to a useable level the amplified and filtered receive signals are transmitted from the amplifier and filter circuit 64 through a line 66 to the detector and logic circuit 26 the channel one and channel two signals are also transmitted through lines 68 and 70 through an inhibit sense circuit 72 the oscillator and sync generator 20 produces a receiver synchronization signal which is.