Detectors 167
Detectors 167 : Particle Detectors: (4.18) Magnetic Detector: (2) How it works: That radio signal in the excitation coil aids the reversal of the magnetization of the iron band as it passes under the audio pickup coil. This works by agitating the magnetic domains to reduce reluctance in the iron band. Changes in the amplitude of the radio signal change the rate of magnetic reversal in the moving iron band. This causes variation in the magnetic flux through the audio pickup coil causing the current in the audio pickup coil to vary. The audio pickup coil is connected to a telephone receiver which converts the varying current to sound. From a technical standpoint, several subtle prerequisites are necessary for operation. The strength of the magnetic field of the permanent magnets at the iron band must be of the same order of magnitude as the strength of the field generated by the radio frequency excitation coil, allowing the radio frequency signal to significantly contribute to the total the magnetic field. Also, the radio signal applied to the excitation coil roughly matches the impedance of the excitation coil which is very low and requires a special tuner design considerations. The impedance of the telephone earphone must roughly match the impedance of the audio pickup coil, which is a few hundred ohms. The iron band moves a few millimeters per second. Its sensitivity does not approach that of a good mineral detector, yet it is a reliable and surprisingly sensitive detector for radio
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