A Radio-frequency Measurement System for Metallic Object Detection Using Pulse Modulation Excitation

Guns and knives have imposed a significant threat to public safety. Based on the electromagnetic characteristics of the materials of guns and knives’, techniques concerning Electromagnetics (EM) e.g. walk-through metal detection equipment and X-ray and THz screening system have been used in an effort to detect and characterise them. Different EM frequencies for metallic object detection have demonstrated different merits. This paper reports the ongoing investigation of RF measurement for metallic object detection using pulse modulation excitation which consists of multiple frequency harmonics with the centre frequency varying within 1-12 GHz. The frequency response and resonance frequency behaviour of object position, object shape, rotation and multiple objects have been tested and analysed. A system utilising the pulse-modulated RF mode and sweep-frequency mode has been set up to implement sensing of metal items at a standoff distance more than 1 meter. Through a series of experimental investigations, it can be found that the positions of the metal items have significant effect on the spectrum profiles of received RF signals. Such effect presents a modulation pattern in frequency domain, in consequence of which new features have been investigated in order to implement detection and characterisation of metallic objects.