The following are patents on security-type metal detectors, primarily the walk-through variety. An early walk-through detector was described in the April 1926 issue of Radio News.
Download	Patent #	Title	Description
434K	US3676772	Metallic Intrusion Detector System	Pass-through type
565K	US3950696	Trapezoidal Coil Configuration for Metal Detector in the Shape of an Inverted U	Walk-through type
1506K	US4012690	Device for Selectively Detecting Different Kinds and Sizes of Metals	Walk-through type
495K	US4605898	Pulse Field Metal Detector with Spaced, Dual Coil Transmitter and Receiver Systems	Walkthrough PI.
242K	US4779048	Metal Detector for Detecting Metal Objects	Pass-through type
763K	US4821023	Walk-Through Metal Detector
1024K	US4866424	Metal Detector Coil	Walk-through type
1183K	US4906973	Walk-Through Metal Detector	White's
284K	US5121105	Metal Detector	Walk-through type
1015K	US5521583	Metal Detection System	Walk-through type, see US5680103
1128K	US5680103	Metal Detection System	Walk-through type, see US5521583
409K	US5726628	Metal Detector System	Walk-through type
650K	US5790685	Apparatus and Method for Detecting and Imaging Metal	Pass-through type
549K	US6133829	Walk-through Metal Detector System and Method	Fisher.
559K	US6696947	Metal Detector	Foldable security walk-through
183K	US6970086	Wide Area Metal Detection (WAMD) System and Method for Security Screening Crowds
256K	EP0611970	Multiple Aerial for Metal Detector	Cylindrical coil for walk-through
850K	WO09742527	Metal Detector System	Walk-through type, with circuit details