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Metal Detector Patents

Frequency Shift
US3355658 (515K) Differentiating Metal Detector for Detecting Metal Objects and Distinguishing Between Detected Diamagnetic and Non-Diamagnetic Objects Gardiner, VLF BFO
US3467855 (635K) Object Detector and Method for Distinguishing Between Objects Detected Including a Pair of Radio Oscillators Basic BFO
US3519919 (229K) Frequency Stabalizing Element for Metal Detectors
US3546628 (329K) Oscillating Metal Object Detector Simplest detector on earth
US3601691 (313K) Metal Detector Responsive to Small Metallic Objects for Differentiating Between Ferrous and Non-Ferrous Objects Gardiner, Z-response
US3626279 (522K) Metal Detector Utilyzing Radio Receiver and Harmonic Signal Generator Harmonic technique increases sensitivity
US3662255 (801K) Apparatus for Locating Concealed or Buried Metal Bodies and a Stable Inductor Usable in Such Detectors Garrett Zero-Drift BFO
US3742341 (582K) Inductively Coupled Metal Detector Arrangement Appears to be an off-resonance technique
US3823365 (1096K) Metal Detecting Apparatus Having Improved Ground-Effect Immunity Uses induction balance
US3875498 (529K) Metal Detector for Distinquishing Between Precious Metal Objects and Other Metals D-Tex patent, your real basic BFO.
US3896371 (902K) Metal Detector With a Resonating Circuit Being Driven by a Frequency Higher Than Its Natural Resonance Frequency A.H. Electronics, off-resonance type (AH Pro)
US3961238 (345K) Selective Metal Detector Circuit Having Dual Tuned Resonant Circuits Gardiner, very basic Z-response
US3986104 (621K) Dual Frequency Metal Detector System Gardiner
US4130792 (694K) Metal Detector With Feedback Tuning Z-response with feedback
US4196391 (1202K) Metal Locator with Stereotonic Indication of Translateral Position Stereo BFO.
US4204160 (516K) Metal Detector With Automatic Optimum Sensitivity Adjustment
US4255710 (1276K) Plural Search Frequency Directional Metal Detector Apparatus Having Enhanced Sensitivity Uses two gated oscillator frequencies (I think).
US4263553 (779K) Discriminating Metal Detector With Compensation for Ground Minerals A.H. Electronics, off-resonance detector
US4321539 (1232K) Digital BFO Metal Detecting Device with Improved Sensitivities at Near-Zero Beat Frequencies
US4439734 (1134K) Metal Object Locator Including Frequency Shift Detector Induction balance and ground balance.
US4678992 (925K) Electronic Metal Detector A.H. Electronics, off-resonance type (Super Pro?)
US5025227 (148K) Metal Detection Circuit Appears to be off-resonance
US7068028 (117K) Method and Apparatus for Metal Target Proximity Detection at Long Distances Appears to be a variation of energy theft, which is akin to frequency shift methods.

Induction Balance
US3405354 (1710K) Apparatus for Limiting Phase-Angle Response Range, Particularly in Eddy Current Testing Apparatus Uses syncronous demodulation to determine target phase, see 3848182.
US3471772 (874K) Instrument for Measuring the Range and Approximate Size of Buried or Hidden Metal Objects Synchronous detectors, lots of fundamental theory & equations.
US3471773 (436K) Metal Detecting Device with Inductively Coupled Coaxial Transmitter and Receiver Coils Basic TR with 4B-style coil (NOT coaxial!).
US3686564 (1061K) Multiple Frequency Magnetic Field Technique for Differentiating Between Classes of Metal Objects Multi-frequency detector from 1972.
US3826973 (1282K) Electronic Gradiometer Actually, a metal detector.
US3835371 (1330K) Apparatus for Detecting the Presence of Electrically Conductive Material Within a Given Sensing Area Focuses on submersible probe & cabling method
US3848182 (728K) Apparatus for Limiting Phase-Angle Response Range, Particularly in Eddy Current Testing Apparatus See 3405354.
US3872380 (484K) Metal Detector Distinguishing Between Different Metals by Using a Bias Circuit Actuated by the Phase Shifts Caused by the Metals Gardiner, basic phase response IB.
US4016486 (664K) Land Mine Detector with Pulse Slope, Width and Amplitude Determination Channels
US4024468 (1347K) Induction Balance Metal Detector with Inverse Discrimination White's TR Discriminator (1975), includes good description with phase diagrams and circuitry with component values.
US4030026 (2003K) Sampling Metal Detector White's, the basis of most of their early-80's analog detectors.
US4053828 (683K) Metal Detector with First and Second Nested Rectangular Coils Not too practical for a handheld.
US4096432 (319K) Metal Detector for Discriminatory Detection of Buried Metal Objects Basic phase response IB.
US4099116 (840K) Metal Detector With Phase Related Selective Discrimination Circuit Nautilus, feedback method for ground balance and discrimination.
US4110679 (1271K) Ferrous/Non-ferrous Metal Detector Using Sampling White's
US4128803 (779K) Metal Detector System With Ground Effect Rejection PNI (the old Bounty Hunter), probably the Red Baron series (RB3/5/7).
US4249128 (1021K) Wide Pulse Gated Metal Detector With Improved Noise Rejection White's
US4263551 (1277K) Method and Apparatus for Identifying Conductive Objects by Monitoring the True Resistive Component of Impedance Change in a coil System Caused by the Object Title says it all, looks at target response vs. frequency
US4300097 (882K) Induction Balance Metal Detector with Ferrous and Non-ferrous Metal Identification Techna, now First Texas Mfg. (Bounty Hunter)
US4303879 (1576K) Metal Detector Circuit with Mode Selection and Automatic Tuning 1979 Garrett patent (ADS?)
US4325027 (1352K) Metal Detector for Locating Objects with Full Sensitivity in the Presence of Distributed Mineral Material Compass
US4334191 (1176K) Metal Detector Circuit Having Momentary Disabled Output Garrett
US4334192 (1267K) Metal Detector Circuit Having Automatic Tuning With Multiple Rates Garrett, probably their Master Hunter VLF.
US4344034 (1218K) Selective Ground Neutralizing Metal Detector Gardiner patent for phase discriminator.
US4348639 (1008K) Transmitter-Receiver Loop Buried Metal Object Locator with Switch Controlled Reference Vooltage Discovery Electronics' two-box detector.
US4423377 (876K) Compact Metal Detector of the Balanced Induction Type Garrett, handheld with integrated double-D coil. See also 4488115.
US4470015 (3160K) Metal Detector System With Undesirable Target and Mineralized Ground Discrimination Teknetics, lots of diagrams and waveforms.
US4486712 (941K) Frequency Dependent Pulsed Gain Modulated Metallic Object Detector
US4486713 (1210K) Metal Detector Apparatus Utilizing Controlled Phase Response to Reject Ground Effects and to Discriminate Between Different Types of Metals Tesoro, lots of signal & phase diagrams
US4488115 (758K) Low Battery Voltage Indicator Circuit for a Metal Detector Garrett, see also 4423377
US4507612 (6059K) Metal Detector Systems for Identifying Targets in Mineralized Ground Teknetics, tons of info.
US4514692 (734K) Metal Detector and Discriminator Using Differentiation for Background Signal Suppression Fisher, basis for early X models.
US4563645 (337K) Inductively Balanced Oscillatory Coil Current for Metal Detection
US4628265 (1126K) Metal Detector and Classifier with Automatic Compensation for Soil Magnetic Minerals and Sensor Misalignment Fisher
US4659989 (654K) Inductively Balanced Metal Detector Circuit with Orthogonal Balancing Signals and Including Phase and Polarity Detection
US4677384 (1728K) Target-Identifying Metal Detector Teknetics
US4700139 (1235K) Metal Detector Circuit Having Selectable Exclusion Range For Unwanted Objects Garrett patent with a pretty good explanation of I&Q signal processing.
US4709213 (2644K) Metal Detector Having Digital Signal Processing Garrett
US4783630 (1073K) Metal Detector With Circuits for Automatically Screening Out the Effects of Offset and Mineralized Ground White's
US4868910 (2838K) Metal Detector with Microprocessor Control and Analysis White's, ton's of info on their target ID.
US4881036 (352K) Phase Shift Compensation for Metal Detection Apparatus
US4894618 (1325K) Metal Detector Using Cross-Correlation Between Components of Received Signals Minelab, good explanatory text.
US4912414 (1055K) Induction-Type Metal Detector with Increased Scanning Area Capability Describes a large array for use with a submersible, includes IB and PI methods.
US4942360 (1275K) A Method and Apparatus of Discrimination Detection Using Multiple Frequencies to Determine a Recognizable Profile of an Undesirable Substance Minelab, multiple frequency.
US4975646 (325K) Detector System for Recognizing a Magnetic Material Multi-frequency
US5148151 (1317K) Metal Detector Having Target Characterization and Search Classification Garrett patent on VDI, explanantion on phase response and VDI flowchart.
US5506506 (1669K) Metal Detector for Detecting and Discriminating Between Ferrous and Non-ferrous Targets in Ground Minelab, combines synchronous demodulators with PI, calls it multi-frequency. Compare with US5576624
US5523690 (1055K) Metal Detector with Bivariate Display White's patent detailing VDI, includes 6805 assembly code. (see also 5596277)
US5596277 (959K) Method and Apparatus for Displaying Signal Information from a Detector White's patent detailing VDI, includes 6805 assembly code. (see also 5523690)
US5642050 (2199K) Plural Frequency Method and System for Identifying Metal Objects in a Background Environment Using a Target Model White's multi-frequency detector (see 5654638).
US5654638 (2306K) Plural Frequency Method and System for Identifying Metal Objects in a Background Environment White's multi-frequency detector (see 5642050).
US5691640 (980K) Forced Balance Metal Detector
US5721489 (690K) Metal Detector Method for Identifying Target Size Garrett's target ID patent, lots of signal analysis and flow charts. See also 5786696.
US5729143 (502K) Metal Detector With Nulling of Imbalance Bucking signal calibrated for amplitude & phase
US5786696 (656K) Metal Detector for Identifying Target Electrical Characteristics, Depth and Size Garrett's target ID patent, lots of signal analysis and flow charts. See also 5721489.
US5969528 (559K) Dual Field Metal Detector Garrett's two-box add-on.
US6172504 (922K) Metal Detector Target Identification Using Flash Phase Analysis White's, applies phase info to what's essentially a flash ADC ; see 6421621.
US6421621 (716K) Metal Detector Target Identification Using Flash Phase Analysis White's, applies phase info to what's essentially a flash ADC ; see 6172504.
US6583625 (723K) Metal Detector and Method in Which Mineralization Effects Are Eliminated
US6879161 (132K) Method and Apparatus for Distinguishing Metal Objects and Employing Multiple Frequency Interrogation White's
US6911823 (168K) Metal Detector Employing Static Discrimination White's
US7078906 (105K) Simultaneous Time-Domain and Frequency-Domain Metal Detector Combo PI & phase sampling
US7088103 (106K) Metal Detector Having a Plurality of Phase Discrimination Regions with Corresponding Selectable Exception Spaces Therein White's
US7126323 (193K) Systems and Methods for Synchronous Detection of Signals
US7432715 (227K) Method and Apparatus for Metal Detection Employing Digital Signal Processing Minelab
US7579839 (44K) Metal Detector Minelab, multi-frequency demod technique
EP0580396 (738K) Metal Detector with Display White's VDI

Pulse Induction
US3707672 (388K) Weapon Detector Utilyzing the Pulsed Field Technique to Detect Weapons on the Basis of Weapons Thickness
US4868504 (1050K) Apparatus and Method for Locating Metal Objects and Minerals in the Ground with Return of Energy from Transmitter Coil to Power Supply Fisher Impulse PI, recycles power from the coil.
US4894619 (495K) Impulse Induced Eddy Current Type Detector Using Plural Measuring Sequences in Detecting Metal Objects
US5047718 (481K) Improving the Discrimination of an Impulse Technique Metal Detector by Correlating Responses Inside and Outside of a Cut-Off Peak Area
US5414411 (678K) Pulse Induction Metal Detector White's, block level.
US5537041 (2369K) Discriminating Time Domain Conducting Metal Detector Utilizing Multi-Period Rectangular Transmitted Pulses Minelab pulse.
US5576624 (2032K) Pulse Induction Time Domain Metal Detector Minelab, compare with US5506506
US6326790 (271K) Ground Piercing Metal Detector Having Range, Bearing, and Metal-Type Discrimination A PI probe
US6326791 (1491K) Discrimination of Metallic Targets in Magnetically Susceptible Soil
US6452396 (1408K) Method for Detecting the Metal Type of a Buried Metal Target Periscope
US6452397 (1378K) Ground Piercing Metal Detector Method for Detecting the Location of a Buried Metal Object Periscope
US6456079 (1430K) Circuit for Detecting the Metal Type of a Metal Target Object Periscope
US6529007 (1830K) Temperature Compensation for Ground Piercing Metal Detector Periscope
US6586938 (730K) Metal Detector Method and Apparatus
US6636044 (258K) Ground Mineralization Rejecting Metal Detector (Receive Signal Weighting) Minelab, multi-width pulse
US6653838 (706K) Ground Mineralization Rejecting Metal Detector (Transmit Signal) Minelab, multi-width pulse
US6686742 (284K) Ground Mineralization Rejecting Metal Detector (Power Saving) Minelab, multi-width pulse
US6690169 (56K) Interference Cancelling Metal Detector Including Electronic Selection of Effective Sensing Coil Arrangement Minelab
US6724305 (1022K) Pulse Induction Silverware Detector
US6853194 (466K) Electromagnetic Target Discriminator Sensor System and Method for Detecting and Identifying Metal Targets
US6927577 (115K) Digital Nulling Pulse Inductive Metal Detector
US6967574 (287K) Multi-Mode Electromagnetic Target Discriminator Sensor System and Method of Operation Thereof
US7078906 (105K) Simultaneous Time-Domain and Frequency-Domain Metal Detector Combo PI & phase sampling
US7148691 (156K) Step Current Inductive Antenna for Pulse Inductive Metal Detector
US7474102 (89K) Rectangular-Wave Transmitting Metal Detector Minelab PI, see 7791345
US7652477 (128K) Multi-Frequency Metal Detector Having Constant Reactive Transmit Voltage Applied to a Transmit Coil Minelab
US7710118 (117K) Resonant Pulse Induction Metal Detector that Transmits Energy from High Voltage Flyback Pulses Discriminating PI
US7791345 (118K) Rectangular-Wave Transmitting Metal Detector Minelab PI, see 7474102
EP0654685 (605K) Arrangement and Method for Detecting Metal Objects PI with DD coil
EP0732600 (109K) Active Impulse Magnetometer Looks more like a PI method

US3549985 (301K) Metal Detector Device Having a Disk-ShapedHead for Housing a Coil System Seems to mostly cover plastics & fillers
US3753185 (462K) Metal Detector Search Coil Bill Mahan, D-Tex
US3882374 (276K) Transmitting-Receiving Coil Configuration IB
US4255711 (1162K) Coil Arrangement for Search Head of a Metal Detector Compass, concentric IB
US4276484 (624K) Method and Apparatus for Controlling Current in Inductive Loads Such as Large Diameter Coils Pulse method
US4293816 (613K) Balanced Search Loop for Metal Detector White's concentric loop
US4345208 (405K) Anti-falsing and Zero Nulling Search Head for a Metal Detector Daytona search coil
US4552134 (405K) Equipment for Determining the Position of a Metal Body in a Medium With Low Electric Conductivity Particular IB coil arrangement
US4862316 (735K) Static Charge Dissipating Housing for Metal Detector Search Loop Assembly White's concentric loop
US4890064 (439K) Metal Detector Sensing Head with Reduced Eddy Current Coils Minelab
US5038106 (1142K) Detector of Metalliferous Objects Having Two Pairs of Receiving Loops Symmetrical and Orthogonal to a Driving Loop see US5039946
US5039946 (877K) Metalliferous Objects Detector Having a Pair of Angularly Positioned Driving Loops and a Pair of Parallel, Coaxial Receiving Loops see US5038106
US5245307 (614K) Search Coil Assembly for Electrically Conductive Object Detection
US5498959 (411K) Metal Detector With Multipolar Windings Shaped So As To Eliminate the Neutralizing Effects When Several Metal Masses Are Passing Through Simulataneously Walk-through type coil arrangement
US5859532 (429K) Method of and Measuring Arrangement for Metal Detection With a Coil Device Having Several Separately Controllable Regions Coil arrangements for walk-through type
US5863445 (1216K) Etched Coil Unibody Digital Detector Handheld wand.EP0249110
US6791329 (779K) Portable Metal Detection and Classification System Coil methods for a powered rover
US6822429 (154K) Inductive Sensor Arrangement Comprising Three Sense Coil Cooperating withSaid Three Field Coils to Perform Three Field/Sense Coil Pairs and Method for Detecting of Ferrous Metal Objects Say that 3 times fast.
US7075304 (96K) Variable Damping Induction Coil Metal Detection Uses a MOSFET for the damping resistor
US7176691 (147K) Switched Coil Receiver Antenna for Metal Detector
EP0249110 (465K) Sensors for Metal Detectors Some various coil arrangements
EP0764856 (80K) Sensor for a Metal Detector Several IB coil arrangements

Eddy techniques
US3337796 (698K) Eddy Current Testing Device with Means for Sampling the Output Signal to Provide a Signal Proportional to the Instantaneous Value of Said Output Signal at a Particular Phase Helluva title, the basis for most modern sampled discriminators.
US3478263 (2061K) Wide Frequency Range Eddy Current Testing Instrument
US4006407 (1225K) Non-destructive Testing Systems Having Automatic Balance and Sample and Hold Operational Modes
US4095180 (1107K) Method and Apparatus for Testing Conductivity using Eddy Currents
US4188577 (1973K) Pulse Eddy Current Testing Apparatus for Magnetic Materials, Particularly Tubes
US4191922 (1204K) Electromagnetic Flaw Detection System and Method Incorporating Improved Automatic Coil Error Signal Compensation
US4230987 (785K) Digital Eddy Current Apparatus for Generating Metallurgical Signatures and Monitoring Metallurgical Contents of an Electrically Conductive Material
US4303885 (1196K) Digitally Controlled Multifrequency Eddy Current Test Apparatus and Method
US4424486 (810K) Phase Rotation Circuit for an Eddy Current Tester
US5508610 (1155K) Electrical Conductivity Tester and Methods Thereof for Accurately Measuring Time-Varying and Steady State Conductivity Using Phase Shift Detection
US5952879 (218K) Device for the Simultaneous Demodulation of a Multifrequency Signal, Particularly for an Eddy Current Measurement

US3676772 (434K) Metallic Intrusion Detector System Pass-through type
US3950696 (565K) Trapezoidal Coil Configuration for Metal Detector in the Shape of an Inverted U Walk-through type
US4012690 (1506K) Device for Selectively Detecting Different Kinds and Sizes of Metals Walk-through type
US4605898 (495K) Pulse Field Metal Detector with Spaced, Dual Coil Transmitter and Receiver Systems Walkthrough PI.
US4779048 (242K) Metal Detector for Detecting Metal Objects Pass-through type
US4821023 (763K) Walk-Through Metal Detector
US4866424 (1024K) Metal Detector Coil Walk-through type
US4906973 (1183K) Walk-Through Metal Detector White's
US5121105 (284K) Metal Detector Walk-through type
US5521583 (1015K) Metal Detection System Walk-through type, see US5680103
US5680103 (1128K) Metal Detection System Walk-through type, see US5521583
US5726628 (409K) Metal Detector System Walk-through type
US5790685 (650K) Apparatus and Method for Detecting and Imaging Metal Pass-through type
US6133829 (549K) Walk-through Metal Detector System and Method Fisher.
US6696947 (559K) Metal Detector Foldable security walk-through
US6970086 (183K) Wide Area Metal Detection (WAMD) System and Method for Security Screening Crowds
EP0611970 (256K) Multiple Aerial for Metal Detector Cylindrical coil for walk-through

Mine Detection
US5307272 (1198K) Minefiled Reconnaissance and Detector System Pulsed radar
US5680048 (955K) Mine Detecting Device Having a Housing Containing Metal Detector Coils and an Antenna Appears to combine a metal detector and GPR in one search head.
US7532127 (936K) Motion and Position Measuring for Buried Object Detection Mine detector

US3836960 (528K) Sensor System VHF/UHF
US3976564 (395K) Combination Digger and Sifter for Use With Metal Detector
US4006481 (941K) Underground, Time Domain, Electromagnetic Reflectometry for Digging Apparatus High frequency wide spectrum detector mounted to a digging tool
US4529937 (458K) Metal Detector With Spring Loaded Hinged Support
US4540943 (435K) Belt-Supported Swingable Metal Detector
US4560935 (356K) Remote Actuator for Metal Detector Discriminating Adjust Switch
US4594559 (213K) Metal Detector Audio Amplifier
US4641091 (332K) Device for Testing and Calibrating Treasure Hunting Metal Detectors
US4644290 (365K) Metal Detector Audio Amplifier
US4719421 (639K) Metal Detector for Detecting Product Impurities Auto phase adjustment
US4779777 (410K) Support Bracket for Metal Detector
US4797618 (718K) Caddy for Metal Detector
US4983281 (210K) Metal Detector Scoop Sifter
US5045789 (1747K) Detector for Detecting Foreign Matter in Object by Using Discriminant Electromagnetic Parameters
US5138262 (406K) Metal Detector Having Detachable Battery and Speaker Housing Garrett
US5247257 (680K) Electronic Metal Detector Return Signal Phase Changer A nail and two coils, weird. I don't know what to make of this one.
US5501283 (571K) Hole Cutting Device for Recovering Targets Located with a Metal Detector Audio Amplifier
US5696490 (167K) FM (VHF) Infrared Wireless Digital Metal Detector
US5896031 (876K) Quad Coil Vibration Cancelling Metal Detector
US5963035 (846K) Electromagnetic Induction Spectroscopy for Identifying Hidden Objects Multi-frequency method, interesting data
US5994897 (970K) Frequency Optimizing Metal Detector
US6791329 (190K) Portable Metal Detection and Classification System Detector on wheels
US6838886 (214K) Method and Apparatus for Measuring Inductance Vehicle method
US6870370 (132K) Electromagnetic Induction Detection System Airborne method
US7081754 (88K) Metal Detection System With a Magnetometer Head Coupleable to Conventional Footwear and Method of Use Metal detector on your shoe
US7123016 (457K) Systems and Methods Useful for Detecting Presence and/or Location of Various Materials A mish-mash of techniques, see 6724191
US7132943 (142K) Moving Belt Sensor
US7288927 (163K) Remote Substance Identification and Location Method and System Supposedly an infrared molecular locator (LRL?)
US7310586 (139K) Metal Detector with Data Transfer Minelab, transfer of operating parametrics
US7575065 (176K) Metal Detector with Excavation Tool Garrett pinpointer
EP0790507 (226K) Metal Detector with Pivoting Detector Coil... Lockable coil pivot

US269439 (130K) Apparatus for Finding Torpedoes 1882, C.A. McEvoy (First US detector patent?)
US1126027 (167K) Apparatus for Detecting Pipe Leads or Other Metallic Masses Embedded in Masonry 1915, Dr. Max Jüllig (Figure 8 coil)
US1812392 (383K) Method of and Apparatus for Locating Terrestrial Conducting Bodies 1931
US1890786 (383K) Radio Distance or Location Finder 1932
US2066135 (288K) Apparatus for Locating Bodies Having Anomalous Electrical Admittances 1936, 2-box locator, much like Fisher's
US2066561 (201K) Metalloscope 1937, Fisher 2-box
US2139460 (577K) Means and Methods for Geophysical Prospecting 1938
US2129058 (382K) Transformer for a Metal Locator 1936, describes an adjustable DD coil
US2160356 (460K) Geophysical Instrument 1939, another 2-box
US2167630 (707K) Electrical Prospecting Method and Apparatus 1938, bore hole instrument
US2201256 (847K) Electrical Apparatus and Method for Locating Minerals 1940, 2-box variation
US2220070 (542K) Method and Apparatus for Magnetically Exploring Earth Strata 1940, bore hole instrument
US2268106 (255K) Radiowave Prospecting 1941
US2208029 (232K) Electrical Prospecting Apparatus 1946
US2447316 (593K) Variable Frequency Oscillatory System 1948, BFO I think
US2451596 (685K) Unitary Balanced Inductor System 1948, concentric coil
US2608602 (479K) Detecting Device 1952, bore hole unit, describes phase discrimination
US3015060 (491K) Method and Means of Prospecting for Electrically Conducting Bodies 1961
US3020470 (655K) Submerged Body Detection System 1962
US3105934 (839K) Method and Apparatus for the Remote Detection of Ore Bodies Utilizing Pulses of Short Duration to Induce Transient Polarization in the Ore Bodies 1963
US3471773 (214K) Metal Detecting Device with Inductively Coupled Coaxial Transmitter and Receiver Coils 1969

US22053909A1 (312K) Interference Cancelling Metal Detector Bruce Candy
US22175817A1 (379K) Pulse Induction Silverware Detector
US23016131A1 (379K) Steerable Three-Dimensional Magnetic Field Sensor System for Detection and Classification of Metal Targets
US23034778A1 (865K) Portable Metal Detection and Classification System
US23107377A1 (379K) Metal Detector
US23160617A1 (1011K) Metal Detector Employing Static Discrimination Mark Rowan (White's?)