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