United States Patent 3631442

Article theft detection system wherein a passive nonlinear radio transponder is attached to each article to be protected, and an exit way is monitored by a loop antenna which is energized by currents of two different frequencies. The transponders radiate the difference frequency when subjected to a field of the two energizing frequencies. Receiving means connected to the loop antenna detects signals having a frequency equal to the difference between the two energizing frequencies, and actuates an alarm.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
340/572.3, 340/572.6, 342/42, 343/787
International Classes:
G01S13/75; G06K7/10; G08B13/24; (IPC1-7): G08B13/24
Field of Search:
340/258D,258,252TR 333
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US Patent References:

Foreign References:
Primary Examiner:
Caldwell, John W.
Assistant Examiner:
Trafton, David L.
Parent Case Data:

This application is a continuation-in-part of U.S. application for Pat. Ser. No. 680,666, filed Nov. 6, 1967 and now abandoned.
What is claimed is

1. A system for sensing passage of objects through a surveillance area and useful for detecting theft of an object from an area, comprising:

2. A system according to claim 1 wherein said energy component producing means comprises at least two emitting means each of which produces a said energy component and each of which comprises a combination of a coil winding and a capacitor, which combination is resonant at the frequency at which the emitting means produces its said energy component and wherein each emitting means receives no energy from any other emitting means concurrently producing an energy component.

3. A system according to claim 2 wherein said emitting means which produce energy components concurrently have no mutual inductance.

4. A system according to claim 2 wherein each said emitting means further comprises a filter coupled in electrical series with said combination, which filter has a narrow passband centered about the resonant frequency of said combination to prevent energy from any other concurrently producing emitting means from being received by the emitting means of said combination.

5. A system according to claim 2 further comprising a deactivating device for producing a recognizable change in a merchandise marker to permit deactivation of the marker of a good the removal of which from the designated area has been authorized.

There are in existence several systems for detecting or preventing the theft of articles of value. One of these corresponding with U.S. Pat. No. 3,292,080, granted to E. M. Trikilis, Dec. 13, 1966, makes use of a magnetometer and utilizes a magnetized object which identifies the article unless checkout procedure has removed the magnetism from the object. The magnetized object is attached to or becomes a part of the merchandise or article of value, and by energizing the magnetometer system as it passes through the doorway, is detected. If the magnetized object has been demagnetized it causes no magnetic signal as it passes through the doorway and is not detected. Demagnetizing is done in the process of checking out the merchandise. Thus by the checkout procedure an individual has free passage with the merchandise that has been paid for or recorded by the clerk. Any additional merchandise not paid for and however concealed radiates a magnetic influence, and energizes the magnetometer at the doorway, creating an awareness of security department personnel that something is being stolen.

Another system involves radioactive material which emits nuclear radiation. When the label containing the magnetic material is removed from the merchandise, the radiation is no longer emitted, and therefore radiation detectors situated in the doorway are not energized. On the other hand, if the radiation emitters remain on the merchandise, doorway sensors of nuclear radiation react, and security personnel are in a position to prevent the theft.

In another system currently being employed in a men's wear department in Macy's in New York City, the operator uses a radio frequency generating device embedded in a rubber pad. The radio frequency emitting device is fastened to the men's clothing, and if not removed, will energize radio frequency detecting antennae at the doorway. In the normal course of events, when the merchandise is sold, a special fastener is unlocked and the radio frequency emitter is removed from the clothing at the time it is sold, permitting the buyer to pass through the doorway without attracting the attention of the store detective.

All of the foregoing systems have severe difficulties of one kind or another. The Trikilis system unfortunately requires a rather large piece of ferromagnetic material for the marking of the merchandise. If too small a piece of ferromagnetic material is used, ambient variations in the magnetic field are greater than the changes caused by the Trikilis merchandise marker. In the case of the radioactive dot, there is a severe health problem involving danger to people from the nuclear radiation, and involving danger to those who remove the markers and store them. The system in use in Macy's Store unfortunately is limited by the extreme costliness of the radio frequency transmitter, and the limited period of time during which its emission can be maintained by the little batteries with which it is provided. True, larger radio frequency emitting pads could be made, but these tear or injure the clothing, and are impractically bulky.

I have discovered a practical solution to the problems presented but not solved by the workers in the prior art as described above. As a matter of convenience, I choose to employ electromagnetic radiation. However, because of the inconvenience of supplying energy in a contraband marking, the energy to be radiated from the contraband marked device is delivered, instead, from structural members of my sensing doorway.

I have found it extremely difficult to reradiate or reflect energy in a distinctive manner from any merchandise marker for the reason that all solid bodies and all electrically conductive masses (including the human body which is largely composed of salt water) also reflect or disperse electromagnetic radiation and therefore must be considered in the recognition of any merchandise marking. A human being reflects more electromagnetic energy than any practical size of merchandise marker.

I have solved the problems just described by my discovery of an extremely simple device which can receive energy and reemit it, receiving the energy in a frequency spectrum entirely distinct from the frequency spectrum which is reemitted. I do this by making use of the properties of electrically and electromagnetically nonlinear systems. In general, it is the property of a nonlinear system that if a frequency F is imposed at an energy level at which the nonlinearity of the system becomes important, the system will generate frequencies 2F, 3F, 4F, etc. Similarly, if I impose on a nonlinear system signal sources which deliver approximately equal energy in each of two frequencies, the nonlinear system will generate other frequencies, not originally present. If the frequencies which I impose are F1 and F2, the nonlinear system will generate signals having frequencies F1 +F2, F1 -F2, F1 +2F2, 2F1 +2F2, and various other combinations of sums and differences of multiples of the frequencies which I impose.

It is an essential part of my invention that I have discovered a merchandise marker which constitutes a nonlinear system and which therefore can generate phenomena such as those which have just been discussed. In making my discovery I have overcome certain basic difficulties relating to the concurrent delivery of energy at different frequencies. One of these difficulties lies in the fact that the output members of oscillators and the like always are at least a little bit nonlinear. Therefore two frequencies, concurrently delivered from any emitter, will usually generate summation and difference tones as above related, generating these to an appreciable extent and tending to conceal the summation and difference tones produced by my merchandise markers. I have discovered a way to avoid this problem by emitting the two frequencies of electromagnetic energy from two different sources such as, for example, two large and separate coils in the vicinity of my doorway structure. The large coils in the doorway vicinity are arranged so that they have no mutual inductance. Accordingly therefore energy from one coil does not flow into the system connected to the other coil and vice versa. In such a system as the one I have discovered, summation and difference tones are not produced by any nonlinearities in my signal source oscillators. On the other hand, even though the two emitter coils have no mutual inductance to each other, there is available (in the space where both their magnetic fields exist concurrently) energy at both frequencies. Energy at both the frequencies can be concurrently absorbed by a suitably placed small magnetic object or receiver. If the small object or receiver is in some way adapted to have what would commonly be called an overload characteristic, it exhibits nonlinearity, and therefore radiates summation and difference tones.

I now summarize the elements of my invention. I have discovered a theft detection and prevention system which comprises a surveillance doorway containing among other things emitters of two frequencies of electromagnetic energy adapted to emit an electromagnetic energy into the same region of space, and so arranged that the two emitters do not radiate energy to each other. My system also includes an electromagnetic signal detection means situated at or near the surveillance doorway designed and adapted to receive energy at a summation or difference tone resulting from the concurrent action of the energy of both the frequencies imposed on a merchandise marker device. My system comprises, among other things, a plurality of merchandise marking devices capable of receiving energy at more than one frequency, and reradiating it at other frequencies resulting from the concurrent action of those frequencies which are incident upon it. The system which I have discovered includes means and arrangements to prevent the generation of any effects due to the concurrent action of several frequencies from occurring anywhere except in my merchandise marker itself. My invention also includes a means of altering the electrical or electromagnetic characteristics of my merchandise markers during checkout procedure so that the alteration is recognizable as an indication that the merchandise has been properly sold.

I now turn to FIG. 1 which is a general view of the manner in which my system operates in a store to prevent theft of merchandise. Merchandise 1 is provided with contraband marker elements 2. The checkout stand area 3 contains a deactivating device 4 which is capable of changing the electromagnetic properties of the contraband marker elements 2. An energizing and detecting system 5A situated in the 5B and 5C vicinity of the outgoing doorway 6 detects the contraband marker elements 2, and identifies those which have not been subjected to change at the checkout stand area 3 by the deactivating device 4. In the use of my system, one way traffic, enforced by perhaps a turnstile 7, takes care of persons entering the store, prohibiting the carrying of merchandise from the store to areas outside the store except through my outgoing doorway 6. The turnstile 7 is provided at the entry portal 8.

I turn now to FIG. 2 which illustrates a form of contraband marking element suitable for use in my system. An easily saturable high permeability filament or narrow ribbon of specialized magnetic material such as the one known by the trade name Superpermalloy, is shown at 9. The filament extends parallel to, and is so situated as to collect the magnetic flux from two pole piece coupons 10 which are also composed of high quality magnetic material of the type having a very low coercive force and a very high maximum magnetic permeability The coupons 10 are preferably composed of materials having a maximum permeability in the vicinity of 50,000 or thereabouts. Attached to the coupons 10 I provide masses or rigid plastic substance such as polymerized methyl methacrylate 11. In use, the device is assembled between layers of paper 12 (or plastic) illustrated in exploded view (removed from the vicinity of the filament 9 and coupons 10). The filament 9 and coupons 10 are not shown in exploded form, but are illustrated realistically. In use, the filament 9 is spaced from the coupons 10 by a few thousandths of an inch, the space being occupied by a lubricating particle suspension such as silicone oil with magnesium oxide particles in it. Any other suitable lubricant may be employed, together with particles of suitable size. For example, petroleum lubricant and carbon particles are satisfactory. Fluorocarbon oil with bentonite clay suspended in it is suitable. In use of the FIG. 2 device, the space between the filament 9 and the adjacent layer of paper 12 is also filled with a suitable particle suspension lubricant, to space the filament 9 apart from the paper 12 by an appropriate distance.

In use, the contraband assembly described in FIG. 2 encounters, at the outgoing doorway 6 of FIG. 1, a combination of electromagnetic fields producing an oscillating component of magnetic field parallel with the axis of the filament 9. The oscillating component of magnetic field, as provided in the outgoing doorway 6, includes contributions of two separate frequencies. The magnetic fields thus provided have a component parallel to the axis of the filament 9 as shown in the FIG. 2 device, and are of sufficient magnitude to bring about a substantial degree of magnetic saturation of the filament 9 parallel to its axis. Because of the nonlinearity of the magnetic phenomena occurring in the filament 9, summation and difference tones are produced and radiated in the form of electromagnetic radiation from the device shown in FIG. 2. The above-described phenomena occur when merchandise 1 (FIG. 1) carrying a contraband marker element 2 shown in detail in FIG. 2 is taken through the outgoing doorway 6 (FIG. 1 ) without paying for it. When, on the other hand, the customer pays for the merchandise 1 (FIG. 1) the merchandise 1 (FIG. 1) is presented in the vicinity of the deactivating device 4 (FIG. 1) in the checkout stand area 3 (FIG. 1). The deactivating device 4 (FIG. 1) delivers an extremely strong magnetic field, a field so strong that it is sufficient to induce a very large magnetic flux not only through the filament 9, but also in the coupons 10 of the device of FIG. 2. The large magnetic flux induced in the coupons 10 (FIG. 2) by the deactivating device 4 (FIG. 1) would normally bring about an elongation (or in some instances possibly a shortening) of the material composing the coupons 10 (FIG. 2) in the direction in which the magnetic flux is induced. However the plastic substance 11 attached to each coupon 10 is rigid and nonmagnetic. The plastic substance 11 being firmly attached to the coupons 10 resists the dimensional change which would otherwise occur due to a strong magnetic flux in the coupons. The clamping effect which the plastic substance 11 thus exerts corresponds with a mechanical strain imposed on the magnetic material of the coupons 10. The mechanical strain being beyond the elastic limit of the said magnetic material, it undergoes cold-work which destroys its superior magnetic properties, degrading its maximum permeability from the vicinity of 50,000 to the general vicinity of one or two thousand.

A FIG. 2 device assembled as described in the foregoing paragraph and deactivated as described, still has demonstrable nonlinear magnetic properties. However the doorway field intensity required to induce nonlinear behavior of the filament 9 is substantially altered, for the reason that the maximum permeability of the coupons 10 being lowered, they do not collect magnetic flux from the outgoing doorway 6 environment and feed it into the filament 9 as efficiently as they did before their magnetic properties were degraded. Thus it is possible for an energizing and detecting system 5 existing in the vicinity of the outgoing doorway 6 to determine the presence of unsold merchandise 1, and at the same time be sensitive to the fact that the same individual, or one nearby, is also carrying merchandise which has been properly paid for and carries deactivated contraband marker elements 2 (FIG. 1). (I note that the contraband marker elements 2, as actually illustrated in FIG. 1, are not deactivated, being shown inside the store area.)

FIG. 3 illustrates a modified form for the coupons 10 (FIG. 2). In this modified form the coupons 10 are not attached to any plastic clamping substance 11 over their entire surface, but material of a different nature, either more or less magnetic than the material of the coupons, or not magnetic at all, is deposited in a periodically spaced pattern in a plurality of closely spaced stripes 13 equidistant from each other on the specially arranged coupon 14, which has properties generally similar to the properties of the coupons 10 of FIG. 2. Because of the mass of such periodically spaced stripes 13, and because of their other properties by which they are differentiated from the magnetic material composing the specially arranged coupon 14, the specially arranged coupon 14 in combination with the stripes 13, exhibits a mechanical resonance tending to vibrate in such a manner that the material situated at the stripes 13 undergoes a minimum of movement and/or dilatation. If mass is the predominant characteristic of the material at the stripes 13, the stripes 13 will correspond with a minimum of movement. If mechanical stiffness predominates, the stripes 13 will correspond with very little change of dimension at the frequency of the resonance, and with the specially arranged coupon 14 vibrating in the resonant mode. Because of the influence of the periodically spaced stripes 13, the specially arranged coupon 14 will always exhibit a sharply determined mechanical resonance in the manner just described.

When using contraband marker elements 2 manufactured generally in accord with FIG. 1, but provided with specially arranged coupons 14, I choose to energize the deactivating device 4 (FIG. 1) at the frequency corresponding with the resonance, or I make the deactivating device 4 resonance seeking with respect to the desired mode of mechanical motion. Inducing the resonant motion, the deactivating device 4 (FIG. 1) causes mechanical energy to build up in the specially arranged coupon 14 until the amplitude of movement and the amplitude of stress and strain involved in the resonant oscillations approaches the elastic limit of the magnetic material composing the specially arranged coupon 14. As I have described before in connection with the deactivation of coupons such as the coupons 10 (FIG. 2), the cold-work result form the movement causes the magnetic properties of the specially arranged coupons 14 to be degraded from the general vicinity of a maximum permeability of 50,000 to a maximum permeability in the vicinity of one to two thousand. As before, the contraband marker elements 2 (FIG. 1) in which coupons of whatever type have been degraded, are recognizable, and may be differentiated from other contraband marker elements 2 (FIG. 1) which have not passed through the deactivating process, and not had their coupons degraded.

Although the provision of periodically deposited stripes 13 on the specially arranged coupon 14 helps to define and select a particular resonance at which the spacially arranged coupon 14 will oscillate, it is nevertheless true that the coupon without stripes 13 and without the plastic substance 11 (FIG. 2) can also be induced to oscillate in a resonance mode. In fact resonant oscillations can be induced at a wide variety of modes comprising an extensive plurality of possible choices of resonant frequencies. This, in fact, is the chief difference between an ordinary unclamped coupon such as the coupon 10 of FIG. 2 (but without plastic substance 11 and without stripes 13 as provided in FIG. 3) and the specially arranged striped coupon 14 of FIG. 3. Because of the stripes, the specially arranged coupon 14 of FIG. 3 prefers a particular mode of resonant oscillation and the striped structure 13 tends to suppress the other modes which are a feature of an unclamped and unstriped coupon. In fact the convenience of the stripes lies in this, that the otherwise extremely large diversity of possible oscillatory frequencies is reduced by the stripes 13 to one chosen and preferred mode and frequency. Through the provision of this feature, a specially arranged coupon 14, because of its thickness, mechanical characteristics, and because of the periodicity of the stripes 13, is distinctly recognizable and can be differentiated at the outgoing doorway 6 (FIG. 1).

Thus it is possible, using my system, and using the provisions of my FIG. 3 to distinctly characterize the contraband marker elements 2 (FIG. 1) being employed by Woolworth's, or for example by Sears Roebuck. In fact, using the recognition capabilities intrinsic in contraband markers thus manufactured, the outgoing doorway 6 energizing and detecting system 5 (FIG. 1) can report at the Sears Roebuck store when it detects merchandise 1 that was stolen at Woolworth's, and determine that it is Woolworth merchandise that is being observed.

I turn now to FIG. 4 in which there is illustrated an electromagnetically functionable nonlinear device of another type. The ring-shaped conductor 15 may be made, if desired, of a flat piece of metal, or if desired, may be composed of a wire. In any event the ends of the ring-shaped conductor 15 do not join, but (as shown in inset A) are separated by a space filled with substance such as barium titanate 16. The barium titanate filled space 16 is preferably of appreciable area and thin. As is well known, the electrical polarization of the barium titanate 16 is a nonlinear function of the electric field acting on it. Accordingly therefore, the assembly shown in FIG. 4 radiates energy at frequencies other than those imposed on it when it is energized by a magnetic vector 17 corresponding with a cyclical variation of magnetic intensity in the direction indicated. If a single frequency F is imposed as a result of induction from the electromagnetic induction in the ring-shaped electrical conductor 15, the frequencies which are generated as a result, and which may therefore be radiated, are 2F, 3F, 4F, and so on. If the magnetic vector 17 comprises electromagnetic energy at two frequencies F1 and F2, and, particularly, if these are of approximately equal intensity, the nonlinear behavior of the barium titanate layer 16 results in the generation of frequencies such as F1 +F2, F1 -F2, 2F1 +F2, F1 +2F2, and various other combinations of sums and differences of multiples of the frequencies F1 and F2.

In the same manner that the contraband marker elements 2 shown in FIG. 1 may be composed of an assembly such as I have previously presented and described in FIG. 2, the contraband marker element 2 (FIG. 1) may be composed of the structure such as I have described in my FIG. 4. The same uses and results are obtained by employing a FIG. 4 device, which is in many ways an equivalent to the FIG. 2 device in respect to its function as a contraband marker element. Additional features in connection with the use of my FIG. 4 device become evident in the immediately following description.

In the previous description of FIG. 4, I have recited only the essential components, those which pertain to its electrical and signal inducing behavior, by which it serves to identify merchandise 1 (FIG. 1) that is stolen. As in the case of the FIG. 2 device, a checkout stand deactivating arrangement can be employed in the general manner shown at reference numeral 4 in FIG. 1. In the use of the contraband marker element 2 of the type set out in FIG. 4, the deactivating device 4 (FIG. 1) comprises an electromagnetic energy source which radiates, at least sometimes, electromagnetic energy corresponding with the frequency of mechanical resonance of the barium titanate mass 16 and the nearby portions of the attached metal ring-shaped conductor 15. In this form of deactivation, the energy of mechanical vibration induced by the deactivating device 4 (FIG. 1) fractures the barium titanate mass 16, thus destroying or noticeably changing the behavior of this type of electromagnetic marker. By this change I can recognize that the contraband marker element 2 (FIG. 1) was deactivated, and therefore determine that the attached merchandise 1 FIG. 1) was sold.

Another technique of deactivation which may be employed in connection with the FIG. 4 device makes use of the constriction of the ring-shaped conductor 15 at the point 18 as shown in the inset B. In deactivating, I may, if I choose, make use of this constriction 18 by inducing on the ring-shaped conductor 15 enough current to melt or destroy the electrically conducting material present at the constriction 18. If it is desired to make the constriction 18 sensitive and easily destroyed, the material present in the constriction 18 may, in fact, be composed of a substance or substances less well adapted to conduct electricity than is the main portion of the ring-shaped conductor 15. By such a choice, heat or other alteration will occur readily at the constriction 18 causing the circuit involving the ring-shaped conductor 15 and the barium titanate mass 16 to open up with the result that the contraband marker element 2 (FIG. 1) will no longer function to produce summation and difference frequencies, and therefore is not detected by the energizing and detecting system 5 (FIG. 1) of the outgoing doorway 6 of FIG. 1.

I turn now to FIG. 5, in which I illustrate a further variation of contraband marker element 2 (FIG. 1). In inset A of FIG. 5 I show a conductor 19 and a mass of barium titanate 20 at a separation between the ends of the conductor 19, as shown in inset B. In addition, a quantity of ferromagnetic material 21 (shown in an inset B) is disposed in such a manner that it closes a magnetic circuit surrounding the current flowing in the conductor 19 in a manner to substantially increase the inductance exhibited by the one turn loop of the conductor 19. As a result of the use of ferromagnetic material 21 and because of the relatively large electrical capacity of the gap containing the barium titanate 20 (as compared with a gap containing ordinary dielectric) the system illustrated in this figure is, in fact, an inductance and capacitance loop which, because of the ordinary considerations of communications engineering, has a resonance frequency of

where L is inductance in microhenries, C is capacitance in microfarads

In view of the presence of the ferromagnetic material, the above-described resonance is not as sharp as resonances of air core coils containing large amounts of electrically conductive material but containing no ferromagnetic material. For the reason that the resonance of the system described in FIG. 5 has an appreciable width, I can, if I choose, energize it with more than one frequency, the said frequencies differing appreciably, and yet expect that both frequencies will lie substantially within the resonance. The operation of my system as set out in FIG. 1, employing contraband marker elements 2 (FIG. 1) but of the special type provided in FIG. 5 works in a manner generally similar to the description I have given in my discussion of the operation of my system with the contraband marker element of FIG. 4, but will require that the energy sources at the energizing and detecting system 5 (FIG. 1) of the outgoing doorway 6 (FIG. 1) supply frequencies falling within the capacity and inductance resonance of the system for the greatest efficiency of energy delivery to the marker. The vector arrow 17 through the center of the loop formed by the conductor 19 has exactly the same significance as the vector arrow 17 in FIG. 4.

In FIG. 6 I illustrate a contraband marker comprising a flat coil of one or more turns short circuited on itself. An equivalent to the illustrated flat coil 22 would be, for instance, a copper washer occupying the same region of space, and having in it an amount of copper equal to the total amount contained in the wire of the illustrated coil 22. Ferromagnetic elements 23 and 24 (shown in the inset) are disposed to link the magnetic flux developed by the coil 22, and are chosen of material of extremely low magnetic coercive force. Additionally the ferromagnetic elements 23 and 24 are deliberately taken in a form having an insufficient amount of ferromagnetic material, creating a strong likelihood that magnetic saturation will occur. By the occurrence of magnetic saturation, which is a nonlinear process, the flux changes associated with the nonlinearity cause radiation from the electrically conducting loop 25. The frequencies so radiated correspond with modulation products, serving the same purposes as the modulation products developed in connection with the uses of my other contraband marker elements 2 (FIG. 1).

The ferromagnetic elements 23 and 24 are shown in a form adapted to serve the purpose of linking the magnetic flux induced in the presence of my short-circuited coil 22, but do not have to be bent sharply to go around the flat coil 22. Instead I lay a piece 24 flatwise immediately below the flat coil 22 and another piece 23 similarly above it. The two pieces 23 and 24 approach each other very closely at their extremities, permitting the easy transfer of magnetic flux from one piece into the other, thus allowing the circulation of magnetic flux around the conductor. The marker element illustrated in FIG. 6 is not provided with any deactivation capabilities. Instead the user has to remove this type of marker label from the merchandise at the time of sale. This marker element of FIG. 6 is described for the purpose of illustrating how a system not involving deactivation can be combined in my invention. In use, the energizing and detecting system 5 (FIG. 1) in the vicinity of the outgoing doorway 6 (FIG. 1) produces and detects from this contraband element (FIG. 6) a signal showing that the merchandise 1 (FIG. 1) being taken out still has the contraband marker element 2 (FIG. 1) on it. Merchandise from which the clerk has removed the contraband marker element of FIG. 6, of course, does not give this effect at the doorway.

In FIG. 7A I show another type of contraband marker element corresponding with a longitudinally extending strip or rod of ferromagnetic material 26 capable of responding at the frequencies F1 and F2 delivered at my energizing and detecting system 5 (FIG. 1) in the vicinity of the outgoing doorway 6 (FIG. 8 1). Linking the equatorial region of the longitudinally extending ferromagnetic material 26 I provide an electrical conductor 27 which makes one or more turns around the equatorial region of the said ferromagnetic material 26. The terminations of the electrical conductor 27 are connected together through a nonlinear electric element 28 comprising a germanium rectifier junction, a copper oxide rectifier junction, a silicon rectifier junction, or suitable other more or less unilaterally electrically conducting arrangement. In use, the knee of the voltage versus current characteristic for the nonlinear elements represents a nonlinearity which imposes its effect on any current induced in or flowing through the electrical conductor 27. THe nonlinear effect thus imposed reacts on the magnetic field in the ferromagnetic element 26 causing summation and difference frequencies to be magnetically radiated, as is the case with the contraband marker elements 2 (FIG. 1) previously described. The utilization of summation and difference frequencies is, in fact, the same. Deactivation is produced at the deactivating device 4 (FIG. 1) by inducing through the diode element 28 a large enough electrical current to destroy it. In the destroyed form, the nonlinear element either loses its directional characteristic, which removes the nonlinear behavior, or on the other hand, it may break up and become an open circuit, resulting in the passage of no current at all in the electrical conductor 27 thereby removing the nonlinear effect originally present due to the nonlinear electrical element 28.

Referring further to FIG. 7A, I may, if I choose, employ a nonlinear element sufficiently durable that it can resist the work of my deactivating device 4 (FIG. 1) which I provide in the checkout stand area 3 (FIG. 1). In this case, the utilization of my system proceeds through the removal of the contraband marker element of FIG. 7A by the clerk at the checkout stand area 3 (FIG. 1) at the time merchandise is purchased. Otherwise the system functions generally in the same manner as it does in connection with my other contraband marker devices.

In a further modification of my FIG. 7A marker element which I have illustrated as FIG. 7B, I provide the same longitudinally extending ferromagnetic element 26, the same electrical conductor 27, extending one or more turns around the girth of the longitudinally extending ferromagnetic element 26 in the vicinity of its equator, but the diode or nonlinear electrical conductor 28 which I afforded in my FIG. 7A is modified (in FIG. 7B) to comprise, instead, two diodes 29 and 30 connected in parallel, and aiding. The two diodes 29 and 30 are disposed differently, the diode 29 having a much larger electric current carrying capacity than the other diode 30. The much larger current capacity of the diode 29 is so chosen that the deactivating device 4 (FIG. 1) operating in the checkout stand area 3 (FIG. 1) cannot cause enough electric current to flow in the conductor 27 to damage the diode 29. On the other hand, the diode 30 which has less current carrying capacity is destroyed. In addition, to cause the electric current delivered by the conductor 27 to be shared in a predetermined manner between diodes 29 and 30, I also provide resistors R1 and R2, each in series with the corresponding diodes 30 and 70 29.

In use, the effect 137 the modified form 7B marker device is that deactivation between deactivating device 4 (FIG. 1) employed in the checkout stand area 3 (FIG. 1) results in a predetermined and predictable change in the properties of the contraband marker element corresponding with this figure, but leaves it still able to deliver a radiation effect corresponding with modulation products, at the doorway area. Like my other contraband marker corresponding with FIG. 2 this contraband marker element as shown in FIG. 7B affords recognition of stolen merchandise and at the same time affords, at the outgoing doorway, recognition of the fact that contraband marking, in deactivated form, is present on the merchandise being carried by the customer through the outgoing doorway 6 (FIG. 1).

I turn my attention to the energizing and detecting system 5 (FIG. 1) situated in the outgoing doorway 6 (FIG. 1). Because there are three perpendicular coordinates available in space of three dimensions, I can arrange for the two energizing systems and detecting devices to work in a noninteracting manner. In fact, it is a characteristic of my plan that within the limits of accuracy of adjustment of the position and orientation of my electromagnetic radiating and receiving components, the two radiating components radiate independently, neither one being capable of transmitting energy into the other one, and further, the detecting or receiving pickup does not receive energy directly from either of the radiating devices. These arrangements of course are valid only when the space in the doorway is ample, there being no contraband marker elements 2 (FIG. 1) in it. The manner in which I achieve the type of arrangement which has been generally recited above is depicted in more detail in FIG. 8.

In FIG. 8 I have pictured two pedestals 31, each containing near its center a pair of sending coils 32. All the sending coils 32 are connected in parallel (or they could have been connected in series). For illustration only, I will suppose that the frequency by which these sending coils 32 are energized in 21 kilohertz. Each such sending coil 32 is separately tuned to exhibit the highest possible at 21 kilohertz. For illustration only, the coils may be composed of 99 turns of No. 20 copper wire wound on a 1-inch diameter coil form in a single layer to produce 99 turns in a total length of 311/2 inches. Such a coil may be resonated to 21 kilohertz by the use of an electrical capacity of not less than 1 microfarad and not more than 1.1 microfarad. The combination of one of these coils 33 with it resonating capacitor 34 (as shown in the inset), when energized at the resonant frequency, represents an entirely resistive impedance and in the illustrative case exhibits a resistance between 100 and 150 ohms. A parallel combination of four such resistive loads has a combined effect adapted to efficiently load the voice coil outputs of some available audio amplifiers.

Similarly, there are situated at the bottom and at the top of each of the pedestals 31, coils 35 intended for transmitting another chosen frequency such as (for illustration only) 24.5 kilohertz. The four coils 35 which are intended for 24.5 kilohertz radiation may be constructed similarly and resonated similarly, but, of course, resonate with a correspondingly smaller electrical capacity for each coil. The combination of the first group of four coils 32 is connected to a source of electrical energy 36 at 21 The combination of the second group of four coils 35 is connected to a separate, entirely independent, source of electrical energy 37 at 24.5 kilohertz. Because of the arrangement which I have chosen for the first group of coils 32 and for the second group of coils 35, there is no appreciable mutual inductance acting to deliver 21 kilohertz energy into the 24.5 kilohertz, or vice versa.

At four other locations I present four more coils 38 with their axes perpendicular to the plane of the paper. Because all the contributions of the first group of four coils 32 and the second group of four coils 35 lie in the plane of the paper, the four coils 38 with their axes perpendicular to the plane of the paper do not receive energy neither at 24.5 kilohertz, not at 21 kilohertz. The four coils 38 with their axes perpendicular to the paper are resonated at 3.5 kilohertz by choosing an appropriate electrical capacitance. In order to achieve good sensitivity in these coils, and in order that they may be resonated efficiently at the frequency of 3.5 kilohertz, more copper is required in the winding, preferably four layers of No. 20 wire, each layer containing 99 turns more or less. The capacity required to resonate such a coil is in the general vicinity of 2 microfarads for 3.5 kilohertz.

I call attention to the fact that the cores of these windings have not been specified thus far. It is a preferred choice to wind them on nonmagnetic, electrically nonconducting material, for the reason that ferromagnetic material (because of its nonlinear properties) imparts to my system interactions between the energy sources, interactions which I desire to avoid. Electrically conducting material, on the other hand, destroys the quality of the inductive performance of all the coils. As a matter of fact an air core coil of 99 turns, made in the manner that I have described, has a Q in the vicinity of 500 at 21 kilohertz when wound on a wooden core. The resonance cannot be found, nor the inductance measured well enough to determine the Q if it is wound on an electrical conductor as a core.

The combination of the four coils, as described, with their axes perpendicular to the paper (each coil resonated at 3.5 kilohertz by appropriate electrical capacitance) delivers its output to the ingoing end of a high gain tuned amplifier 39 adapted to selectively receive and amplify electrical signals at 3.5 kilohertz. The amplifier 39 delivers its output to an alarm mechanism 40, or to a carrier frequency module, which I discuss further on. To achieve a closer impedance match with respect to the commonly prevailing input resistance of the amplifiers that are the most convenient, I may choose to vary from the connections shown in FIG. 8, and connect the four receiving coils 38 (the ones with their axes perpendicular to the paper) in series. The resistive component of these coils (with their resonators connected) comes out for each such resonated system in the vicinity of 100 ohms, with the result that the series of four of them are a close match to the communications impedance figure of 500 ohms, a common choice for amplifiers, filters, etc.

I turn now to FIG. 9 presented for the purpose of diagrammatically assisting in the explanation of the manner of functioning of the energizing and detecting system 5 (FIG. 1) which I have particularly detailed and described in connection with FIG. 8. In FIG. 9 the axis X may be taken to represent the action of the 21 kilohertz radiator, the perpendicular axis Y illustrates the action of the 24.5 kilohertz radiator, and the axis Z represents the receiving sensitivity or direction of the 3.5 kilohertz receiving coils 38 of FIG. 8. The vector θ is illustrated in a direction not parallel to nor perpendicular to any of the three axes. The vector θ represents the direction in which a contraband marker element 2 (FIG. 1) is capable of receiving and reradiating energy. Because the vector θ has an appreciable component in all three axes, the contraband marker element 2 (FIG. 1) oriented in accord with this vector is able to receive energy concurrently at 21 kilohertz, and likewise at 24.5 kilohertz. For similar reasons, if the contraband marker element 2 (FIG. 1) reradiates at 3.5 kilohertz (not being deactivated) then detection axis Z is so directed with respect to the vector θ that the said detection system is not insensitive to radiation emitted by the contraband marker element 2 (FIG. 1).

The user, considering the information presented in connection with FIG. 8, the information just presented in connection with FIG. 9, will realize that the reception of a 3.5 kilohertz in my system is a distinctive and an exclusive evidence of the presence of contraband marker elements 2 (FIG. 1). One or more such elements must be in the domain of energy radiation sensitivity provided by the arrangements shown in my FIG. 8 to deliver a 3.5-kilohertz signal. Other entities than contraband marker elements are not entirely without effect, but they do not present the same effects.

To aid the understanding of another modification of my system which I have described, I turn again to FIG. 9. In FIG. 9 I have represented the directions of action of the energy source frequencies X and Y (21 and 24.5 kilohertz sources) and the direction of sensitivity of the system that detects the difference tone Z in the form of three perpendicular axis. To the worker skilled in the art, it is evident that if contraband vector θ is exactly perpendicular to either of the signal source axes X or Y, I eliminate the energy corresponding with the vector to which the vector θ is perpendicular. Furthermore if the vector θ lies in the X-Y plane, it is perpendicular at all times to the axes Z which therefore prohibits the reception of any energy in my signal receiving system 38, FIG. 8. It is, in fact, true that the vector θ must have appreciable and comparable components or direction cosines aligned with all three of the vectors X, Y, and Z. For those directions θ which do not fulfill these conditions, either the difference tone signals are not produced or they are not observed (if produced) by the contraband marker element 2 (FIG. 1) The fact that there are so many blind spots and so many requirements on the direction of contraband, causes my system, conceived as in the foregoing, to sometimes fail to recognize contraband markers passing through the outgoing doorway 6 (FIG. 1). It still remains a fact that nothing other than a contraband marker will ring the alarm. However, I have discovered a way to reduce the inconvenience resulting from the above-noted limitations (which now and then permit a contraband marked piece of stolen merchandise to get through).

The user will note in FIG. 8 that in the foregoing I have excluded the energy from the 21 kilohertz source from getting into the 24.5 kilohertz source by arranging for separate radiators, and arranging that these be noninteracting because of their perpendicularity arrangement. Another approach to excluding wrong pathways of signal energy is quite applicable in the frequency range which I have chosen, an approach not dependent on geometry. My modification permits advantages in the simplification of the doorway structure.

The system which I contemplate for the reduction of the number of blind spots in respect to the direction of the vector θ (FIG. 9) substitutes rigorously designed wave filters, containing passive elements only. These perform the function performed by the geometric isolation in the system of FIG. 8. Such wave filters can be designed for the range of frequency in the vicinity of 20 to 50 kilohertz without the use of ferromagnetic material or anything else which would impose a nonlinearity. The wave filters thus used, if provided in a sufficient number of sections, propagate the desired energy substantially without loss and are able to reject the unwanted signal frequencies to whatever extent is desired, through the use of a sufficient number of networks. A properly designed M or pi derived filter network will exclude unwanted frequencies by over 100 decibels in just a few networks.

Lattice-type filters may be employed for single frequency rejection and are extremely effective. In fact, the only serious limitation of the rejection brought about by a lattice-type filter is imposed by variation in frequency of the signal which it is desired to reject. A lattice-type filter, for example, may comprise two electrical capacitances and two inductive elements as the four components of a bridge. The input to the bridge and the output to the bridge have a ratio which theoretically is infinite at the frequency at which it balances. Thus it is theoretically possible to exclude a single frequency to any extent, by a single network of such a filter. At the same time a single network lattice filter can transmit very efficiently energy corresponding with signal frequencies that are substantially different from the signal frequency at which the bridge balances.

For 20 kilohertz or more, substantially perfect inductances (inductances with a Q in the realm of thousands) can be delivered in the space of a few cubic inches, and need not contain more than an ounce or two of copper wire. Again, in the frequency spectrum involving a metal box comprised of iron or copper, and with a coil spaced from the walls, inside the box, the coil neither radiates nor absorbs electromagnetic energy appreciably in this kilohertz range. Capacitances constructed of aluminum foil and wound with such a dielectric as wax paper (or mylar or polystyrene) form a substantially perfect electrical performance in my preferred frequency range. It is, accordingly, entirely feasible to contemplate the substitution of rigorous filtering in place of my previous geometric means of arranging radiator coils so that energy is not transferred from one system to another. Moreover, the use of well designed filters has a further advantage, that the presence of conducting bodies of any description in the doorway 6 (FIG. 1) does not cause energy to flow from one system to the other, since the wave filters function independently of whatever bodies are situated in the doorway 6 (FIG. 1). On the contrary my geometric arrangement of coils is sensitive to the presence of electrically conducting bodies in the doorway 6 and the favorable results which I achieve by making these coils 32, 35, and 38 (FIG. 8) perpendicular are partly destroyed whenever a large electrically conducting body passes through the outgoing doorway 6 (FIG. 1).

I turn now to FIG. 10 which illustrates the plan comprised in a general way in the foregoing discussion. In FIG. 10, for simplicity I illustrate one common radiating and receiving means 41, and one only, since this shows the flexibility of my modified plan most clearly. In the block diagram, the user will note that there are provided three distinct wave filters, each connected at its input to a separate electrical entity. The electrical entity to which the first two wave filters are connected is in each instance an oscillator. For convenience, the filters 42 and 43 are also designed by the symbol F1 and F2 to indicate the center of a pass band which each of the said filters 42 and 43 selectively transmits. The third filter 44 is designated by the symbol F1 -F2 to indicate the fact that the center of its pass band is chosen at the difference frequencies corresponding with the difference between the two frequencies F1 and F2. The filters in question are deliberately taken from designs which permit extremely strong selectivity and extremely high exclusion of the unwanted frequencies.

As an example of a frequency corresponding with a capability of extremely strong filtering, F1 may be 31 kilohertz, F2 may be 21 kilohertz, and F1 -F2, in fact, 10 kilohertz. The frequencies can be very stringently filtered against one another and, in fact, exclusivity can be achieved to whatever extent is required. I therefore indicate these entities as being each connected to a single electronic device in the doorway detecting and energizing system 41. A suitable doorway sensing and detecting device 41, one adapted for the purpose is a flat wound coil 41 diagrammatically shown in FIG. 10. Such a flat wound coil serves effectively because the two input energy sources 46 and 47 cause a concurrent influence on the contraband at the frequencies F1 and F2 whenever a contraband element has a significant component of its vector θ in a direction not in the plane of the coil. In a completely reciprocal manner, the illustrated doorway coil 42 is able to receive energy at the difference tone F1 -F2 with good efficiency, and can do so whenever the contraband marker element 2 (FIG. 1) exhibits and appreciable component perpendicular to the plane of the doorway (shown in FIG. 10) (at the time the contraband element 2 (FIG. 1) is passing through the plane of the said doorway).

I refer again to FIG. 10. In this figure it will be noted that I have provided two frequency sources F1 and F2, and two filter systems. It is obvious that if the frequency sources which deliver energy at F1 and F2 are adjusted so that the frequency F1 =F2, and furthermore, if I impose the requirement that these two alternating current energy sources be in phase, then, in this degenerate case, the entire system comprising the frequency sources delivering energy at the two frequencies F1 and F2 has the same effect as one oscillator and one filter. Accordingly therefore I achieve the same result if I simply omit the filter F1 and the oscillator 46. In a system comprised by such an omission, since F1 =F2, the quantity F1 -F2 has no significance as alternating current for the reason that F1 -F2 equals zero. However in modulation products, as has been stated, earlier one of the functions that is generated is F1 +F2. For the case in which F1 =F2, F1 +F2 is of course 2F.

In the modification of my system which I am now describing with the help of FIG. 10, I envision omitting the oscillator 46 and the filter 42. I provide the substitution of a filter adapted to pass the frequency 2F1 instead of a filter 44 (as illustrated) to pass the frequency F1 -F2. The recognition of contraband marked merchandise by this modified system is identically the same as has been described in the other embodiments of my invention. From an engineering standpoint it is required that the filter 43 of FIG. 10, be adapted to particularly stringent rejection of the frequency 2F. In a lattice filter designed for single frequency rejection elimination of the unwanted frequency 2F1 from the output of this filter can be accomplished to more than 100 decibels in two meshes, providing the stability of the frequency of the oscillator 47 is sufficiently good. This is easily arranged by employing crystal control to stabilize the oscillator 47. I envision the use of a temperature insensitive cut of the quartz crystal and, if necessary, I employ a temperature controlled environment to further improve the frequency stability of the oscillator 47. The stability of oscillators has been controlled within one part per billion over long periods by the careful use of these techniques. Since I do not need such extreme frequency control, the adequacy of the methods which I propose is quit obvious.

In the use of my antishoplifting systems there is a problem of communicating the warning signal indicating that merchandise is being stolen, and bringing the indication to the attention of security guards who are not, necessarily, at the same place. To make this procedure convenient in finished buildings where the wiring is already in place, I propose the use of ordinary carrier frequency signaling techniques that are well known in the art, and propose that the carrier frequency signals be inserted on the electric power system.

Since my warning devices are electrically powered, it is convenient to insert the carrier warning signal on the cord through which the power requirements of the system are served, making communications connections of a separate nature unnecessary. The electronic equipment necessary to put the carrier frequency warning message into the power cord will generally be a part of, or will be situated close to the other parts of the antishoplifting system. In fact all these things may be on the same panel rack or may be built up in the same stack of shielded boxes, a proves convenient. I visualize such carrier frequency systems as a valuable and useful feature in combination with the other elements of my invention. In FIG. 10, the carrier frequency module is, as desired, the element 48.

In FIG. 10 the operator will note that there are six electrical connections, comprising three pairs, going from the systems: (a) 46 and 42, (b) 47 and 43, and (c) 48 and 45. My U.S. Pat. No. 2,520,677 Aug. 29, 1950) makes a similar use of six wires in the form of three pairs, and provides an especially effective means for filtering out the noise from the signal frequency F1 ±F2 (F1 -F2, as used in the discussion in this patent application). I contemplate the use of all the same means and methods for improving the signal to noise ratio in my antishoplifting system, and employ the same in combination with the other features of my antishoplifting system to better reject unwanted noise and electrical disturbances of all kinds.

I refer once more to FIG. 10, and particularly I employ the device of FIG. 10 with the omission of elements 43, 44, 45, 47, and 48. I further describe the filter F1 (element 42) as a nonsignificant component comprised in this use of my FIG. 10 device as simply a pair of wires going straight through from left to right. In effect I omit the function of this filter. In this use of my FIG. 10 device I also construe the oscillator 46 as one emitting relatively very strong electrical oscillations, and one which may at times be adjusted or at least have its frequency reset to another value as required. Further the oscillator 46 may be a "warble" oscillator adapted to cyclically retraverse a small range of frequency.

In the use which I am now describing for my FIG. 10 device, I insert the coil identified in FIG. 10 as "doorway" at the point shown for the device 4 in FIG. 1. The coil 41 is assumed to be taken to a proper scale so that it will fit in the space provided at location 4 in FIG. 1. My FIG. 10 device so arranged is, in fact, suitable to perform the deactivating function. To assure the upward radiation of a strong electromagnetic effect through the belt 2A of the checkout stand 3 shown in FIG. 1, I arrange the design of the checkout stand so that there are no closed metallic loops between the device 4 and the merchandise 1 with contraband marker 2. I further designate that the plane of my FIG. 10 coil 41 will be the same as the plane of the largest side of the box-shaped space designated at numeral 4 in FIG. 1. For this use, and for all the other uses of the FIG. 10 device, it is understood that the mechanical coil support which is illustrated in FIG. 10 is an electrically nonconducting material, and a nonferromagnetic material.

In addition to the systems described in the preceding, we have found an especially useful way to practice this invention. If we employ an extremely favorable high permeability ferromagnetic material, as for example, a substance having a maximum permeability of 400,000 or thereabouts and a coercive force of 0.02, and if we choose a very slender cross section compared with length, as for example a cross-sectional area of 0.0004 square centimeters, and a length of 4 centimeters or more, the same being comprised in a ribbon not thicker than 0.00125 centimeters thick, and if such a contraband marker element is presented with its axis approximately parallel to the oscillating magnetic field in a doorway such as is illustrated in FIG. 1, the oscillating magnetic field having an intensity of the order of magnitude of three oersteds (such a contraband element, being generally similar to element 9 in FIG. 2) the magnetic element so chosen returns harmonic frequencies of a very high order, extending up to and including 1.6 megacycles when excited by a frequency such as 60 cycles per second.

If a contraband element as above described, and generally similar to element 9 of FIG. 2 and particularly represented by element 49 of FIG. 11 is accompanied by other ferromagnetic elements also of a very slender nature, such other ferromagnetic elements being disposed close to and parallel with ferromagnetic elements which were first described, very valuable and useful results are obtained. The additional ferromagnetic elements 50 and 51 of FIG. 11 are chosen to have distinctive magnetic properties, properties not the same in the two additional ferromagnetic elements, and neither of the two additional ferromagnetic elements 50 and 51 are at all similar to the first ferromagnetic element 49. The ferromagnetic element 50 may be chosen from among those substances high in iron content which have a coercive force in the general vicinity of 15 oersteds.

The ferromagnetic element 51 may be chosen from among ferromagnetic substances high in iron which have a coercive force of approximately 100 oersteds. Other elements, not shown, may be chosen having still higher magnetic coercive force characteristics. The magnetic element 50 is of such a cross section (as for example less than 0.0004 square centimeters) that if it is left as strongly magnetized as possible, the number of lines that it will deliver is insufficient to saturate the first magnetic element 49. The cross-sectional area of the ferromagnetic element 51 is so chosen that if it is left as fully magnetized as possible, and if at the same time the element 50 is also as fully magnetized as possible, and in the same direction, the lines carried by both these ferromagnetic elements are but little more than sufficient to magnetically saturate the magnetic element 49.

The ferromagnetic elements 49, 50, and 51 are shown in FIG. 11 separately, and we have illustrated nothing else, for the purpose of simplicity of the discussion. However, it will be understood that, in the use of the FIG. 11 device consisting of the combination of ferromagnetic elements shown therein, paper cards generally similar to elements 12 of FIG. 2 may be employed to sandwich, support, and conceal the ferromagnetic elements 49, 50, and 51 in a contraband label or marker.

The spectrum of reradiated frequencies which results from the combination of ferromagnetic elements 49, 50, and 51 has three possibilities when the combination of adjacent ferromagnetic elements 49, 50, and 51 is carried through a doorway such as is illustrated in FIG. 1. The first possibility represents the type of reradiation that occurs when the ferromagnetic elements 50 and 51 have been degaussed and when the ambient or zero value of the magnetic field in the doorway is neutralized to have approximately no component parallel to the axis of the oscillating field components.

The second possibility occurs when the condition of the doorway is generally the same but the contraband element shown in FIG. 11 is presented in the condition in which element 50 is approximately fully magnetized but the element 51 is not magnetized. This condition is achieved by imposing a magnetic field sufficient to magnetize the element 50 but not adequate to magnetize the element 51.

A third condition of the arrangement shown in FIG. 11 exists when both the ferromagnetic elements 50 and 51 are magnetized, and are left as strongly magnetized as possible in the same direction. In this case also it is understood that the ambient condition of the doorway in FIG. 1 is the same as was previously described in connection with the spectrum condition number one referred to.

A fourth magnetic state of the arrangement shown in FIG. 11 can be obtained by arranging for the ferromagnetic elements 50 and 51 to exist in magnetized condition, but magnetized with opposite polarization. This state is achieved by first imposing a very strong magnetic field which leaves both the elements 50 and 51 magnetized in the same direction, and afterward applying a weaker field sufficient to reverse the magnetization of the element 50 (in view of its lower coercive force) but not sufficient to reverse the magnetization of the ferromagnetic element 51.

Referring now to spectrum condition number one, the expected output consists entirely of odd harmonics of the power frequency of 60 cycles. This conclusion is particularly rigorous for the case in which the loop antenna which receives the energy is chosen with a very insufficient number of turns and produces in an approximately rigorous manner an electrical voltage proportional to the time derivative of the surface integral of the magnetic flux threading through the loop antenna. The loop antenna may be element 5B of FIG. 1, for example.

Spectrum condition number two deviates from spectrum condition number one in that even harmonics appear and represent an important contribution to the energy.

Spectrum condition number three corresponds with "silence" in the sense that the combination of elements does not radiate. The voltage wave delivered by the output in the four described spectrum conditions (in the order above presented) is shown in FIGS. 12A, 12B, 12C, and 12D. In FIG. 12A the operator will note that the successive alternating cups of voltage are evenly spaced. In FIG. 12B the alternating cusps are no longer evenly spaced but are closer to one another in pairs. In FIG. 12C the cusps have in fact disappeared. In FIG. 12D corresponding with spectrum condition four as previously described, the cusps are unevenly spaced, but the degree of uneveness is different from the unevenness shown in FIG. 12B. The distinction between the FIG. 12B information and the FIG. 12D information is that the ratio of energy delivered in even harmonics to that delivered in odd harmonics is significantly different in the two cases. Actually as the condition approaches the disappearance of the cusps, they move up until the positive and negative pulse crowd each other. As the positive pulse moves into the negative pulse, the two cancel and the information gathered by these features disappears.

In the use of the contraband elements of the particularly advantageous type which we have described, we employ again the FIG. 10 arrangement for the electronic energizing and readout at the doorway. In this use of the FIG. 10 arrangement, we omit elements 42 and 46, energizing the doorway with but a single frequency. The element 44 which has been hitherto characterized as a wave filter, we characterize instead as an electronic device for selecting even and odd harmonics present on the ingoing leads to element 44. The device 44 in this arrangement delivers a voltage proportional to the ratio of the selected even and odd harmonics on the wires going out to amplifier element 45. In such a manner of use, the FIG. 10 device and the doorway coil 41 illustrated in connection with it, serve to energize the security readout system and communications system 48 (which relies on the output of the amplifier 45) for the purpose of energizing alarms, lighting lights, etc.

When so used, the element 44 is further qualified to indicated the condition when it receives no signal at all. Accordingly, the device 44 can deliver distinctive signals corresponding with three conditions in which energy is retransmitted from the contraband elements and finally the condition of silence when nothing is retransmitted. This number of possibilities is sufficient for codified identification of merchandise being stolen.

Another very valuable way of using the arrangement of FIG. 11 is accomplished by omitting the ferromagnetic element 51 and choosing ferromagnetic element 50 to have a sufficient cross section that when it is fully magnetized, it is more than adequate to saturate the ferromagnetic element 49. A combination so chosen constitutes a marker that has two conditions that are clearly definable. The first signal producing condition, the unmagnetized one, corresponds with the voltage curve shown in FIG. 12A. The condition of sold merchandise, in which the marker has been commanded to be silent, is shown in FIG. 12C. This condition is brought about at the checkout stand by imposing on the marker (consisting of the elements 49 and 50, as previously set out) a magnetic field sufficiently strong to leave the element 50 in a fully magnetized condition. This modification of the marker has particular merit where it is merely desired to determine whether merchandise has been sold or not as it is carried out the doorway, and where it is not necessary to determine in codified detail what kind of merchandise is involved in a potential theft. For the more complicated problems, we prefer the previously described arrangement with at least three ferromagnetic elements, and for more complicated codes, even as many as four, all being slender and lying in reasonable close proximity to each other and essentially parallel, as shown in general in FIG. 11.

In addition to the use of the systems and apparatus disclosed herein as an antishoplifting means the invention may equally well be utilized in various arrangements for classification, recognition on production lines, security, and for identification of objects such as I.D. cards, canceled tickets, and other such similar applications.