WO1994014143A1

WO1994014143A1 - Dual frequency tag using rf and microwave technology

Info

Publication number: WO1994014143A1
Application number: PCT/US1993/011323
Authority: WO
Inventors: Richard J. Frohbergh
Original assignee: Checkpoint Systems, Inc.
Priority date: 1992-12-04
Filing date: 1993-11-22
Publication date: 1994-06-23
Also published as: AU5728094A

Abstract

A system (102) for detecting the presence of a protected article (104) within a surveillance area (106) includes first (108) and second (114) transmitters for radiating energy at first and second frequencies, respectively; a tag (120) comprising a first circuit (202) for receiving and mixing the energy radiated at the first and second frequencies to thereby generate energy at a third predetermined frequency, and a second circuit (206) which is closely coupled to the first circuit such that the second circuit receives the energy generated by the first circuit at the third frequency, the second circuit having a resonant frequency substantially the same as the third frequency such that the second circuit resonates and thereby radiates energy at the resonant frequency when the second circuit receives the energy generated by the first circuit at the third frequency; and a receiver (124) for detecting in the surveillance area energy radiated by the second circuit at the resonant frequency.

Description

DUAL FREQUENCY TAG USING RF AND MICROWAVE TECHNOLOGY

Field of the Invention

The present invention generally relates to electronic article surveillance (EAS) systems and tags used in EAS systems, and more particularly to EAS systems and tags used therein which employ a combination of both radio frequency (RF) technology and microwave technology. As used herein, RF refers to a limited frequency electromagnetic spectrum extending from approximately 5 MHz to 15 MHz.

Background of the Invention

Electronic article surveillance (EAS) systems are often used for detecting the presence of a protected article within a surveillance area. The surveillance area is established, for example, next to an exit of a store. A tag is attached to an article which is to be protected against unauthorized removal from the store. The tag is deactivated or removed from the article once the article is properly purchased or otherwise authorized for removal from the store, such that the article can be carried through the surveillance area and out of the store without generating an alarm. However, if the tag is not deactivated or removed from the article, then attempts to move the protected article through the surveillance area toward the exit of the store are detected by the EAS system. Specifically, the EAS system detects the presence of the tag attached to the article in the surveillance area. In this manner, the EAS system detects unauthorized attempts to remove protected articles from the store. Generally, there are two types of conventional non-magnetic EAS systems: microwave EAS systems and radio frequency (RF) EAS systems. As used herein, RF refers to a limited frequency electromagnetic spectrum extending from approximately 5 MHz to 15 MHz.

Conventional microwave EAS systems include a microwave transmitter for radiating energy at a first microwave frequency (hereinafter called the first microwave energy) into an area designated as a surveillance area. A tag having a non-linear element (such as a diode) responsive to the first microwave energy receives the first microwave energy (when the tag is in the surveillance area) and, as a result of such reception, radiates energy at a second microwave frequency (hereinafter called the second microwave energy) . A receiver is tuned to receive the second microwave energy and, upon such reception of the second microwave energy, generates an alarm in some manner.

Generally, microwave EAS systems are advantageous because at a given power level, the transmitting range and receiving sensitivity of microwave transmitters and receivers are greater than that of RF transmitters and receivers used in RF EAS systems. Consequently, at a given power level, the size of a surveillance area achievable by a microwave EAS system is larger than that achievable with an RF EAS system. This difference between microwave and RF EAS systems is significant given that the Federal Communications Commission (FCC) effectively limits the power levels at which microwave and RF EAS systems can transmit. Therefore, at the maximum power levels allowed by the FCC, it is possible to protect larger areas (by achieving larger surveillance areas) with microwave EAS systems than with RF EAS systems. Microwave EAS systems have several inherent flaws, however, particularly when compared to RF EAS systems, such as poor pick rate, the difficulty in detecting shoplifters within a large opening, and poor deactivation techniques. Pick rate in microwave EAS systems is poor as compared to RF EAS systems mainly because of the large surveillance areas in microwave EAS systems. Therefore, in microwave EAS systems, several people can be within the protected area at the same time, making it difficult to determine the identity of the shoplifter (i.e., the person having possession of the protected article) when an alarm is generated by the EAS system. Detection is more difficult in microwave EAS systems because the tags used therein are more easily shielded. Tag deactivation in microwave EAS systems is achieved by placing a deactivating device in contact with the tag to thereby destroy one of the tag elements with electrical current from the deactivating device. In RF EAS systems, the tag can be remotely deactivated; that is, the tag can be deactivated without actually touching the tag with a deactivating device. Such remote deactivation allows much more rapid processing at checkout.

Conventional RF EAS systems are conceptually similar to microwave EAS systems, except that the components transmit and receive at RF frequencies. Generally, conventional RF systems are advantageous in that they are more sensitive and more reliable than conventional microwave EAS systems. However, as discussed above, the size of the surveillance area achievable using RF EAS systems is relatively less than the size of the surveillance area achievable using microwave EAS systems.

Therefore, what is required is an EAS system having the large surveillance area capacity of microwave EAS systems and the high performance and reliability characteristics of RF EAS systems. Summary of the Invention

The present invention is directed to a system for detecting the presence of a protected article within a surveillance area. The system includes first transmitting means for radiating energy at a first predetermined frequency into the surveillance area, wherein the first frequency is within a first predetermined frequency range, and second transmitting means for radiating energy at a second predetermined frequency into the surveillance area, wherein the second frequency is within a second predetermined frequency range. The system also includes a tag for being affixed to an article to be protected and capable of being moved with the article into the surveillance area. The tag includes a first circuit having means for receiving energy radiated at the first frequency and at the second frequency, and for mixing the energy received at the first and second frequencies to thereby generate energy at a third predetermined frequency, the third predetermined frequency being within a third predetermined frequency range. The tag also includes a second circuit and means for coupling the first and second circuits such that the second circuit receives the energy generated by the first circuit at the third frequency. The second circuit includes means having a resonant frequency substantially the same as the third frequency for resonating and thereby radiating energy at the resonant frequency when the second circuit receives the energy generated by the first circuit at the third frequency. The system further includes receiver means for detecting in the surveillance area energy radiated by the second circuit at the resonant frequency. Brief Description of the Drawings

The foregoing summary, as well as the following detailed description of the presently preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the present invention is not limited to the particular arrangements and instrumentalities shown. Fig. 1 is a block diagram of an electronic article surveillance (EAS) system according to an embodiment of the present invention;

Fig. 2 is a block diagram of a tag according to an embodiment of the present invention, the tag being utilized in EAS systems such as that shown in Fig. 1;

Fig. 3 is a block diagram of a first transmitter according to an embodiment of the present invention, the first transmitter being a component in the EAS system of Fig. 1;

Fig. 4 is a block diagram of a second transmitter according to an embodiment of the present invention, the second transmitter being a component of the EAS system of Fig. 1; Fig. 5 is a schematic diagram of the tag of

Fig 2 in accordance with a preferred embodiment of the present invention; and

Fig. 6 is a schematic diagram of the tag of Fig. 2 in accordance with an alternate embodiment of the present invention.

Detailed Description of the Preferred Embodiments

Referring in detail to the drawings, wherein like numerals indicate like elements throughout, there is shown in Fig. 1 an electronic article surveillance (EAS) system 102 in accordance with a preferred embodiment of the present invention for detecting the presence of a protected article 104 within a surveillance area 106. The EAS system 102 includes a first transmitting means, such as first transmitter 108 and an accompanying antenna 110, for radiating energy (indicated by arrow 112) at a first frequency into the surveillance area 106. The first frequency is preferably within a first predetermined frequency range. Preferably, the first transmitter 108 is a microwave or ultra-high frequency (UHF) transmitter such that the first predetermined frequency range comprises microwave or UHF frequencies, preferably from approximately 898 MHz to approximately 902 MHz. The first frequency is preferably fixed at approximately 900 MHz. Fig. 3 is a more detailed schematic block diagram of the first transmitter 108 in accordance with a preferred embodiment of the present invention. The first transmitter 108 includes a master oscillator 302 which operates at the first frequency, i.e., approximately 900 MHz. Signals generated by the master oscillator 302 at the first frequency are transferred to a buffer and driver amplifier 304, which operates in a well-known manner to isolate the master oscillator 302 from other components in the first transmitter 108 and to also provide preliminary amplification to the signal from the master oscillator 302. The signal from the buffer and driver amplifier 304 is amplified by a power output amplifier 306 in a well-known manner. The amplified signal from the power output amplifier 306 is transferred to an antenna coupling circuit 308 which operates in a well-known manner to couple and match the amplified output signal to the antenna 110.

The EAS system 102 also includes a second transmitting means, such as second transmitter 114 and an accompanying antenna 116, for radiating energy

(indicated by arrow 118) at a second frequency into the surveillance area 106. The second frequency is within a second predetermined frequency range. Preferably, the second transmitter 114 is also a microwave or UHF transmitter such that the second predetermined frequency range comprises microwave or UHF frequencies, preferably from approximately 907 MHz to approximately 909 MHz. More particularly, the second transmitter 114 is preferably a microwave or UHF sweep transmitter having a sweep frequency range equal to the second predetermined frequency range, or, in the present embodiment, approximately 907 MHz to 909 MHz, such that the second transmitter 114 radiates energy at frequencies within the second predetermined frequency range.

Fig. 4 is a more detailed schematic block diagram of the second transmitter 114 in accordance with a preferred embodiment of the present invention. The second transmitter 114 is similar to the first transmitter 108, except that the master oscillator 402 operates at frequencies within the second predetermined frequency range, i.e., approximately 907 MHz to 909 MHz. The second transmitter 114 includes a buffer and driver amplifier 404, a power output amplifier 406, and an antenna coupling circuit 408, each of which functions in substantially the same manner as the corresponding components of the first transmitter 108. The second transmitter 114 also preferably includes a sweep generating circuit 410 which, in a well-known manner, controls the master oscillator 402 such that the master oscillator 402 sweeps between 907 MHz and 909 MHz (i.e., the second predetermined frequency range) .

The first transmitter 108 includes a first antenna 110 which is preferably a microwave or UHF directional antenna. The second transmitter 114 includes a second antenna 116 which is also preferably a microwave or UHF directional antenna. The first and second antennas 110 and 116 are aimed and focused to provide an electromagnetic field in an area designated as the surveillance area 106. The positioning of the surveillance area 106 in a store or library (or some other area requiring protection against unauthorized removal of articles) , such as next to an exit in a store or library, can be adjusted by appropriate positioning of the first transmitter 108, the second transmitter 114, and a receiver 124 (described below), and by appropriate focusing of the first and second antennas 110 and 116 and an antenna 126 of the receiver 124 in a manner which is well known in the EAS art. Since the first and second antennas 110 and 116 are microwave or UHF antennas, it is possible to precisely focus and contain the energy 112, 118 within the surveillance area 106, thereby reducing false alarms.

As indicated above, at a given power level, the size of the surveillance area 106 depends largely on the transmitting range of the first and second transmitters 108 and 114 and the receiving sensitivity of the receiver 124 (described below) . Since the transmitters 108 and 114 are microwave or UHF transmitters, they have a greater transmitting range (approximately twenty feet at current FCC maximum power levels) than RF transmitters (approximately three to five feet at current FCC maximum power levels) . Therefore, a width D (Fig. 1) of the surveillance area 106 achievable by the EAS system 102 of the present invention is generally greater than surveillance area widths achievable using conventional RF EAS systems. The EAS system 102 also includes a tag 120 which is affixed to an article 104 to be protected, such that the tag 120 is capable of being moved with the article 104 into the surveillance area 106. The tag 120 is affixed to the article 104 using any well known affixing means, such as chemical adhesion, staples, pins, etc. Preferably, the tag 120 is removably affixed to the article 104 such that the tag 120 can be removed from the article 104 by an operator once the operator has authorized the removal of the article 104 from the surveillance area 106 (and from the space enclosing the surveillance area 106, such as a store or library) . Alternatively, the tag 120 is affixed to the article 104 in a generally permanent manner and may then be deactivated by an operator once the operator has authorized the removal of the article from the surveillance area 106 (and from the space enclosing the surveillance area 106) . The tag 120 can be deactivated using a number of different methods, such as exposing the tag to an electromagnetic field at a predetermined power level and frequency, for example, as the article is being scanned at a checkout area. Since the tag 120 can be deactivated without removal from the article 104 and without the need for the deactivating device actually touching the tag 120, the tag 120 can be hidden in or on the article 104, thereby enhancing security. Also, since it is not necessary to remove the tag 120 from the article 104, the time necessary for processing of the article 104 (for example, purchase of the article) is reduced. Referring to Fig. 2 , the tag 120 includes a first circuit, such as a mixer 202, for receiving the energy 112, 118 radiated at the first frequency and at the second frequency, respectively, and for mixing the energy 112, 118 received at the first and second frequencies to thereby generate energy (indicated by arrow 203) at a third predetermined frequency. The third predetermined frequency is within a third predetermined frequency range. The mixer 202 preferably receives the energy 112, 118 radiated at the first and second frequencies when the tag 120 is in the surveillance area 106. Preferably, the mixer 202 subtracts the energy 112 received at the first frequency from the energy 118 received at the second frequency (or vice versa) such that the third predetermined frequency at which the mixer 202 generates energy 203 is substantially equal to the difference between the first and second frequencies. As discussed above, the first transmitter 108 preferably radiates energy 112 at approximately 900 MHz, and the second transmitter 114 preferably radiates energy at frequencies between approximately 907 MHz and approximately 909 MHz. Consequently, the mixer 202 preferably generates energy 203 at frequencies within the third predetermined frequency range of approximately 7 MHz to approximately 9 MHz (that is, RF frequencies) . The mixer 202 is a hybrid microwave (or UHF) and RF device, since the mixer 202 receives and mixes microwave (or UHF) energy 112, 118 and generates RF energy 203.

The tag 120 also includes means, such as a coupling circuit 204, for coupling the mixer 202 and a resonating circuit 206 (described below) such that the resonating circuit 206 receives the energy 203 generated by the mixer 202 at the third frequency.

The tag 120 further includes a second circuit, such as the just-mentioned resonating circuit 206, having a resonant frequency which is substantially the same as the third frequency (preferably approximately 7 MHz to 9 MHz, and more particularly approximately 8.2 MHz) . The resonating circuit 206 resonates and thereby radiates energy 122 at the resonant frequency when the resonating circuit 206 receives the energy 203 generated by the mixer 202 at the third frequency. In a well known manner, the resonating circuit 206 preferably includes an inductance and a capacitance which may comprise one or more capacitor components in parallel or in series with the inductor component. Since the tag 120 and the receiver 124 of the present invention utilize RF technology, the EAS system 102 of the prevent invention achieves the high reliability and the high performance characteristics generally associated with conventional RF EAS systems. The tag 120 (Fig. 2) may be implemented in a number of ways in accordance with the present invention, such as in Fig. 5, which is a schematic diagram of a tag 120' in accordance with a preferred embodiment of the present invention. The tag 120' includes a broad band pickup loop 504 which is formed on a first or front surface 503 of an insulative substrate 502. The insulated substrate 502 is fabricated of material well known in the art having predetermined insulative and dielectric characteristics. The broad band pickup loop 504 is formed on the front surface 503 of the substrate 502 utilizing electrically conductive materials of a known type, such as aluminum, in a manner which is well known in the electronic article surveillance art. The broad band pickup loop 504 includes a solid state mixing diode 506 which is bonded to the front surface 503 of the substrate 502 in a known manner such as by chemical adhesion, etc. The broad band pickup loop 504, including the mixing diode 506, represent the mixer 202 (Fig. 2) in that the broad band pickup loop 504 receives energy radiated at selected microwave frequencies, and the mixer diode 506 mixes the energy received at the selected microwave frequencies. In particular, the broad band pickup loop 504 receives the energy 112, 118 radiated at the first and second frequencies, respectively, and the mixer diode 506 mixes (preferably subtracts) the received energy 112, 118 to thereby generate the energy 203 at the third predetermined frequency (that is, the difference of the first frequency and second frequency) . The tag 120' also includes circuitry (such as the capacitor components and the inductor component described above) for establishing an RF resonating section 520 which represents the resonating circuit 206 (Fig. 2) . Referring to Fig. 5, the inductor component is formed by a coiled portion 512 of a conductive pattern 508 formed on the front surface 503 of the substrate 502. A capacitor component is formed by a first plate 510 of the conductive pattern 508 in combination with a second plate (not shown) formed on an opposite or rear surface (not shown) of the substrate 502, wherein the second plate (not shown) is aligned with the first plate 510 and wherein the substrate 503 is the capacitor dielectric. Other capacitor components may be formed on the tag 120' in a similar manner. As those skilled in the art will appreciate, the number and size of the inductor component and the capacitor components are determined based upon the desired resonant frequency of the resonating circuit 206 and the need to maintain a low induced voltage across the plates of the capacitors. The tag 120', in particular the resonating circuit 206, is described in U.S. Patent No. 5,103,210 entitled "Activateable/ Deactivateable Security Tag For Use With An Electronic Security System" which is incorporated herein by reference.

As implied in Fig. 2, the coupling circuit 204 may be a physical component, although those skilled in the art will appreciate that the mixer 202 can be coupled to the resonating circuit 206 without the use of a physical component. Referring to Fig. 5, for example, the broad band pickup loop 504 is electromagnetically coupled to the RF resonating section 520 (which represents the resonating circuit 206 in Fig. 2) by virtue of the close physical proximity of the broad band pickup loop 504 to the RF resonating section 520. Therefore, the RF resonating section 520 receives the energy 203 generated by the broad band pickup loop 504 at the third predetermined frequency by virtue of the inherent electromagnetic coupling between the broad band pickup loop 504 and the RF resonating section 520. Upon receiving the energy 203 at the third predetermined frequency, the RF resonating section 520 (representing the resonating circuit 206 of Fig. 2) resonates and thereby radiates energy 122 at the resonant frequency as described above.

Fig. 6 is a schematic diagram of a tag 120' ' in accordance with an alternate embodiment of the present invention. The tag 120" includes a multi-turn loop 604 which is formed on a first or front surface 603 of an insulated substrate 602. The insulated substrate 602 is fabricated of material well known in the art having predetermined insulative and dielectric characteristics. The loop 604 is formed utilizing electrically conductive materials of a known type, such as aluminum, in a manner well known in the art. The loop 604 includes an outer turn 606 and one or more inner turns 608. The loop 604 also includes a solid state mixing diode 610 which is bonded to the front surface 603 of the substrate 602 in a well known manner. The outer turn 606 of the loop 604 and the mixing diode 610 operate in a manner similar to the broad band pickup loop 504 and the mixing diode 506 of the tag 120' of Fig. 5, and therefore, the outer turn 606 of the loop 604 and the mixing diode 610 represent the mixer 202 (Fig. 2) . In a manner similar to that described above, the entire loop 604, including the outer turn 606 and the inner turns 608, a plate 612, and additional conductive patterns (not shown) formed on a second or back surface (not shown) of the substrate 602 form circuitry (such as the capacitor components and the inductor component described above) for establishing an RF resonating section (not numbered) which represents the resonating circuit 206 (Fig. 2). In the tag 120' ' of Fig. 6, electromagnetic coupling between the mixer 202 (represented by the outer turn 606 of the loop 604 and the mixing diode 610) and the RF resonating section (represented by the loop 604 and the plate 612) is achieved by using a single conductor (i.e., the loop 604) to operate at both the microwave and the RF frequencies as described above. Referring again to Fig. 1, the EAS system 102 also includes receiver means, such as receiver 124, for detecting in the surveillance area 106 energy 122 radiated by the resonating circuit 206 of the tag 120 at the resonant frequency (preferably approximately 7 MHz to 9 MHz, and more particularly approximately 8.2 MHz). The receiver 124 includes an antenna 126 which is preferably an RF antenna that is optimized to receive energy at frequencies within the third predetermined frequency range. Upon detecting the energy 122 radiated by the resonating circuit 206 of the tag 120 at the resonant frequency, the receiver 124 preferably sends a signal to an alarm processing device 128. The alarm processing device 128 may be separate from, or part of, the receiver 124. Upon receiving the signal from the receiver 124, the alarm processing device 128 takes appropriate action (such as sounding either an audible, visual or silent alarm, or combination thereof) to notify appropriate personnel of the existence of the tag 120 and the protected article 104 attached thereto in the surveillance area.

As shown in Fig. 1, the first transmitter 108 and the second transmitter 114 are positioned on one side of the surveillance area 106 and the receiver 124 is positioned on the opposite side of the surveillance area 106. It will be understood by those skilled in the art that in the present invention the receiver 124 can be located close to the first transmitter 108 and/or the second transmitter 114 without adversely affecting the performance of the EAS system 102 since the first and second frequencies at which the first and second transmitters 108, 114 transmit are significantly different than the third frequency at which the receiver 124 receives. Thus, the first transmitter 108, the second transmitter 114, and the receiver 124 may be positioned in many other configurations without departing from the scope of the present invention. For example, in an alternate embodiment the first transmitter 108 and the second transmitter 114 are positioned on opposite sides of the surveillance area 106 and the receiver 124 is positioned on either side of the surveillance area 106, or inside the surveillance area 106.

The EAS system 102 operates as follows. The first transmitter 108 radiates energy 112 at the first predetermined frequency (approximately 900 MHz) into the surveillance area 106, and the second transmitter 114 radiates energy 118 at the second predetermined frequency (that is, at swept frequencies between approximately 907 to approximately 909 MHz) into the surveillance area 106. The mixer 202 of a tag 120 located within the surveillance area 106 receives and subtracts the energy 112 radiated at the first predetermined frequency from the energy 118 radiated at the second predetermined frequency (or vice versa) to thereby generate energy 203 at the third predetermined frequency, which is equal to the difference between the first and second predetermined frequencies (approximately 7 MHz to 9 MHz) . The coupling circuit 204 in the tag 120 couples the mixer 202 to the resonating circuit 206 in the tag 120 such that the resonating circuit 206 receives the energy 203 generated by the mixer 202 at the third predetermined frequency. The energy 203 at the third predetermined frequency causes the resonating circuit 206 to resonate at its resonant frequency, which is substantially the same as the third predetermined frequency (approximately 7 MHz to 9 MHz, and more particularly approximately 8.2 MHz), such that the resonating circuit 206 radiates energy 122 at the resonant frequency. The receiver 124 detects the energy 122 radiated by the resonating circuit 206 of the tag 120 at the resonant frequency and sends a signal to the alarm processing device 128, which takes appropriate action to notify appropriate personnel of the presence of the tag 120 in the surveillance area 106.

It should be noted that the EAS system 102 of the present invention greatly reduces false and phantom alarms which result from inadvertent illuminating of items (such as windows, doors, etc.) having resonant frequencies approximately equal to the third predetermined frequency. In conventional (both microwave and RF) EAS systems, the transmitters illuminate the items (that is, cause the items to resonate at their respective resonant frequencies) because the transmitters radiated energy at the respective resonant frequencies of the items. In contrast, in the EAS system 102 of the present invention, the first and second transmitters 108, 114 do not transmit at the resonant frequency of the items (that is, the third predetermined frequency) , but at the much different first and second predetermined frequencies. Therefore, the items are not illuminated by the first or second transmitter 108, 114 and false and phantom alarms are avoided.

It should be understood that the present invention includes the EAS system 102, and also the tag 120 which forms a part of the EAS system 102. While preferred embodiments of the present invention have been described and certain modifications thereto suggested, it will be recognized by those skilled in the art that other changes may be made to the above-described embodiments of the invention without departing from the broad, inventive concepts thereof. It should be understood, therefore, that the invention is not limited to the particular embodiments disclosed, but covers any modifications which are within the scope and spirit of the invention as defined by the appended claims.

Claims

1. A system for detecting the presence of a protected article within a surveillance area, the system comprising: first transmitting means for radiating energy at a first predetermined frequency into the surveillance area, the first frequency being within a first predetermined frequency range; second transmitting means for radiating energy at a second predetermined frequency into the surveillance area, the second frequency being within a second predetermined frequency range; a tag for being affixed to an article to be protected and capable of being moved with the article into the surveillance area, the tag comprising a first circuit having means for receiving energy radiated at the first frequency and at the second frequency and for mixing the energy received at the first and second frequencies to thereby generate energy at a third predetermined frequency, the third frequency being within a third predetermined frequency range, the tag also comprising a second circuit and means for coupling the first and second circuits such that the second circuit receives the energy generated by the first circuit at the third frequency, the second circuit having a resonant frequency substantially the same as the third frequency for resonating and thereby radiating energy at the resonant frequency when the second circuit receives the energy generated by the first circuit at the third frequency; and receiver means for detecting in the surveillance area energy radiated by the second circuit at the resonant frequency.

2. The system of claim 1, wherein the first frequency range is generally equal to the second frequency range.

3. The system of claim 1, wherein the first and second transmitting means comprise first and second microwave transmitters, respectively, such that the first and second frequencies comprise first and second predetermined microwave frequencies, respectively.

4. The system of claim 1, wherein the mixing means comprises means for subtracting the energy received at the first frequency from the energy received at the second frequency such that the third predetermined frequency at which the mixing means generates energy is substantially equal to the difference between the first and second frequencies.

5. The system of claim 4, wherein the third predetermined frequency range is 7 MHz to 9 MHz.

6. A security tag for use with an electronic security system for detecting the presence of the security tag within a surveillance area, the system having first transmitting means for radiating energy at a first predetermined frequency into the surveillance area, second transmitting means for radiating energy at a second predetermined frequency into the surveillance area, and receiving means for detecting in the surveillance area energy radiated at a resonant frequency which is substantially the same as a third predetermined frequency, the first, second, and third frequencies being within first, second, and third predetermined frequency ranges, respectively, the security tag comprising: a first circuit having means for receiving energy radiated at the first frequency and at the second frequency and for mixing the energy received at the first and second frequencies to thereby generate energy at the third predetermined frequency; a second circuit; means for coupling the first and second • circuits such that the second circuit receives the energy generated by the first circuit at the third frequency; wherein the second circuit comprises means for resonating and thereby radiating energy at the resonant frequency when the second circuit receives the energy generated by the first circuit at the third frequency.

7. A security tag for use with an electronic security system for detecting the presence of the security tag within a surveillance area, the system having first microwave transmitting means for radiating microwave energy at a first predetermined microwave frequency into the surveillance area, second microwave transmitting means for radiating microwave energy at a second predetermined microwave frequency into the surveillance area, and receiving means for detecting in the surveillance area energy radiated at an RF resonant frequency, the first and second microwave frequencies being within first and second predetermined microwave frequency ranges, respectively, the security tag comprising: a first circuit having means for receiving microwave energy radiated at the first microwave frequency and at the second microwave frequency and for mixing the microwave energy received at the first and second microwave frequencies to thereby generate energy at an RF frequency which is substantially the same as the resonant frequency; a second circuit; and means for coupling the first circuit and the second circuit such that the second circuit receives the energy generated by the first circuit at the RF frequency; wherein the second circuit comprises means for resonating and thereby radiating energy at the resonant frequency when the second circuit receives the energy radiated by the first circuit at the RF frequency.

8. The security tag of claim 7, wherein the RF frequency is in a range from 7 MHz to 9 MHz.