AU695927B2 - Device for detecting an article provided with an electronic security tag - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANIDARD PATENT

ORIGINAL

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Name of Applicant: Actual Inventors: Address of Service: Invention Title: ESSELTE METO INTERNATIONAL GimbH Jurgen SCI-JOON and Michael Plivid CROSSFIELD SI-IELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "1DEVICE FOR DETECTING AN ARTICLE PROVIDED WITH AN ELECTRONIC SECURITY

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The following statement is a full description of this invention, including the best method of performing it known to us:la Device for Detecting an Article Provided with an Electronic Security Tag This invention relates to a device for detecting an article provided with an electronic security tag in an interrogation zone, comprising at least one transmit device emitting an alternating magnetic field causing the electronic security tag to deliver a characteristic signal, and at least one receive device producing an alarm on receiving the characteristic signal.

Devices for the electronic protection of articles have found widespread use in department stores and ware- Shouses Inventory losses caused by thefts can be sub- *stantially reduced by installing such devices. Devices of this type are conventionally located in the entrance and exit areas of department stores and warehouses in order to indicate the unauthorized removal of electronically protected merchandise from the sales or warehouse area.

One of the factors determining the detection rate of S an electronically protected article is the width of the zone subject to surveillance. In order that the presence of an electronically protected article be detected in the interrogation zone at a high rate of detection, devices referred to as antennas are conveniently arranged at a S" 25 minimum possible relative distance in the exit area of a department store or a warehouse.

*to However, a closely spaced arrangement of antennas in the entrance and exit area spoils the visual impression 3 conveyed to the customer on entering the department S" 30 store: It suggests a feeling of confinement and being locked in. As a result, the entrance area has a less inviting effect on customers. This impression is stronger still if the antennas are not transparent. Moreover, closely spaced antennas obstruct customer traffic in the entrance and exit areas.

-2- It is an object of the present invention, at least in the preferred embodiment, to overcome or substantially ameliorate one or more of the disadvantages of the prior art.

According to the invention there is provided a device for detecting an article provided with an electronic security tag in an interrogation zone, comprising at least one transmit device emitting an alternating magnetic field causing the electronic security tag to deliver a characteristic signal, and at least one receive device producing an alarm on receiving the characteristic signal, characterised in that the at least one tramsit device includes a coil having several truns, that the turns lie essentially in one plane, and that the turns are subdivided into at least two groups, said groups being arranged in a spaced relationship to each other providing a gap there between.

In its preferred embodiment, the present invention provides a widened interrogation zone and sufficiently high detection rate. In particular, it is proposed that the transmit device include a coil having several turns, that the turns lie essentially in one plane, and that the turns be subdivided into at least two groups, with the (two) groups being arranged at a defined relative distance.

By means of the device of the present invention, the field strength of the magnetic field in the interrogation zone can be substantially increased without this involving an increased power consumption relative to a corresponding detecting device that does not have the spatial separation of the groups of turns.

*20 When viewed from the other side, this means that the width of the zone that can be reliably interrogated by the device of the present invention can be selected larger than o: the zone that a prior-art device has so far been able to monitor reliably.

Due to the subdivision of the turns of the transmit device into at least two groups as disclosed by the present invention with the (two) groups being spaced at a defined relative distance, the mutual influence of the turns and thus the inductivity of the coil are substantially reduced. The reduction of the inductivity of the coil is accompanied by an increase in the quality factor Q of the coil, resulting in particular in reduced losses in the high-frequency range. The quality factor Q is defined as follows: Q(o) coL/R (where co. is the frequency, and R the resistance dependent on the frequency Co).

The device according to the preferred embodiment of the present invention results A? in an appreciably reduced inductivity relative to the close arrangement of the individual turns as hitherto known and employed. It is therefore possible to increase the number 2a of coil turns of the transmit device and thus the field stren-,gth within the interrogation zone without this involving an increased load resistance relative to the solution known in the art.

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a S .9 9S*99* 9 I848I-06DOCLt(' 3 The use of litz wire instead of customary cables having a single metal wire has proved to be of particular advantage in accordance with a further feature of the device of the present invention. The skin effect is hardly noticeable with litz wire which is composed of wire mesh comprising a plurality of thin wires, whilst in the use of conventional wire the skin effect results in a severely increased resistance at high frequencies.

In a further feature of the device of the present invention, the turns are arranged in a plane lying normal to the direction of passage through the interrogation zone. In a first aspect, therefore, the turns of the coil may be arranged in such a way that they lie side by side as seen when looking at the antenna in a top plan view. Thus, the dimensions of the external turns, in particular, the turns of the outer group, are larger than those of the internal turns, that is, the turns of the inner group. Without doubt, this arrangement has the enormous advantage of enabling the width of the antennas to be selected very small.

In a second aspect, the possibility exists to arrange i :the turns in such a way that they lie one behind the other as seen when looking at the antenna in a top plan view. Although this arrangement necessitates wider antennas, this need is compensated for in that it offers the advantage that the turns of both or all groups have o the maximum possible extent with a predetermined antenna size.

o.ti In an advantageous further feature of the device of 30 the present invention, spacing means are provided for spatially separating the (two) groups of turns from each other and maintaining them at a constant relative distance.

To increase the stability of the antenna, an alternative solution proposes the following: When fastening the turns of the transmit device, a groove formed by rib members disposed on either side is provided at least in the two side areas of the antenna, the groove serving as a substitute for the spacing means. A rod made of a hard 40 material is inserted into this groove following assembly, Moreover, it is advantageous to configure the device of the present invention in such a way that the transmit device and at least one receive device are arranged in an ante~a Corming a unit. Depending on the area subject to surveillance, it is possible to accommodate in one antenna one transmit device and two complete receive devices which are in turn combined from several individual 4 receive coils. Each receive device detects separately the characteristic signals of security tags present in two adjacent interrogation zones.

As an alternative, it is of course also possible to arrange the transmit device and the receive device(s) in separate antennas.

In a particularly advantageous embodiment of the device of the present invention, it is proposed that in cases where an integral configuration is chosen the antenna be comprised of an outer section and an inner section, with the transmit device being accommodated in the outer section and the receive device(s) in the inner section of the antenna. In particular, the antenna is provided with a perforated front and rear side lending it a modern, open design having a high aesthetic appeal. This configuration also affords technical advantages: The material consumption for the receive device(s) can be substantially reduced due to its (their) low surface area.

From EP 0 428 384 Bl, a universal antenna assembly for use in electronic article surveillance systems is S.known. The antenna is composed of a base adapted to be anchored to the floor, an upper section having two parallel side sections, as well as a center section 25 rigidly connected with the base and an upper section.

Further, means are provided for joining the two housing halves in predetermined positions. The turns of the transmit and receive coils extend above the base. While the turns of the transmit device extend in the peripheral areas of the upper section, the turns of the receive coils are located in the center section of the antenna.

The physical composition of the antenna constructed in accordance with the present invention differs from this known construction in a decisive way. The antenna 35 is composed of two half shells preferably manufactured by injection-molding techniques. The half shells form a circumferential frame with a center section dividing the area predetermined by the frame into two halves. The S. turns of the transmit device are arranged in the frame of 40 the antenna; for the transmit device, therefore, the maximum available surface area of the antenna is utilized approximately. By contrast, the receive coils are preferably located in the center section of the antenna.

It will be understood, of course, that the new physical composition of the antenna is not limited to the subdivision of the turns of the transmit coil into two groups, but is generally suitable for use with antennas of any desired configuration.

The receive device is advantageously configured such as to include at least one receive coil. Advantageously, the receive coil is wound such as to be multipolar; preferably, it has a quadrupole characteristic. Such a configuration enables both the influence of the transmit device and the influence of electromagnetic fields originating from outside the interrogation zone far fields) on the receive coils to be re'duced considerably.

A further advatnage becomes apparent when using several receive coils: In such an arrangement, the individual receive coils are relatively tuned such that the detection rate is sufficiently high in all areas of the interrogation zone. For example, a receive coil shaped as a "figure of eight" has a zero detection rate on a line extending through the point of intersection between the upper and the lower loop of the "figure of eight" and parallel to the upper or lower edge of the antenna. By superposing another "quadrupole offset figure-of-eight" upon this receive coil, so that the center area of one of the two loops of the "quadrupole offset figure-of-eight" is superposed upon the area in which the o 15 two loops of the first "figure of eight" intersect, it is ensured that there is no place where the detection rate of the receive coils is equal to zero.

An additonal advantage in the use of several receive coils is afforded in that an additional protection against false alarms is obtained as a result of the different detection sensitivities of the coils in different areas of the interrogation zone. In this connection, reference is made in particular to false alarms produced by intereference having its source in the immediate proximity of the interrogation zone. The increased safety results from the fact that it is highly improbable for an intereference to interfere with all of the differently shaped receive coils in the same manner.

The device of the present invention is suitable for utilization with any possible configuration of security tag. Thus, the electronic security tag may involve an element fabricated from a material of high permeability and low coercive force (magnetically soft meaterial). Moreover, the device of the present invnetion detects security tags made of a material having magnetostrictive properties. The same applies in principle to security tags having a resonant circuit. Security tags having the above-described properties are known, for example, from the following European patent specifications: EP 0 295 028 BI, EP0 181327Bl, andEP 0 093 281B1.

Sa Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". 9* S 4 4.

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The present invention will be explained in more detail in the following with reference to the accompanying drawings. In the drawings, FIG, 1 is a schematic view of the device constructed in accordance with the present invention; FIG, 2 is a partial sectional view of the device of the present invention illustrating a preferred embodiment thez4 of; FIG, 3 is a top plan view of the antenna; FIG. 4 is a sectional view taken along the line IV-IV of FIG. 2; FIG. 5 is a sectional view taken along the line V-V of FIG. 2, FIG. 6 is a sectional view taken along the line VI-VI of FIG. 2; FIG. 7 is a sectional view taken along the line VII- VII of FIG. 2; atoms*FIG. 8 is a sectional view taken along the line VIII- VIII of FIG. 2; FIG, 9 is a sectional view taken along the line IX-IX of FIG. 2; FIG. 10 is a sectional view taken along the line X-X atom of FIG. 2; a 4 aFIG. ii1 is a sectional view taken along the line XI- X1 of FIG. 2; FIG. 12 is a sectional view taken along the line XII-XII of FIG. 2; FIG, 13 is a partial sectional view of the device of the present invention illustrating a further embodiment thereof, FIG. 14 is a sectional view taken alon~g the line XIV-XIV of FIG. 13; FIG, 15 is a sectional view taken along the line XV- XV of FIG. 13; and FIG, 16 is a sectional view taken along the line XVI-XVI of FIG. 13.

7 FIG. 1 is a schematic illustration of the device 1 of the present invention for detecting the presence of an article provided with a security tag 2 in an interrogation zone 3. The interrogation zone 3 is formed by two antennas 4 mounted in parallel arrangement and comprising a transmit device 5 and a rq-ceive device 6. current is delivered to the t-ansmit device 5 from a power supply 7 under computer control in such a way that the transmit device emits an alternating magnetic field (several alternating magnetic fields) with the desired frequency (frequencies) into the interrogation zone 3, The f requency (frequencies) is (are) tuned to the individual characteristics of the security tag 2 employed. Considering that the detection rate depends, inter alia, on the position of the security tag 2 relative to the alternating magnetic field in the interrogation zone 3 the characteristic signal being at a maximum level when the magnetic field and the security tag 2 are in parallel alignment, and being at zero level when aligned vertically to each other an additional requirement is to select the field strength of the magnetic field such that the security tag 2 emits a sufficiently strong character- :istic signal detectable by the receive device 6 also in 25 the positions in between. A particular method of detecting a zecurity tag 2 in the interrogation zone 3 is known, for example, from EP 0 153 286 Bl. The contents of too 0this European patent specification are to be added to the disclosure content of the present. i.1vention by express reference.

The receive device 6 arranged in one or in both too. antennas 4 of the device 1 of the present invention is (are) connected to a computing/control device 8. This computing/control device 8 evaluates the signals of the receive device(s), causing an alarm device 9 to produce an alarm when a characteristic signal is detecoted.

FIG, 2 illustrates a preferred embodiment of the device I of the present invention in partial sectional view. The antenna 4 consists roughly speaking of two 40 partial sections: a frame-type outer section 10 and an inner section 11 disposed between the upper and the lower part of the frame. The transmit device 5 is arranged in the outer section 10, and the receive device 6 in the inner section 11. Both the outer section 10 and the inner section 11. are comprised of two joinable, shell-shaped frame halves 23, 24 serving to accommodate the transmit device 5 and the receive device(s) 6. Mounting means 17 are provided on either side of the outer section 10. The mounting means 17 are fixedly located in position by 8means of floor fasteners 22 comprising, for example, a bolt anchored to the floor, arnd a nut.

In the following, first the structure of the pref erred transmit device 5 and then the structure of the preferred receive device 6 will be described.

The transmit device 5 comprises eighteen turns 12 subdivided into two groups 13a, 13b of turns 12 according to the present invention. In the embodiment shown, either group 13a, 13b has the same number of turns 12.

In general, however, it is possible to subdivide the turns 12 into groups 13a, 13b of any desired constellation.

A gap 15 between the two groups 13a, 13b of turns 12 is created by spacing means 14 arranged at a defined relative distance along the coil turns 12. The spacing means 14 have a bore 28 eatch in the area of the gap created by them, which bore serves to receive a screw 29, The screws 29 serve the added function of connecting the two frame halves 23, 24.

Tn the embodiment shown, the turns 12 are arranged in 6 4 a side-by-side relationship as seen when looking at the ~front or rear side of the antenna 4 in a top plan view~.

~As mentioned in the foregoing, the transmit device 5 of the present invention will provide equally excellent 25 operation if the turns 12 are arranged one behind the other as seen when looking at the front or rear side of the antenna 4.

ofThe assembly of the receive device 6 and the turns 12 ofthe transmit device 5 is performed by proceeding 46:. 30 according to the following steps: 1. The receive coils 18, 19, 20 of the receive device a. are mounted in the inner section 11 of the antenna 4.

**too:2. The frame front half 23 and the frame rear half 24 are pre-adjusted using alignmrent means 32.

35 3. The inner group 13b of turns 12 is mounted in the outer section 10 of the antenna, with rib members 31 6 06providing a lateral boundar~y for the turns 12.

4. The spacing means 14 are inserted anid secured to the two frame halves 23, 24 by means of the screws 29 and 36. in this arrangement, the spacing meanz, 14 perform the added function of maintaining a defined relative distarnce of the two frame halves.

9 The outer group 13a of turns 12 is then inserted in the gap between the two frame halves 23, 24, with the spacing means 14 serving to laterally secure the turns 12 in place.

6. A sealing strip 27 is fitted into the circumferential gap between the two frame halves 23, 24. This results in securing the outer group 13a of turns 12 in their lateral position.

Where necessary, the turns 12 of the groups 13a, 13b are electrically connected in the individual steps.

According to the present invention, the inductivity of the coil embodying the transmi t device 5 is substantially reduced by subdividing the turns 12 and providing the gap 15 between the groups 13a, 13b., The gap 15 between th~e two groups 13a, 13b makes it possible to inc'rease the number of turns 12 and thus the field strength in the interrogation zone 3 without incurring additional power losses.

The transmit device 5 is fed with ctirrent from the power supply 7 under computer control, such that an alternating magnotic field with the desired frequencies is emitted into the- interrogation zone 3. As follows from EP 0 153 286 Bi, these frequencies are, for example, one low frequency in thle Hertz range, and two high fre- 25 quencies in the kilohertz range.

The computing/control device 8 is accommodated in the lower part of the antenna 4 in the form of a computer card, being freely accessible from the outside after opening a f lap 16. The lower part of the antenna 4 is co~~rdin such a way that a clearance is maintained between the outer group 13a of turns 12 and the floor.

This clearance 30 provides sufficient space for accommo- 0: dating electric cables routed to the antenna 4 from outside. Further, sufficient space is available between the groups 13a, 13b of turns 12 and the flap 16 with the flap 16 closed, enabling the electric cables to be routed readily from outside to the computing,! control device 8, which is a computer card in this embodiment, The concealed routing of the electric cables contributes to the harmnonic overall appearance of the antenna 4.

As mentioned in the foregoing, the receive device 6 is arranged in the inner section 11 of the antenna 4.

According to a particularly favorable embodiment, the receive device 6 is comprised of a combination of three receive coils 18, 19. 20. The receive coil 18 is made up of two loops 21 arranged horizontally side-by-side and 10 connected in series, such that current flows through the turns in opposite directions. While the receive c'~iil 18 embodies a horizontally extending "figure of eight",. the receive coil 19 is a vertically extending "figure of eight", and the receive coil 20 is effectively composed of two 'figures of eiqghtl arranged one below the other, which is referred to as an offset figure of eight. The selected shapes of the receive coils 18, 19, 20, particularl~y when used in combination, ensure a sufficient detection rate for an electronic security tag 2 in the entire area of the interrogation zone 3. The degree of coupling between all three receive coils or between them and the transmit device is low. Further, as described in the foregoing, they are characterized by their relative insensitivity to far fields originating from outside the interrogat ion zone.

Pins 33 are arranged in the area of the inner section These pins 33 serve the purpose of securing the turns 12 of the receive coils 18, 19, 20 in the desired 2 0 positions.

The shape of the receive coils 18, 19, 20 is selected such that they have the sensitivity of guadrupole fields.

With this constellation, a sufficient sensitivity of the receive device 6 in the mid-area of the antenna 4 is S accomplished, also when the dimensioning is small. In iddition, interference signals from far fields can be uvessed to a sufficiently high degree, resulting in an opt3~detection rate for the security tags present in the mnt i.'..-,Ation zone.

30 When the receive device 6 receives a signal subsequently recognized by the computing/ control device 8 as a characteristic signal emitted by a security tag 2, it delivers a pulse to the alarm device 9 which then indicates the unauthorized presence of an electronically protected article in the interrogation zone 3, such indication occurring audibly by means of two buzzers 25 and/or visually by means of a flashing light 26. The alarm device 9 is clearly shown in FIG. 3 which is a top plan View of the antenna 4.

40 Further, FIG, 3 illustrates the recess between the of two shell-shaped frame halves 23, 24. The recess is sealed by an elastic closure means in the form of a sealing strip 27. The mounting means 17 arranged in the lower area of the outer section 10 of the antenna 4 are anchored to the floor by means of the floor fasteners 22.

Adjusting screws 37 serve for fine alignmrent of the antenna 4.

11 FIGS. 4 to 12 illustrate the sections identified in FIG. 2 by the corresponding roman numerals. FIG. 4 illustrates a section through an upper corner of the outer section 10 of the antenna 4 The rib members 3 1 prfovided on t1'e insides of the two frame halves 231, 24 serve to provide a lateral boundary for the turns 12 of the inner group 13b. Another function they serve is to contribute to the stability of the outer section 10 of the two frame halves 23, 24. The two f rame halves are screwed together by means of screws 29 extending through bores 28.

FIG. 5 shows a cross-section through the upper corner of the outer section 10 of the antenna 4. The two frame halves 23, 24 have bores 28 for receiving the screws 29.

The bores 28 and the screws 29 are arranged in the area of the gap 15 between the two groups 13a, 13b of turns 12. Further, part of the screws 29 fixedly locates the spacing means 14 in the desired positions. The spacing means 14 are elements made of plastics having a section perforated by the bore 28 in the area of the gap 15 as well as a slot fn-r receiving the turns 12 in the area of the outer group 13a of turns 12. The recess left by the 000 two frame halves 23, 24 at the edge of the outer section is sealed by an elastic closure means referred to as a sealing strip :FIG. 6 illustrates a crozs-section through the outer *~**section 10 of the antenna 4 in the area of the lateral mounting means 17. FIG. 7 relates to a parallel section in the area of a spacing means 14.

*ss 30 FIG. 8 represents a section throughi the bottom porto tion of the outer section 10 of the antenna 4. In addi- :::s:tion to the parts described with reference to the preceding drawings, FIG. 8 shows the clearance 30 for accommodating the electric cables.

FIG, 9 shows a section through the innev. section 11 of the antenna 4, taken along the line IX-IX of FIG. 2.

in addition to showing the frame rear half 24 and the frame front half 23, this Figure also shoa the arrangement of the turns 12 of the receive coils 18, 1.9, 40 The individual turns 12 are held in clamping means FIG, 10 shows a cross-section of the area in which the audible indicating portion of the alarm device 9 is arranged. In particular, the two buzzers 25 are illustrated in this Figure, FIG. 11 shows a section through the visual indicating portion of the alarmi device 9. in a housing cover~ed by a 12 colored transparent cap 34, one (or two) flashing light(s) 26 is (are) arranged producing, in combination with the two buzzers 26, clear visual and, respectively, audible alarm signals as soon as the computing/ control device 8 establishes the presence of the characteristic signal of a security tag 2 in the interrogation zone 3.

FIG. 12 shows a section taken along the line XII-XII of FIG. 2 in the area of one of the six alignment means 32 provided in the inner section 11 of the antennma 4. As mentioned previously, these alignment means 32 are members intended to facilitate the joining together of the two frame halves 23, 24 following assembly of the receive device(s) 6. In the embodiment shown, the alignment means 32 is comprised of two members adapted to fit into each another snugly, one of these members being provided on the inside of the frame front half 23, while the other element is~ provided on the inside of -the frame rear half 24.

FIG. 13 shows a further embodiment of the device of the present invention. it differs from the embodiment previously described merely by the manner in which the turns 12 of the transmit device 5 are fastened. In the lower and upper area of the frame, the fastening function is performed )oy the spacing means 14 as before. In the lateral areas, the spacing means are replaced by two rib members 38, 39 extending between the two groups 13a, 13b **of turns 12. This construction results in a significantly enhanced stability of the antenna 4, protecting it from external impact. The stability of the antenna 4 can be further optimized by the insertion of a rod 40 fabri- ~.cated from a hard material into the space between the two rib members 38, 39, FIG. 14, FIG. 15 and FIG, 16 of the drawings illus- *trate the sections identified in FIG. 13. All further details of the embodiment of FIG. 13, FIG. 14, FIG. and FIG. 16 of the drawings correspond to the details shown in the preceding Figures.

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Claims (9)

1. 2. The device according to claim 1, characterized in that the turns are arranged in a plane lying normal to the direction of passage through the interrogation zone.
2. 3. The device according to claim 1 or claim 2, characterized in that spacing means are provided which spatially separate the (two) groups of turns from each other. o. 4. The device according to claim 1, characterized in that two rib members forming a groove are provided between the two groups of turns. The device according to claim 4, characterized in that a rod fabricated from a hard too: material is provided which is insertable into the groove formed between the two rib 'toomembers.
3. 6. The device according to claim 1 or claim 2, characterizeed in that the at least one transmit device and at least one receive device are arranged in an antenna forming a unit,
4. 7. The device according to claim I or claim 2, characterized in that the at least one transmit device and the at least one receive device are arranged in separate antennas. 8, The device according to claim 1 or claim 2, characterized in that the turns of the coil of the at least one transmit device are fabricated from litz wire.
5. 9. The device according to claim 6, characterized in that the antenna is comprised of an outer section and an inner section, with the transmit device being accommodated in the outer section and the at least one receive device in the inner section of the antenna. The device according to claim I or claim 9, characterized in that the at least one receive device covers a substantially smaller surface area than the at least one transmit device. -14-
6. 11. The device according to claim 10, characterized in that the at least one receive device includes a coil having at least one turn bent such as to have essentially a quadrupole characteristic.
7. 12. The device according to claim 1, characterized in that the at least one receive device has the shape of a "figure of eight", with the two portions of the "figure of eight" being arranged side-by-side or one below the other,
8. 13. The device according to claim 1, characterized in that the electronic security tag is an element fabricated from a material of high permeability and low coercive force (magnetically soft material), or that the material has magnetostrictive properties, or alternatively, that the security tag is an element having a resonant circuit.
9. 14. A device for detecting an article provided with an electronic security tag substantially as herein described with reference to Figure 1 or Figures 2 to 12 or Figures 13 to 16 of the accompanying drawings. .DATED this 15th day of July, 1998 ESSELTE METO INTERNATIONAL GmbH Attorney: JOHN B. REDFERN Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS 0** S Abstract of the Disclosure The invention is directed to a device for detect- ing an article provided with an electronic security tag in an interrogation zone The device is com- prised of at least one transmit device emitting an alternating magnetic field causing the electronic secu- rity tag to deliver a characteristic signal, and of at least one receive device producing an alarm on re- ceiving the characteristic signal. In order to be able to extend the width of passage of the interrogation zone while other parameters remain unchanged, it is proposed according to the present inven- tion that the transmit device include a coil having several turns that the turns (12) lie essentially in one plane, and that the turns (12) be subdivided into at least two groups (13a, 13b), with the (two) groups (13a, 13b) being arranged in a spaced relationship to each other providing a gap (15) therebetween. 4 e 4 44 *4 4