1 2
DETECTION TAG support, but this is used for short-circuiting a capacitor and
not for increasing the capacitance of said capacitor.
BACKGROUND OF THE INVENTION Moreover, an additional connection through the support is
The invention relates to a detection tag for a resonance required,
detection system, comprising a support consisting of elec- 5 Preferred embodiments are specified in the subordinate
tricaliy insulating material and a resonant circuit supported claims.
thereby, which has an inductive element formed by a con- DESCRIPTION OF THE DRAWINGS ductor track disposed on the support in a predefined patter,
and a capacitive element formed by at least two capacitor The invention will be described below in more detail with
electrodes which are kept apart by the support and are 10 reference to the drawings, in which:
constructed as electroconductive electrode regions, the ends FIG. 1 shows an embodiment of the invention with two
of the track being connected to the one and to the other possible resonant frequencies;
capacitor electrode, respectively. mG 2 mustrates ^oHia embodiment of the invention
A detection tag of this type is disclosed by the laid-open 15 wim two possible resonant frequencies; and
European Patent Application 0 463 233 A2. mG 3 depicts a embodiment of me invention with
The support of this known tag is provided on one side four possible resonant frequencies;
with a conductor track according to a spiral and rectangular mG 4 shows { another embodiment of me invention.
pattern and on both sides is provided with electroconductive
regions which form capacitor electrodes or plates of 4 20 DESCRIPTION OF PREFERRED
capacitors. The capacitive element of the support is formed EMBODIMENTS
by a connection in parallel of 2 branches, in which two ~, . x ^. , . iL - . ..
capacitors connected in series are incorporated. Said capaci- , TMe Section tags shown m the figures can be used in an
r , .. ♦ J * .u J t ,1. • 1 _ t electronic detection system (not shown). It is generally
tive element is connected to the ends of the spiral track, as , ^ ^ '. _v . .. . ? . \
. . . . . . . known that a system of mis type is used m shops to protect
a result of which a resonant circuit is obtained having a i< it . , } tlt . \\, ~ A , Ji ■ .
.. , , , . .. , 0 the articles present there against theft. An electronic protec
resonant frequency which differs from a detection frequency „. * c ^. ■ j j f , f
.... j • ^ .. ft . „ . , ^. , . tion system of this type is described, for example, in U.S.
which is used in an anti-theft system The detection tag is ' JU j£ 03 U63 ^
activated by a capacitor being short-circuited in order to tune'
the resonant circuit to the detection frequency. If the acti- The known anti-theft system comprises a transmitter for
vated detection tag has to be deactivated, a following 30 emitting to, and generating electromagnetic fields in. a
capacitor is short-circuited, so that the resonant frequency of detection zone. Preferably a radio-frequency electromag
the tag once more differs from the detection frequency of the netic field having a Fedefined frequency, hereinafter called
anti-theft system. detection frequency. A frequency of 8.2 MHz is a suitable
„ „ , . _ . ,x . .. , frequency, although other frequencies may also be used.
The capacitors to be short-circuited are provided with an 2. , . . , ,
indentation, as a result of which the corresponding plates are 35 The electronic protection system further comprises a
situated at a reduced mutual distance locally. The first receiver for Pref»« °f a detection tag in the
capacitor is short-circuited by electromagnetic energy being detecb011 MBe< by reason of Jtms ^ ^vwg a resonant
supplied with a frequency which corresponds to the current frequency which is virtually identical to the detection fre
resonant frequency of the tag and at a level which is quency of me electtomagnetic field Tlus tag is brought into
sufficiently high to produce a discharge transversely to the 40 resonance by the electromagnetic field, which is detected by
support at the indentation of the capacitor in question. receiver.
Short-circuiting of the second capacitor is effected in cor- The European Patent Application 0 463 233 describes an
responding manner activable anti-theft tag which can be attached to an article to
The known tag has the drawback that as a result of using bf Protected: ^ f*°tection tag consists of a support made
indentations in the capacitors the resonant frequencies are 45 of electrically insulating material which supports a resonant
not precisely defined so that high energy levels or an TMcmt- The inductive portion ot the resonant circuit is
, .,,„■ „ „ formed mainly by a conductor track disposed on the support
additional tuning action are necessary. . ..m .. . . , .
in a spiral patter. The capacitive portion, supported by the
SUMMARY OF THE INVENTION support, of the resonant circuit is formed by a capacitor
50 which in its initial state with the spirally wound coil has a
The object of the invention is to provide a detection tag of ^ resonant frequency which differs from the detection
the type mentioned in the preamble, which overcomes me frequency of the protection system. Said known detection
abovementioned drawback. tag is provided with means for altering the capacitance of the
This object is achieved according to the invention by at capacitor, in such a way that in the activated state the
least one electroconductive island region being disposed on 55 resonant frequency of the resonant circuit is equal to the
the support so as to be adjacent to. and in the same plane as, detection frequency, while in the deactivated state the reso
one of the capacitor electrodes, those edges of the island nant frequency is again altered to a third frequency value. As
region and the capacitor electrode, which face one another, long as the product has not been paid for at the till, the
being situated at a discharge gap distance. detection tag has a resonant frequency which is equal to the
This arrangement has the advantage that, for the purpose «) detection frequency of the security system, while after
of predefining a discharge path by means of the discharge payment the tag is set to a deactivated state, in which the
gap, neither the quality factor nor the resonant frequency of resonant frequency of the resonant circuit once more differs
the resonant circuit of the detection tag are adversely from the detection frequency of the security system, so that
affected. Both variables remain precisely defined, even after no theft detection will take place when the article with the
the discharge, and indeed are not subject to scatter. 65 detection tag is taken through the detection zone.
It should be noted that the European Patent Application 0 The known alteration of the capacitance value of the
458 923 does disclose a discharge along the surface of the capacitive element of the resonant circuit is effected accord
