Note: Descriptions are shown in the official language in which they were submitted.
~233~
ELECTRONIC ARTICLE SURVEILLANCE SYSTEM
HAVING LOW PROFILE A21TENNAS
BACK&ROUIID OF THE INVENTION
Field of the Invention
. . _ . .
This invention relates to electronic article
surveillance systems and more particularly to elec-
tronic article surveillance systems employing micro-
wave transmitter frequencies.
Description of the Prior Art
Systens for deterring or detecting theft of
articles, commonly known as electronic article sur-
veillance (or EAS) systems, have come into increasing-
ly widespread use, especially in retail stores. Such
systems generally include one or more tags which are
attached to articles to be protected against thet,
one or more transmitters and associated antennas which
radiate signals into a protected area or surveillance
zone, and a receiver and associated antenna which will
detect the presence of a tag in the surveillance zone
and cause an alarm ~o be actuated. The surveillance
zone is usually located near an exit from the pro-
tected premises. The desired surveillance zone may be
an exit doorway, for example.
Electronic article surveillance systems in
commercial use may be classified into two groups on
the basis of transmitter frequency or frequencies em-
ployed. The first group consists of systems employing
ICI Americas Inc.
Docket ~lo. 1620 (Canada)
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one or more microwave frequencies. A "microwave fre-
quency" as used herein is a radio frequency higher
than 500 megahertz (MHz). The second group consists
of systems utilizing lower frequencies. The present
system belongs to the first group, i.e., those employ-
ing microwave frequencies.
References illustrating microwave EAS
systems include U.S. Patent 3,895,368 to Gordon et
al., and U.SO Patent 4,063,229 to Welsh et al. The
Gordon et al. system includes both a ~icrowave trans-
mitter and a low frequency signal generator.
Minasy U.S. Patent 3,493,955 is illustrative
of systems using frequencies below the microwave
range.
Microwave systems constitute a large majori-
ty of the electronic article surveillance systems
currently in use. An advantage of microwave systems
over systems employing lower frequencies is that
microwave systems have a longer range than lower fre-
quency systems and can therefore be used to protect
wider doorways. This is a major advantage, particu-
larly for protecting stores in shopping malls.
Presently known microwave EAS systems have
certain limitations. One is that antennas now used
for such systems requ.ire appreciable depth. As a
consequence, the ant~nnas are generally housed in free
standing pedestals of considerable thickness, which
are ordinarily placed near an exit doorway of a pro-
tected premises. Such installations use valuable
floor space and are conspicuous. Unobtrusive instal-
lations are not possible.
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~2335~
Another drawback of microwave EAS systems is
that all such systems known to date have a problem of
"over-ranging" to a greater or lesser extent. "Over-
ranging" arises because a trans~itter signal or sig-
nals are radia~ed beyond the intended surveillancezone and are picked up, either by tags in the interior
of the store or by spurious metal objects, such as
strollers or baby carriages either inside or outside
the store. The remote tag or spurious metal object
may reradiate a signal back to ~he receivert possibly
triggering a false alarm. "Over-ranging" continues to
be somewhat of a problem despite the use of transmit-
ter antenna arrangements designed to confine the
transmltter signals to the desired surveillance æone
and despite the use of means within ~he receiver for
discriminating between valid and spurious return
signals.
Welsh et al. U.S. Patent 4,063,229, cited
supra, also discloses (at column 6, line 5~ to column
7, line 4) various for~s of antennas and associated
accessories (including reflectors) which may be used
in microwave E~S systems.
Microstrip antennas hav~ been the subject of
a number of patents and other publications in the last
decade or so, use of microstrip antennas to date has
been confined largely to aircraft, missiles, and other
military applications. Patch antennas having an air
dielectric have also been developed in recent years,
but have not achieved either widespread usage or the
prominence in the literature that microstrip antennas
hav~ achieved.
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~l~335
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U.S. Patent No. 4, 366,484 to Weiss et al.
describes a temperature compensated radio frequency
antenna having two metallic members and a resonant
cavity therebetween which is partially filled with air
and partially filled with a dielectric material.
U.S. Patent No. 4,291,312 to Kaloi discloses
various forms of dual ground plane microstrip anten-
nas, including arrays having a plurality of radiating
elements and a feed network on one side of a dielec-
tric substrate, and a ground planes on both sides ofthe substrate.
Other U.S. patents illustrating microstrip
antennas include the following: 3,803,623 to Charlot;
3,811,128 to Munson; 3,921,177 to Munson, and
3,987,455 to Olyphant.
SUM~RY OF THE INVENTION
According to this invention, a microwave
frequency electronic article surveillance system is
provided with antenna means comprising an antenna
element which includes a metallic patch and a metallic
ground plane spaced therefrom. The patch and the
ground plane together form a signal radiating or
receiving structure. Either the transmitter antenna
means 9 or the receiver antenna means, or (preferably~
- both, are in accordance with this invention.
To minimize "over-ranging", i~ is preferred
to use a plurality of antenna elements spaced apart
along the normal direction of ~ravel of an article
through the surveillance zone.
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In preferred embodiments of this invention,
the transmitter antenna means includes at least one
antenna array for each transmitter frequency, and the
receiver antenna means includes at least one antenna
array. Each antenna array includes at least two thin
metallic patches arranged in spaced apart side-by-side
relationship along the normal direction of travel, and
a common metallic ground plane behind and spaced from
the metallic patches. An array is in effect an
assembly of a plurality of antenna elements in which
each element comprises a patch and the portion of the
common ground plane which is behind the patch. Each
antenna element forms a cavity resonator. All ele-
ments on an array have the same resonant frequency.
The antenna elements are preferably circularly
polarized.
An EAS system according to this invention
also includes transmitter means for generating at
least one microwave signal, tag means comprising at
least one ~ag adapted to be attached to a protected
article and operable to receive said at least one
microwave signal and to reradiate a return signal, and
receiver means for detecting said re~urn signal and
actuating an alarm when a tag is present in the
surveillance zone. The transmitter, tag and receiver
may all be ~onventional components of an EAS system,
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
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FIG. l is a front elevational view showing a
preferred arrangement of transmitter antenna and re-
ceiver antenna arrays in accordance with this inven-
tion.
FIG. 2 is a side view, taken along line 2-2
of FIG. 1.
FIG. 3 is a side view, taken along line 3-3
of FIG. 1.
FIG. 4 is a view looking upward, taken along
line 4-4 of FIG. 1.
FIG. 5 is a view looking upward, along line
4-4 of FIG. 1, of a modified antenna arrangement
according to this invention.
FIG. 6 is a schematic diagram of an elec-
tronic article surveillance system which includes the
transmitter and receiver antennas of this invention.
- FIG. 7 is an isometric view of a tag or
passive transponder used in the system shown in FIG.
6.
FIG. 8 is a front elevational view of a
transmitter antenna array according to this invention.
FIG. 9 is a front elevational view of a
receiver antenna array according to this invention.
FIG. 10 is an enlarged front elevational
view of a portion of the array shown in FIG. 8.
FIG. 11 is a vertical sectional view taken
along line 11-11 of FIG. 10.
FIG. 12 is a schematic diagram of the feed
nPtwork for a transmitter antenna array.
FIG. 13 is a schematic diagram of the feed
network for a receiver antenna array.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described with
particular reference to an electronic article surveil-
lance system having two transmitters which radiatesignals on two frequencies in the microwave rangP.
The two frequencies used herein for illustration are
905 ~z and 925 MHz.
Referring now to FIGS. 1 to 4, the transmit-
ter antenna means of the present invention comprises
four low profile patch type transmitter antenna arrays
101, 102, 103, 104. The arrays are mounted on either
side of doorway 106, which is an exit doorway in wall
108 of a protected premises. Two of these arrays, 101
15 and 102, are concealed in cabinet 109 on one side of
doorway 106. The other two arrays, 103 and 104, are
concealed ln cabine~ 110 on the opposite side of door-
way 106. Cabinets 109, 110 are non-metallic so as not
to interfere with signal propagation.
Each array comprises a plurality of square
metal patches, mounted on and spaced from a common
rectangular metal ground plane, as will be described
in detail with reference to FIGS. 8, 10 and 11. The
patches on each array are spaced apart along the
normal direction of travel of an article through the
surveillance zone, as may be seen in FIGS. 2 and 3.
The ground planes of arrays 101, 102 preferably lie in
a common vertical plane. Likewise, the ground planes
of arrays ln3, 104 also preferably lie in a common
vertical plane. Bo~h vertical planes are parallel to
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3~335~
the normal direction of travel through the surveil-
lance zone.
Arrays lOl, 102, 103, 104 are shallow,
- typically no more than 2 inches (5.1 cm) thick. Con-
S sequently, cabinets 109, 110 need not be more than
about 3 inches (7.6 cm) thick. Cabinets 109, 110 can
therefore be installed unobtrusively at the sides of
doorway 106, and they do not materially decrease the
width of the doorway.
Alternatively, the arrays 101, 102, 103, 104
may be placed in free standing pedestals on either
side of the doorway. Pedestals are preferred when a
conspicuous installation is desired, while the illus-
txated arrangement affords an unobtrusive installa-
tion.
Arrays 101 and 103 (one on each side of
doorway 106) radiate one transmitter frequency signal
fl, say 905 M~Iz. The other two arrays 102 and 104
(one on each side of doorway 106) radiate the other
transmitter frequency signal f2, say 9~5 MHz. A pair
of arrays which radiate the same transmitter frequency
signal (e.g., arrays 101 and 103) are preferably
placed at different elevations as shown. Both trans-
mitter frequency signals are radiated from both walls
to minimize the effect of "shadows" due to the pres-
ence of persons or objects within the surveillance
zone (since persons and objects act as shields for
signals in the microwave range) and thereby to assure
that both signals are radiated throughout the surveil-
lance zone.
~LZ3~S40
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The receiver antenna means of the presentinvention comprises a single low profile patch type
receiver antenna array 111 which is concealed in
- cabinet 112 in the ceiling of doorway 106, or suspend-
ed close to the doorway. The ground plane of array
111 lies in a horizontal plane. Cabinet 112 is
ordinarily not more than 3 inches thick.
Location of a pair of transmitter antennas
on each side of a doorway and a receiver antenna in
~he ceiling, as described above, is not a novel
feature of this invention. Such an arrangement is
described in the Fancher application.
The normal direction of travel of a protect-
ed article through the surveillance zone (i.e., door-
way 106) is from the interior of the protected prem-
ises through doorway 106 to the outside. This direc-
tion is essentially horizontal. If a protected
article having a tag attached thereto passes through
doorway 106, the tag will pick up transmitter signals
from transmitter antenna arrays 101, 102, 103, 104 and
reradiate a return signal to receiver antenna array
111, causing an alarm to sound as will be described
later.
FIG. 5 shows an alternative arrangement of
antenna arrays. In this arrangement, two transmitter
antenna arrays 101, 102, one for each transmitter
frequency, and a receiver antenna array 111, are all
placed in cabinet 112, which is located in the ceiling
above doorway 106 as shown in FIG. 1. The ground
planes of all three arrays preferably lie in a common
horizontal plane. The patches of all three arrays are
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spaced apart along the normal direction of travel of
an article through the surveillance zone.
FIG. 6 illustrates a preferred electronic
article surveillance system in which the antennas of
the present invention may be utilized.
Referring to FIG. 6, the present system has
transmitter means comprising transmitters 121, 122
which generate microwave signals. The frequencies of
the signals generated by transmitters 121, 122 are
preferably close together, that is, they preferably
differ from each other by no more than about three
percent. For purposes of illus~ra~ion herein, trans-
mitters 121, 122 generat~ signals having frequencies
o 905 MHz and 925 MHz, respectively. One of th~se
signals, say ~he 925 MHz signal, may be modulated by a
low frequency audio tone (2 kilohertz, for example).
The other signal is preferably an unmodulated continu-
ous wave signal.
The signal generated by transmitters 121,
122 (also deslgnated as TR-l and TR-2, respectively)
- are fed through cables 123, 124 to antenna arrays 101,
102, and through cables 125, 126 to antenna arrays
103, 104. The cables may be conventional low loss
coaxial cables. Antenna arrays 101, 102 are located
on one side of ~he surveillance zone (i.e., doorway
106) and antenna arrays 103, 104 are located on the
opposite side of the surveillance zone. The signals
fed to antenna arrays 101, 102 (which are on the side
of doorway 106 more remote from the transmitters 121,
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122) m~y be amplified by linear amplifiers 127, 128.
The linear amplifiers 127, 128 compensate for the
losses in cables 123, 124 and include filtering to
eliminate noise and reduce undesired intermodulation
products.
Transmitters 121, 122 and amplifiers 127,
128 have appropriate power supplies (typically provid-
ing a low D.C. voltage, e.g., 18 volts derived conven-
tionally from 110 volt or 220 volt AC power lines) not
shown.
Transmitter antenna arrays 101, 102, 103,
104 (which collectively constitute transmitter antenna
means~ radiate their respective microwave signals into
the surveillance zone, i.e., doorway 106.
The system of this invention also includes
one or ~ore tags 130~ which may be a~tached to pro-
tec~ed articles. A tag 130, when in the surveillance
zone, receives microwave signals from antennas 101,
102, 103 and 104 and reradiates one or more return
signals in response thereto. ThP re~urn signals are
received by receiver antenna array 111. ~he preferred
tag is a passive transponder.
FIG. 7 illustrates the preferred tag 130 in
detail. Tag 130 comprises a flat metal antenna loop
132 with a gap on one side, a~d a non-linear impedance
element 134 which may be a semiconductor diode, sup-
ported on or embedded in a non-conductive (e.g.,
plastic) substrate 136. Antenna loop 132 provides a
folded dipole configuration. The overall length and
shape of antenna loop 132 are prefera~ly chosen to
make it resonant at a mean center frequency, which is
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the arithmetic mean between the two microwave frequen-
cies generated by transmitters 121, 122. The mean
center frequency in the illustrated embodiment is 915
~z (transmitter frequencies in the illustrated
embodiment are 905 MXz and 925 MHz as previously
noted).
Because of its non-linear nature, tag 130
responds to the transmitter signals (905 MHz and 925
MHz in the illustrated embodiment) by reradiating a
small return signal at the sum frequency signal of
1830 MHz, in addition to other return signals (includ-
ing the second harmonic frequencies of 1810 MHz and
1850 MHz) of lesser amplitude. The sum frequency
signal (and other signals) reradiated by tag 130 will
include any modulation which is present in either of
the transmitter signals.
Returning now to FIG. 6, receiver 140 is
coupled through cable 142 to receiver antenna array
111. Receiver 140 is also coupled to alarm 144.
One may use compact transmitters 121, 122
located inside cabinets 109, 110, and a compact
receiver 140 located inside cabinet 112, instead of
remotely located transmitters and receiver as shown in
FIG. 6, if desired. (In the embodiment of FIG. 5,
compact transmitters and a compact receiver may be
located in cabinet 112.) The alarm 144 is usually
placed at a location remote from the surveillance zone
(it may be placed in the store offices, for example).
Such a receiver responds to a sum frequency
return signal (1830 ~z) to the exclusion of the
transmitter signals (905 and 925 MHz) and their
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~233~0
harmonics. In other words, receiver 140 excludes the
adjacent second harmonics (1810 and 1850 MHz) as well
as higher`harmonics. When a sum frequency signal
(1830 ~z) exceeding a predetermined threshold ampli-
tude is received by receiver 140, the receiver re-
sponds by admitting the signal to a demodulator or
detector circuit. The threshold amplitude is a
safeguard against false alarms due to spurious signals
received from outside the surveillance zone. When one
of the two transmitter signals is modulated, as is
preferred, the return signal is also modulated.
System reliability and sensitivity are further en-
hanced in this case by having the receiver 140 supply
an output signal to alarm 144 only when the frequency
of the modulating tone signal detected matches the
frequency being transmit~ed as modulated from the
transmitter for a predetermined fixed interval which
is indicative of the actual presence of a tag 130 in
the surveillance zone.
The preferred transmitter antennas will now
be described with reference to FIGS. 8, 10 and 11.
Since all four transmitter antenna arrays 101, 102,
103, 104 are structurally alike except for slight
differences in size (arrays 102, 104 are slightly
smaller than arrays 101, 103 because they radiate a
signal of slightly higher frequency), only array lOl
will be described in detail.
Referring to FIG. 8, transmitter antenna
array 101 comprises a plurality of horizontally spaced
thin flat square sheet metal te.g.. aluminum or mild
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steel) patches (two patches 151, 152 are shown) sup-
ported on, spaced from and parallel to a thin rectan-
gular sheet metal (e.g., aluminium) ground plane 154.
- Air occupies the space between patches 151, 152 and
ground plane 154. The patches 151, 152 are arranged
in spaced relationship along the longitudinal axis of
the ground plane 154.
Patches 151, 152 are approximately but not
exactly one-half wavelength on each side. Good
results have been obtained with patches 0.46 wave-
l~ngth on each side.
A preferred receiver antenna array 111 will
now be described with reference to FIG. 9. Receiver
antenna array 111 has a plurality of horizontally
spaced thin square metal (e.g., brass) patches 181,
- 182, 183, 184, supported on, spaced rom and parallel
to a thin rectangular sheet metal (e.g., aluminum)
ground plane 186. The patches 181, 182, 183, 184 are
arranged in spaced relationship along the longitudinal
axis of ground plane 186. Receiver antenna array 111
is structurally similar to the transmitter antenna
arrays, except that the receiver antenna patches 181,
182, 183, 184 are about one-half the size of patches
151, 152 in the transmitter antenna arrays 101, 102,
103, 104 in order to achieve the desired resonant
- frequency. Likewise the spacing between patches is
also about half as great as the spacing between
patches of the transmitter antenna arrays. The ground
planes 154, 186 of the transmitter and receiver
antenna arrays 101, 111 respectively are ~ypically the
same size. It is therefore possible to fit four
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receiver antenna patches into ~he same space that isrequired for two transmitter antenna patches.
The structure of a preferred transmitter
antenna element will now be described in further
detail with reference to FIGS. 10 and 11. Referring
now to FIGS. 10 and 11, a conductive center post 155
extends from each patch 151, 152 to the ground plane
154, providing a conductive path therebetween. Only
patch 151 is shown in FIGS. 10 and 11, but patches
151, 152 and the means used to support both patches
151, 152 from ground plane 154 are structurally the
same. Center post 155 may comprise an externally
threaded screw 155a surrounded by a bushing 155b.
Four corner posts or spacers 156 also extend from
patches 151, 152 to ground plane 154. The corner
posts 156 may be made of glass filled phenolic plastic
or other suitable dielectric material. The bushing
155b and corner pos~s 156 maintain the desired spacing
-- between the patch and the ground plane. For patches
radiating signals of 905 MHz and 925 MHz, the pre-
ferred spacing of patches 151, 152 from ground plane
154 is typically about 0.23 inch. The spacing may be
varied, but any such variations affect both resonant
frequency and band width, as is known in the antenna
art. If all other parameters (e.g., patch size and
corner post size and material) remain fixed, the
spacing between patches and ground plane must be held
within tolerance limits of about 0.001 inch for repro-
ducible performance, since very slight changes in
spacing without compensating changes in other parame-
ters result in significant changes in resonant
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frequency. The non-conductive corner posts are in
regions of high electric field and therefore influence
the resonance of the cavity formed by a patch and the
- ~round plane. Neither the materials, nor the form nor
the dimensions of the corner posts can be changed
without affecting the resonance of the antenna, or, if
any such change is made compensating changes must be
made in the other parameters.
Each patch 151, 152 has a pair of holes 157,
158, which constitute feed points for a transmitter
signal, as seen best in FIG. 11. Transmitter signals
may be fed to these feed points via coaxial conductors
159, which may be conventional. Each coaxial connec-
tor 159 has a conductive core l59a, which terminates
in patch 151 (or 152) at a feed point 157 or 158 and
which passes through and is insulated from ground
plane 1~34 (holes in the ground plane are provided for
this purpose). Each connector 159 also has a metal
shield which is coaxial with core 159a and which
terminates in ground plane 154. Feed points 157, 15
are equidistant from ~he center post 155 along ~he
vertical and horizontal axes, respectively, of the
patch. The spacing from the center is determined to
make the inpu~ impedance of the cavity match that of
the cable. Two additional holes 157a~ 158a, at the
same distance from center post 155 as holes 157, 158
but in opposite directions, may be provided if de-
sired. By providing four holes, only two of which are
used as feed points> one may rotate the patch at ~he
time of assembles to obtain the best possible imped-
ance matching.
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- Patches 151, 152 are elliptically or circu-
larly (preferably circularly) polarized in order to
minimize the effect of tag orientation. This is
accomplished by locating the feed points 157, 158 as
just explained and by feeding the signals 90 out of
phase, according to techniques known in the antenna
art.
Each patch 151 or 152 and the portion of the
ground plane 154 behind it acts as a leaky cavity
resonator, i.e., a cavity resonator with so-called
magnetic walls, which is tuned to the transmitter
frequency which the pa~ch radiates. All patches on an
array are tuned to the same resonant frequency.
Patches 151, 152 are roughly one-half wavelength
(actually about 0.45 wavelength3 on each side. The
thickness of patches 151, 152 is not critical.
Usually thin patches, typically about 1/32 inch (about
0.8 mm), will be used.
The primary direction of radiation from each
patch (i.e., the dominant lobe) is along an axis per-
pendicular to the patch. In the embodiment shown in
FIGS. 1 to 4 these primary axes of radiation are hori-
zontal, i.e., across doorway 106 from one side to the
other. In the embodiment shown in FI&. 5, the primary
direction of radiation is vertically downward into
doorway 106. Side lobes may also be radiated. Signal
amplitude is greatest along the primary axis and in
general decreases as the angle from the primary axis
increases. Ground plane 154 prevents back radiation
(i.e., radiation away from the surveillance zone) ex-
cept for a small amount which goes around the edges.
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This is important for stores where tagged goods are
near the doors.
The patches on an array, i.e., patches 151,
152 in the ~mbodiment shown, are spaced apart in the
normal direction of travel of a tag through the sur-
veillance zon~. Since the usual direction of travel
of a tag is horizontal, the patches on an array are
spaced apart horizontally. By pro~iding a plurality
of appropriately spaced patches on each array, it is
possible to minimize radiation of transmitter signals
outside the surveillance zone. The signals radiated
sideways ~and therefore to locations outside the sur-
veillance zone) cancel each other out or at least
minimize each other, so that very weak signals, and in
some directions virtually no signals, are received
outside the surveillance zone. On the other hand, the
patches do radiate vertically throughout the entire
height of doorway 106, since the patches are so placed
that no interference in the vertical direction ~akes
place. A preferxed spacing between patches 151, 152
is about three-quarters of a wavelength center to
center.
A two-patch antenna array 101 a~ shown, in
which the patch-to-patch spacing is three-quarters of
a wavelength, has a half power (-3db) beam width of
40 in the longitudinal direction of the array with
minimal side lobes. The half power beam width in the
transverse direction is about 60. By comparison, a
single patch antenna elPment has a half power beam
width of about 60 measured in either direction.
helical antenna of known form also has a half power
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beam width of about 60. Half power beam width is a
useful measurement for comparing the directionality of
different antennas, although it does not ordînarily
coincide with the actual width over which a signal is
effectively radiated or received.
More than two patches can be used when space
permits. Use of more patches decreases the extent of
overranging beyond the doorway (or beyond the space
between the pedestals when they are used). Use of two
patches as illustrated is based on the assumption that
~he depth of doorway 106 ~i.e., the dimension in the
direction of travel of an article) is two feet.
Each of the arrays 101, 102, 103, 104 may be
replaced by an antenna element having a single patch
151 and a ground plane 154, where the passageway to be
protected is narrow. Such would be the case, for
example, at an escalator landing or in a small shop
situated on a street and having a narrow doorway.
Such installation may also be used where necessitated
by space limitations, as for example, when doorway 106
is in a wall substantially less than two feet thick.
- Use of a single antenna element in place of an array
is not preferred for wider doorways. When a single
antenna element (or a single element on each side of
doorway 106) is used for each transmitter frequency,
the signal of that frequency is radiated outside the
surveillance zone due to the broader pat~ern from that
element, without the extinction or attenuation which
is achieved with a properly designed array.
To recapi~ulate, the resonant frequency of a
cavity formed by a patch and the adjacent ground plane
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is influenced by the patch size, the spacing between
the patch and the ground plane, and the size and
materials of the dielectric spacers (i.e., the corner
posts 156) between the patch and the ground plane.
Experimental adaptation of a design is usually neces-
sary for an exact determination of suitable dimen-
sions. Resonant frequency is sensitive to small
changes in patch size, and is extremely sensitive to
small differences in spacing between the patch and the
ground plane. Furthermore, patches which deviate from
flatness may show mismatch between the electrical
characteristics as measured at the two feed ports, and
fail to give satisfac~ory circular polarization. The
thicknesses of a patch and the ground plane are not
critical initially, but once the design i5 fixed, they
cannot be arbitrarily changed. For convenience they
are made of thin sheet metal. The total thickness of
an array is typically less than one-half inch (1.3
cm), although the feed network (to be described with
reference to FIG. 12~ hich is usually mounted behind
ground plane 154, may take up more space.
Referring again to FI~. 9, patches 181, 182,
183~ 184, and the portions of the ground plane 186
behind the patches, act as cavity resonators tuned to
a frequency reradiated by tag 130. When the transmit-
ter frequencies are 905 and 925 MHz, one of the
reradiated frequencies is 1830 MHz. Receiver antenna
array 111 is advanta~eously tuned to this frequency.
Patches 181, 182, 183, 184 are structurally
similar to patches 151, 152, bu~ are preferably
circularly po~arized in the opposite direction from
~ ~ 3;~S 4 ~
the direction of polarization of the transmitter
patches. If the transmitter patches are polarized to
the right, the receiver patches are polarized to the
- left, and vice versa. Thus, each patch 181, 182, 183,
184 includes a conductive center post 187, non-conduc-
tive corner posts 188 at each corner, and holes or
feed points lB9a, l90a for coaxial conductors 189,
190, respectively. The feed points 189a, l90a are
equidistant from center post 187 along the two princi-
pal a~es (which are at right angles to each other) of
the patch. Two extra holes (analogous to holes 157a,
158a in the transmitter antenna arrays) may be provid-
ed as alternate feed point locations if desired.
The receiver a~tenna array 111 of this
invention is highly selective to a predetermined
frequency. In addition, because the receiver antenna
array ~ypically has four patches instead of two as in
a transmitter antenna array of the same size, the
receiver antenna "pickup" or signal reception zone is
more closely confined to doorway 106 (or other desired
surveillance area) than is the case with the transmit-
- ter antenna arrays. A four-patch receiver antenna
array as shown herein has a half power beam width
angle of about 18, compared to abou-t 40 in the
two-patch transmitter antenna array shown. This makes
it possible to place tagged merchandise close to the
doorway 106 inside the store if desired; even if the
tag picks up transmitter signals from the transmitter
antenna arrays 101, 102, 103, 104, the return signal
from the tag will not necessarily be picked up by the
receiver antenna array 111. Also, a receiver antenna
,~,
,j~.9 ''
1233~
array 111 will not pick up return signals from tags
which are close to the doorway on ~he exterior side
thereof (~or example, tags similar to tag 130 which
were affixed to an article by another store in the
same shopping mall and not removed from article before
it was taken out of store).
The receiver antenna array may have any
desired number of patches, from two on up, instead of
the four patches shown in FI&S. 1 and 8. However, use
of fewer patches results in a broader signal pickup
zone.
FIG. 12 shows schematically the feed network
for supplying a transmitter signal to a transmitter
antenna array. Transmitter antenna array 101 has been
chosen for purposes of illustration. The feed net-
works for all transmitter antenna arrays are the same.
The trans~it~er signal from transmitter 121,
after amplification in amplifier 127, passes through
the feed network shown in FIG. 12 to array 101. This
feed network includes a first stage power divider 170,
a pair of output signal lines 171~ 172 connected to
divider 170, and a pair of secGnd stage power dividers
173, 174 connected to lines 171, 172 respectively.
Lines 171, 172 are of equal length. Coaxial conduc-
tors 175, 176, 177, 178 connect the output sides of
dividers 173, 174 with connectors 159 (see FIG. 11) on
patches 151, 152 and their respective ~round planes
154. Conductors 176 and 178 are one-quarter wave-
length longer than conductors 175 and 177 so that
patches 151, 152 are circularly polarized.
.
, ' -' ' ' ' ,'
3 ~ 5
-23-
. The network for conveying reradiated signals
from receiver antenna array 111 to receiver 140 will
now be described with reference to FI~. 13. Referring
to FIG. 13, reradiated signals picked up by patches
181, 182, 183, 184 are fed to combiners 191, 192, 193,
194 respectively. The output signals from combiners
191, 192 are fed to combiner 195, and the output
signals from combiners 193, 194 are fed to combiner
196. The output signals from combiners 195, 196 are
fed to combiner 198. The output signal from combiner
198 is fed via cable 142 to receiver 140.
Both the transmitter and receiver antenna
feed networks are preferably compact and are located
inside cabinets 109, 110, or 112 behind the ground
planes 154 or 186 of the antenna arrays which they
serve.
Transmitter and receiver antenna arrays as
shown and described herein, and having the character-
istics given in Table 1 below, have been found to
~0 perform satisfactorily when opera~ed with feed net-
works and interconnecting cables having 50 ohm charac-
teristics impedances.
. ~ .
.
-24-
Table 1
Transmitter Receiver
arrays ar~y_
Resonant frequency, MHz 905 925 1830
Patch size (length of each 5.955 5.810 2.880
side), inch (cm) (15.2) (14.8) (7.3)
Distance from feed point 1.075 1.075 0.538
to center of patch, inch (cm) (2.73) (2.73) (1.37)
Spacing from patches 0.230 0.230 0.113
to ground plane, inch (cm~ (0.58) ~0.58) (0.29)
Spacing between adjacent 9.67 9.67 4.84
patches, center-to-center (24.6) (.4.6) (12.3)
Diameter of corner posts, 0.25 0.25 0.25
inch (cm) (0.64) (0.64) (0.64)
Operation
When a purchaser of a protected article pays
for it in the usual way, he or she will do so before
reaching the surveillance zone, and the tag will be
removed at the time of paymen~. The purchaser then
passes freely through the surveillance zone and no
- 30 alarm is sounded.
. If a shoplifter attempts to take a protected
- article out of a store ~or other protected premises)
~: wi~hout having the tag 130 removed, he must pass
through the surveillance zone in passing through
doorway 106. As he does 50, tag 130 picks up trans-
mitter signals radiated from transmitter antenna
arrays 101, 102, 103, 104, and reradiates a return
signal ~typically a sum frequency signal in the dual
~335~
-25-
frequency system illustrated). This return signal is
picked up by receiver antenna 111 and conveyed to
receiver 140. Receiver 140 will recognize the return
signal as valid and cause alarm 14~ to be actuated.
Modifications
The present invention may be utilized
generally with EAS systems having at least one trans-
mitter frequency above 500 MHz. Although the presentinvention may be used in EAS systems having transmit-
ter frequencies lower than 500 MHz 9 such use is
usually impractical because of the large size of
transmitter antenna arrays required. The antennas
described herein could be used with other microwave
EAS systems, such as those described in Gordon et al.
U.S. Patent 3,895,3~8 or Welsh et al. U.S. Patent
4,063,229, both cited supra. However, the antenna
system of the present invention is more effective in
preventing unwanted spread of the surveillance zone in
systems employing two microwave frequencies than it is
in systems utilizing a single microwave frequency
(such as those described in Gordon et al. or Welsh et
al.). This is because the strength of a signal
reradiated by a tag 130 is proportional to the product
of the amplitudes of the transmitter signals which the
tag receives in the case of a dual frequency system.
Transmitter signal amplitude diminishes as one goes
outside the doorway region, and the product of the two
transmitter signals diminishes even more rapidly.
When the EAS system chosen uses both microwave and low
,A~f
-26 -
frequencies, as is the case in the system of the
Gordon et al. patent, the present antenna arrays are
used only for the microwave signal.
Circular or other shaped patches may be used
instead of square patches in both the transmitter and
receiver antenna arrays.
Any desired arrangement of transmitter and
receiver antenna arrays may be used. For example, one
may place both transmitter and receiver antenna arrays
on opposite sides of the surveillance zone, either in
cabinets 109, llO or in pedestals, and dispense with
the overhead or ceiling receiver antenna array. All
arrays should be mounted so that the patches in each
array are spaced apart in the direction of travel
through the surveillance zone. Location of all
transmitter antenna arrays on only one side of the
surveillance zone is not preferred, because both the
human body and objects in the surveillance zone
produce "shadows" which would permit a tag to escape
detection.
Instead of one or more transmitter antenna
arrays, one may provide two or more separate transmit-
ter antenna elements, each of which includes a patch
and a ground plane, for each transmitter frequency.
The elements in such instance are arranged in side-by-
side rela~ionship in the normal direction of travel.
~imilarly, two or more receiver antenna elements may
in principle replace a receiver antenna array.
However, in practice the need to have accurate phase
relationships maintained between patches, and patch
spacing matched to the wavelength of signals in order
.,.,l . ~
-
:1233S4~3
to minimize 'loverranging", makes it much better to use
prefabricated arrays, as shown and described herein.
General
Antenna arrays according to this invention
with their feed networks can be made less than two
inches thick. (The antenna arrays themselves can be
made less than one-half inch thick). Installation of
the system therefore need not significantly decrease
the effective width of thP passageway to be protected.
The system can be installed unobtrusively behind a
doorway jamb as illustrated or it can be installed
conspicuously in free standing pedestals, provided in
either case that the intervening non-conducting
material of construction of the doorway or pedestal is
not water-absorbing and i5 not so close that its
dielectric proper~ies materially affect the operation
of the antennas. If a conceal~d doorway installation
is chosen, the space requirements for the antennas are
minimal.
Receiver antenna arrays according to this
invention ca~ be placed in high doorways or ceilings
without losing their effectiveness and without undue
spread of th~ signal reception zone beyond the door-
way. (The zones of effective signal reception in the
direction of travel may be approximated by planes
which extend from the sides of the receiver antenna
array. These planes make only a small angle with the
vertical in the case of overhead receiver antenna
arrays).
~335
-28 -
The surveillance zone can be confined more
closely to the space desired, i.e., the space within
or close to the exit passageway, than is the case with
previous EAS antenna systems. Transmitter antenna
arrays of this invention can be readily constructed to
give chosen beam patterns which will control the
spread of the surveillance zone.
Similarly, the receiver antenna array
effectively picks up signals only from the surveil-
lance zone. The received strength of signals from
outside the surveillance zone is so low that the alarm
will not sound, due to the arrangement of antenna
patches which nullifies or greatly attenuates signals
from outside the surveillance zone.
The receiver antenna is highly selective for
a desired return si~nal frequency. The receiver
antenna thus enhances the sensitivity of the receiver
itself in discriminating between valid and spurious
return signals.
One may use the antenna of this invention
only for the transmitter antennats) or only for the
receiver antenna, with another type of antenna being
used for the other, if desired. Such installation,
for example, may include an overhead receiver antenna
array according to this invention with helical trans-
mitter antennas on the sides of the surveillance zone.
Of course, some of the advantages of this invention
are lost in such case. For example, if one uses a
receiver antenna of this invention with a prior art
transmitter antenna, one loses some control over
spread of the surveillance zone which the present
.
;
- ~ ,
59L~3
-29-
transmitter antenna affords. If one uses transmitter
antennas according to this invention wi~h prior art
receiver antennas, one confines authentic transmitter
signals to the surveillance zone but does not screen
out spurious return signals from outside the surveil-
lance zone as effectively as one does when using a
receiver an~enna according to this invention. It is
especially important to use receiver antennas accord-
ing to this invention, since the improvement in
reducing 'loverranging" is more marked in the case of
receiver antennas than is the case with transmitter
antennas t as already indicated.
- A two-patch transmitter antenna array
according to the present invention when used for
radiating bo~h ~ransmitter frequency signals of a dual
microwave frequency system cuts the amount of "over-
ranging" o~ each transmitter signal by approximately
half as compared to a helical antenna arrangement.
The antenna means of the present invention, when used
for receiving a single return signal frequency, cuts
the amount of "overranging" in the zone of effective
reception by approximately three quarters. (Compari-
sons are based on comparative ~talf power beam width
angles). When the antenna arrays according to this
- 25 invention are used for both the transmitter signals
and the receiver, the amount of 'loverranging'l is cut
even more than when such antenna arrays are used for
either ~ransmi~ter or receiver antennas alone. The
present invention therefore provides an effective
means for diminishing the extent of "overranging",
:'j~`, J!
,~
2 ~
-30-
which has been a major source of false alarms in
previous EAS systems.
While this invention has been described with
reference to specific embodiments and possible modifi-
cations it will be apparent that other modificationscan be made by those skilled in the art without
departing from the present invention.
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