Note: Descriptions are shown in the official language in which they were submitted.
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EAS SYSTEM ANTENNA CONFIGURATION FOR PROVIDING IMPROVED
INTERROGATION FIELD DISTRIBUTION
FIELD OF THE INVENTION
This invention relates to antenna configurations,
and more particularly to antennas for use with electronic
article surveillance (EAS) systems.
BACKGROUND OF THE INVENTION
An electronic article surveillance system 20 is
shown in schematic terms in Fig. 1. The system 20 is
typically provided at the exit of a retail store to detect
the presence of a marker 22 in an interrogation zone 24
defined between antenna pedestals 26 and 28. When the
system 20 detects the marker 22, the system 20 actuates an
alarm of some kind to indicate that an article (not shown)
to which the marker 22 is secured is being removed from the
store without authorization.
Customarily, each of the antenna pedestals 26 and
28 is generally planar and includes one or more loop
antennas. Signal generating circuitry 30 is connected to
the antenna or antennas in pedestal 26 to drive the antennas
in pedestal 26 to generate an interrogation signal in the
interrogation zone. Also, receiver circuitry 32 is
connected to the antenna or antennas in the pedestal 28 to
receive and analyze signals picked up from the interrogation
zone by the antennas in the pedestal 28.
For purposes of further discussion, a coordinate
system 34, consisting of X, Y and Z axes, mutually
orthogonal to each other, is shown in Fig. 1. The antenna
pedestals 26 and 28 are usually arranged in parallel to each
other, and for the purposes of this and further discussion,
it should be understood that the respective planes of the
pedestals 26 and 28 are parallel to the plane defined by
the
Z and X axes. The Z axis is presented as being a vertical
axis, and the X axis is a horizontal axis extending in the
a
direction of a path of travel through the interrogation zone
24, i.e., parallel to the planes of the pedestals 26 and
28.
The Y axis is also horizontal, but in a direction
perpendicular to the X axis. For some purposes, the X
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direction will be referred to as the "horizontal direction",
the Z direction will be referred to as the "vertical
direction", and the Y direction will be referred to as the
"lateral direction".
The marker 22 typically includes a coil or other
planar element that receives the interrogation signal
generated through the antenna pedestal 26 and retransmits
the signal, in some fashion, as a marker signal to be
detected through the antenna pedestal 28. The amplitude of
the marker signal is, in general, dependent on the
orientation of the plane of the receiving element in the
marker 22. As a practical matter, the orientation of the
plane of the receiving element has three degrees of freedom,
but the response of the marker can be analyzed in terms of
components corresponding to three orthogonal plane
orientations. These will be referred to as a "horizontal
orientation", corresponding to the plane defined by the X
and Y axes, a "vertical orientation", corresponding to the
plane defined by the Z and X axes, and a "lateral
orientation", corresponding to the plane defined by the Z
and Y axes.
For markers used in magnetomechanical EAS systems,
the marker responds to flux that is co-planar with the
marker, but for markers that include a coil, the marker
responds to flux that is orthogonal to the plane of the
coil. Subsequent discussions herein will be based on the
assumption that a magnetomechanical marker is in use.
It is generally an objective in an EAS system that
the system reliably detect any marker in the interrogation
zone, regardless of position in the zone or orientation of
the marker. At the same time, it is highly desirable that
the system not produce false alarms either by interpreting
a signal generated by a non-marker object in or out of the
interrogation zone as coming from a marker, or by
stimulating markers not in the interrogation zone to
generate signals at a level sufficiently high to be
detectable by the receiver circuitry.
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One significant obstacle to achieving these
objectives is the uneven interrogation field distribution
commonly provided by antennas used for generating the
interrogation signal. As a result of the uneven field
distribution, the interrogation field may be strong enough
at some or most locations in the interrogation zone to
provide for detection of a marker, while not being strong
enough at other locations to provide for detection. The
locations in which the field is too weak to provide for
to detection are sometimes referred as "null" areas or "holes".
This problem is aggravated by the fact that the
strength of the signal generated by the marker is dependent
on the orientation of the marker. Accordingly, a marker at
a given location in the zone and oriented in a first manner
may be readily detectable, while if the marker is at the
same location but oriented in a different manner, the marker
would not be detected.
One approach that has been contemplated for
overcoming this problem is simply to increase the overall
strength of the interrogation field, i.e., by increasing the
level of the signal used to generate the interrogating
antenna.
Aside from the increased power consumption
requirements resulting from this approach, there are often
regulatory or other practical constraints on the peak signal
level that can be generated. For example, increasing the
peak field strength could lead to increased false alarms
from either or both of non-marker objects in the
interrogation zone and markers located outside of the
intended interrogation zone.
Further, in addition to the usual desire to
confine the interrogation field to the intended zone, it may
be a regulatory requirement, or desirable for other reasons,
to provide far-field cancellation of the interrogation
signal. This requirement places additional constraints on
the design of the antenna used for generating the
interrogation signal.
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OBJECTS AND SU1~IARY OF THE ING'ENTION
It is accordingly an object of the invention to
provide an antenna configuration for use in an electronic
article surveillance system which results in a relatively
even effective field distribution in an interrogation zone.
It is a further object of the invention to provide
an antenna configuration which produces far-field
cancellation of the interrogation signal.
According to an aspect of the invention, there is
provided an antenna for use in an EAS system, comprising:
first, second, third and fourth loops, all co-planar; and
excitation means for generating respective alternating
currents in said first, second, third and fourth loops, such
that the alternating current in said second loop is about
90° out of phase with the alternating current in said first
loop, the alternating current in said third loop is about
180° out of phase with the alternating current in said first
loop, and the alternating current in said fourth loop is
about 180° out of phase with the alternating current in said
second loop; said four loops collectively including a
plurality of vertical segments and no two vertical segments
in said antenna being vertically aligned w~~_th each other.
According to another aspect of the invention,
there is provided an antenna for use in an EAS system,
comprising: first, second, third and fourth loops, all
co-planar; and excitation means for generating respective
alternating currents in said first, second, third and fourth
loops, such that the alternating current in said second loop
is about 90° out of phase with the alternating current in
said first loop, the alternating current in said third loop
is about 180° out of phase with the alternating current in
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said first loop, and the alternating current in said fourth
loop is about 180° out of phase with the alternating current
in said second loopy said four loops collectively including
at least one pair of vertical segments having respective
alternating currents that are 180° out of ;phase with each
other; and in each said pair of vertical segments the two
vertical segments making up the pair of vertical segments
are displaced horizontally with respect to each other.
According to another aspect of the invention,
there is provided an antenna for use in an EAS system,
comprising: first, second, third and fourth loops, all
triangular and co-planar; and excitation means for
generating respective alternating currents in said first,
second, third and fourth loops, such that the alternating
current in said second loop is about 90° out of phase with
the alternating current in said first loop, the alternating
current in said third loop is about 180° out of phase with
the alternating current in said first loop, and the
alternating current in said fourth loop is about 180° out of
phase with the alternating current in said second loop.
According to a further aspect of the invention,
there is provided an antenna configuration for use with an
EAS system comprising: a first planar antenna arranged in a
first plane, said first planar antenna including at least a
first loop and a second loop arranged in said first plane,
said first and second loops being substantially equal to
each other in area; a second planar antenna including at
least two loops arranged in a second plane that is different
from the first plane and is substantially parallel to the
first plane, said first and second antennas substantially
entirely overlapping in a direction normal to said planes,
said second antenna including a third loop and a fourth
loop, said third and fourth loops being sub>stantially equal
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to each other in area and having a total area that is
substantially equal to a total area of said first and second
loops; first excitation means for generating an alternating
current in said first antenna, said first excitation means
including means for generating respective alternating
currents in said loops of said first antenna such that the
respective alternating currents in said loops of said first
antenna are about 180° out of phase with each other; and
second excitation means for generating respective
alternating currents in said loops of said second antenna,
said respective alternating currents in said loops of said
second antenna being about 180° out of phase with each other
and about 90° out of phase with the alternating current in
said first antenna.
According to still another aspect of the
invention, there is provided an antenna configuration for
use with an EAS system, comprising: a first planar antenna
arranged in a first plane, said first planar antenna
including at least a first loop, a second and a third loop
arranged in said first plane, said first and third loops
having a total area substantially equal to an area of said
second loop; a second planar antenna including at least two
loops arranged in a second plane that is different from the
first plane and is substantially parallel i~o the first
plane, said first and second antennas substantially entirely
overlapping in a direction normal to said planes, said
second antenna including a fourth loop and a fifth loop,
said fourth and fifth loops being substantially equal to
each other in area and having a total area that is
substantially equal to a total area of said first, second
and third loops; first excitation means for- generating an
alternating current in said first antenna, said first
excitation means generating respective altE;rnating currents
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in said first, second and third loops such that the
respective alternating currents in said first and third
loops are in phase with each other, and the respective
alternating current in said second loop is about 180° out of
phase with the respective alternating currents in said first
and third loops; and second excitation means for generating
respective alternating currents in said loops of said second
antenna, said respective alternating currents in said loops
of said second antenna being about 180° out of phase with
each other and about 90° out of phase with the alternating
current in said first antenna.
According to another aspect of the invention,
there is provided an antenna for use in an EAS system,
comprising: first, second, third and fourth loops, all
co-planar; and excitation means for generating respective
alternating currents in said first, second, third and fourth
loops, such that the alternating current in said second loop
is about 90° out of phase with the alternai~ing current in
said first loop, and the alternating current in said fourth
loop is about 180° out of phase with the alternating current
in said second loop; said four loops collectively including
a plurality of vertical segments and no two vertical
segments in said antenna being vertically aligned with each
other; said first, second, third and fourth loops all being
triangular; said first loop including a first horizontal
segment, a second segment extending obliquely downwardly and
leftwardly from a right end of said first :>egment and having
a lower end at a point displaced vertically downwardly from
a mid-point of said first segment, and a third segment
extending obliquely to interconnect said lower end of said
second segment and a left end of said first segment; said
second loop including a fourth segment extending obliquely
in parallel and in proximity to said seconcl segment, a fifth
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segment extending vertically downwardly from an upper end of
said fourth segment, and a sixth segment substantially
aligned with said third segment and extending obliquely to
interconnect respective lower ends of said fourth and fifth
segments; said third loop including a seventh segment
extending obliquely in parallel and in proximity to said
sixth segment, an eighth segment extending horizontally
leftwardly from a lower end of said seventh segment, and a
ninth segment substantially aligned with said fourth segment
and extending obliquely to interconnect respective left ends
of said seventh and eighth segments; said fourth loop
including a tenth segment substantially aligned with said
second segment and extending obliquely in parallel and in
proximity to said ninth segment, an eleventh segment
extending vertically upwardly from a lower end of said tenth
segment, and a twelfth segment substantially aligned with
said seventh segment and extending obliquely in parallel and
in proximity to said third segment to interconnect
respective upper ends of said tenth and eleventh segments:
said first and eighth segments being substantially equal in
length; said fifth and eleventh segments being substantially
equal in length to each other; and said second, third,
fourth, sixth, seventh, ninth, tenth and tcaelfth segments
all being substantially equal in length to each other.
According to another aspect of the invention,
there is provided an antenna for use in an EAS system,
comprising: first, second, third and fourth loops, all
co-planar; and excitation means for generating respective
alternating currents in said first, second, third and fourth
loops, such that the alternating current in said second loop
is about, 90° out of phase with the alternating current in
said first loop, and the alternating current in said fourth
loop is about 180° out of phase with the alternating current
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in said second loop; said four loops collectively including
a plurality of vertical segments and no two vertical
segments in said antenna being vertically aligned with each
other; said first, second, third and fourth loops all being
triangular; said first loop including a first horizontal
segment, a second segment extending obliquely downwardly and
leftwardly from a right end of said first segment and having
a lower end at a point displaced vertically downwardly from
a mid-point of said first segment, and a third segment
extending obliquely to interconnect said lower end of said
second segment and a left end of said first segment; said
second loop including a fourth segment extf~nding obliquely
in parallel and in proximity to said second segment, a fifth
segment extending vertically downwardly from an upper end of
said fourth segment, and a sixth segment substantially
aligned with said third segment and extending obliquely to
interconnect respective lower ends of said fourth and fifth
segments; said fourth loop including a seventh segment
extending obliquely in parallel and in proximity to said
sixth segment, an eighth segment extending horizontally
leftwardly from a lower end of said seventh segment, and a
ninth segment substantially aligned with said fourth segment
and extending obliquely to interconnect re~>pective left ends
of said seventh and eighth segments; said third loop
including a tenth segment substantially aligned with said
second segment and extending obliquely in parallel and in
proximity to said ninth segment, an eleventh segment
extending vertically upwardly from a lower end of said tenth
segment, and a twelfth segment substantially aligned with
said seventh segment and extending obliquely in parallel and
in proximity to said third segment to interconnect
respective upper ends of said tenth and eleventh segments;
said first and eighth segments being substantially equal in
length; said fifth and eleventh segments being substantially
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equal in length to each other; and said second, third,
fourth, sixth, seventh, ninth, tenth and twelfth segments
all being substantially equal in length to each other.
According to another aspect of the invention,
there is provided an antenna for use in an EAS system,
comprising: first, second, third and fourth loops, all
co-planar; and excitation means for generating respective
alternating currents in said first, second, third and fourth
loops, such that the alternating current i:n said second loop
is about 90° out of phase with the alternating current in
said first loop, the alternating current in said~third loop
is about 180° out of phase with the altern<~ting current in
said first loop, and the alternating current in said fourth
loop is about 180° out of phase with the alternating current
in second loop; said four loops collectively including at
least one pair of vertical segments that are vertically
aligned with each other; and in each said pair of vertical
segments respective alternating currents in the two vertical
segments making up the pair of vertical segments are in a
phase relationship substantially different from about 180°
out of phase.
According to another aspect of the invention,
there is provided an antenna for use with an EAS system,
including first and second adjacent co-planar loops, and
excitation means for generating respective alternating
currents in the first and second loops such that the
respective alternating currents in the first and second
loops are 90° out of phase. In certain preferred
embodiments of the invention, the antenna does not include
any loops other than the aforesaid first and second loops,
or at least no other loops that are arranged in the common
plane of the first and second loops.
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Further in accordance with this aspect of the
invention, the excitation means preferably includes a signal
source connected to the first loop for directly generating
the respective alternating current in the first loop, and
the first and second loops are inductively coupled such that
the respective alternating current in the first loop
inductively generates the respective alternating current in
the second loop with a 90° phase offset from the respective
alternating current in the first loop.
According to another aspect of the invention,
there is provided an antenna for receiving an alternating
signal in an EAS system including first and second adjacent
loops with the loops being inductively coupled such that the
alternating signal induces respective alternating currents
in the loops with a 90° phase offset.
According to yet another aspect o f the invention,
there is included an antenna configuration for use with an
EAS system, including a first planar antenna arranged in a
first plane, a second planar antenna including at least two
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loops arranged in a second plane that is substantially
parallel to the first plane, the first and second antennas
overlapping in a direction normal to the planes, first
excitation means for generating an alternating current in
the first antenna, and second excitation means for
generating respective alternating currents in the loops of
the second antenna, the respective alternating currents in
the loops being 180° out of phase with each other and 90°
out of phase with the alternating current in the first
antenna.
Further in accordance with this aspect of the
invention, the first antenna preferably includes at least
two loops arranged in the first plane and the first
excitation means includes means for generating respective
alternating currents in the loops of the first antenna such
that the respective alternating currents in the loops in the
first antenna are 180° out of phase with each other.
According to still another aspect of the
invention, there is provided an antenna for use in an EAS
system, including first, second, third and fourth co-planar
loops, and excitation means for generating respective
alternating currents in the first, second, third and fourth
loops, such that the alternating current in the second loop
is 90° out of phase with the alternating current in the
first loop, the alternating current in the third loop is
180° out of phase with the alternating current in the first
loop, and the alternating current in the fourth loop is 180°
out of phase with the alternating current in the second
loop, and the four loops collectively include a plurality of
vertical sections with no two vertical sections in the
antenna being vertically aligned with each other.
Alternatively, in accordance with this aspect of
the invention, the four loops collectively include at least
one pair of vertical segments having respective alternating
currents that are 180° out of phase with each other, but in
each of such pairs of vertical segments, the two vertical
segments making up the pair of vertical segments are
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displaced horizontally with respect to each other. As
another alternative in accordance with this aspect of the
invention, the four loops collectively include at least one
pair of vertical segments that are vertically aligned, and
in each such pair of vertical segments the respective
alternating currents in the two vertical segments making up
the pair of segments are in a phase relationship that is
substantially different from 180° out of phase. For
example, in each pair of vertically aligned vertical
segments, the respective currents are in phase or 90° out of
phase.
An antenna configuration provided according to the
invention,~in which there are no vertically aligned vertical
segments with "bucking" currents, tends to prevent the
formation of holes due to near-field cancellation, as has
commonly resulted from prior art far-field canceling antenna
configurations.
Further in accordance with the latter aspects of
the invention, the four loops may all be rectangular or may
all be triangular.
In accordance with yet another aspect of the
invention, there is provided an apparatus for receiving a
signal present in an interrogation zone of an electronic
article surveillance system, with the signal alternating at
a predetermined frequency, and the apparatus including a
first receiver coil for receiving the signal and providing
a first receive signal which alternates at the predetermined
frequency, a second receiver coil adjacent-to the first
receiver coil for receiving the signal that is present in
the interrogation zone and providing a second received
signal which alternates at the predetermined frequency, a
receive circuit, and quadrature means for -providing the
first and second received signals to the received circuit ,
with a 90° phase offset between the first and second
received signals. Preferably, the quadrature means includes ,
a first shift circuit that phase-shifts the first received
signal by +45° and a second shift circuit which phase-shifts
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the second received signal by -45°, and the quadrature means
also includes a summation circuit which sums the first and
second shifted signals to produce a sum signal which is
outputted to the received circuit. The first shift circuit
may be a low pass filter and the second shift circuit may be
a high pass filter.
According to a further aspect of the invention,
there is provided an antenna arrangement for use with an EAS
system, including a first planar loop arranged in a first
plane, a second planar loop arranged in a second plane that
intersects the first plane at an angle e, with 0°<e<180°,
and excitation circuitry for generating respective
alternating currents in the first and second loops such that
the respective alternating currents in the first and second
loops are 90° out of phase.
According to still another aspect of the
invention, there is provided an antenna arrangement for use
with an EAS system, including first and second co-planar
loops, and excitation circuitry for generating respective
alternating currents in the first and second loops such that
the respective alternating currents in the first and second
loops are 90° out of phase, the first and second loops being
displaced from each other in a horizontal direction.
According to yet another aspect of the invention,
there is provided an antenna arrangement for use with an EAS
system, including first and second co-planar loops, and
excitation circuitry for generating respective alternating
currents in the first and second loops such that the
respective alternating currents in the first and second
loops are 90° out of phase, the first loop having a contour
that is different from a contour of the second loop.
According to still a further aspect of the
. invention, there is provided anantenna arrangement for use
with an EAS system, including a plurality of co-planar loops
which includes first and second loops, and excitation
circuitry for generating respective alternating currents in
the first and second loops such that the respective
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alternating currents in the first and second loops are 90°
out of phase, with at least two of the plurality of co-
planar loops being substantially triangular.
According to still a further aspect of the
invention, there is provided an antenna arrangement for use
with an EAS system, including first, second and third co
planar loops, and excitation circuitry for generating
respective alternating currents in the first, second and
third loops such that the respective alternating currents in
the first and second loops are 90° out of phase, and the
respective alternating currents in the first and third loops
are 180° out of phase with each other, with the antenna
arrangement having no other antenna loops that are co-planar
with the first, second and third loops.
According to yet another aspect of the invention,
there is provided an antenna arrangement for use in an EAS
system, including first and second adjacent co-planar loops,
and excitation circuitry for generating respective
alternating currents in the first and second loops such that
the respective alternating currents are substantially in
phase during a first sequence of time intervals and are
substantially 180° out of phase with each other during a
second sequence of time intervals interleaved with the first
sequence of time intervals, with the antenna arrangement
having no other antenna loops that are co-planar with the
first and second loops.
According to still another aspect of the
invention, there is provided an antenna configuration for
use with an EAS system, including a first planar antenna
arranged in a first plane, a second planar antenna including
at least two loops arranged in a second plane that is
substantially parallel to the first plane, with the first
and second antennas overlapping in a direction normal to the .
planes, a first excitation circuit for generating an
alternating current in the first antenna only during a first ,
sequence of timeintervals, and a second excitation circuit
for generating respective alternating currents in the loops
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of the second antenna only during a second sequence of time
intervals interleaved with the first sequence of time
intervals, with the respective alternating currents a.n the
loops of the second antenna being about 180 out of phase
with each other.
According to still a further aspect of the
V
invention, there is provided an antenna arrangement for use
with an EAS system, including first, second and third co-
planar loops, with the first loop circumscribing the second
l0 and third loops, and excitation circuitry for generating
respective alternating currents in the first, second and
third loops such that the respective alternating currents
in
the first and second loops are about 90 out of phase, and
the respective alternating currents in the second and third
loops are about 180 out of phase with each other.
According to yet another aspect of the invention,
there is provided an antenna arrangement for use with an EAS
system including first, second and third co-planar loops,
with the first loop circumscribing the second and third
loops, a first excitation circuit for generating an
alternating current in the first loop, only during a first
sequence of time intervals, and a second excitation circuit
for generating respective alternating currents in the second
and third loops, only during a second sequence of time
intervals interleaved with the first sequence of time
intervals, with the respective alternating currents in the
second and third loops being about 180 out of phase with
each other.
According to still a further aspect of the
invention, there is provided an antenna arrangement for use
with an EAS system, including first, second and third co-
planar loops, a first excitation circuit for generating an
alternating current in the first loop, only during a first
sequence of time intervals, and a second excitation circuit
for generating respective alternating currents in the second
and third loops, only during a second sequence of time
intervals interleaved with the first sequence of time
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intervals, with the respective alternating currents in the
second and third loops being about 180° out of phase with
each other, and the antenna arrangement having no other
antenna loops that are co-planar with the first, second and
third loops.
According to yet another aspect of the invention,
there is provided an antenna arrangement for use with an EAS
system, including first and second co-planar loops, a first
excitation circuit for generating an alternating current in
the first loop, only during a first sequence of time
intervals, and a second excitation circuit for generating an
alternating current in the second loop, only during a second
sequence of time intervals interleaved with the first
sequence of time intervals, with the first loop being
substantially triangular. As alternatives to the just-
mentioned aspect of the invention, the first loop may have
an area that is substantially larger than an area of the
second loop, and the first and second loops may be arranged
in a plane that is vertically oriented.
According to still another aspect of the
invention, there -is provided an antenna arrangement for use
with an EAS system, including a first planar loop arranged
in a first plane, a second planar loop arranged in a second
plane that intersects the first plane at anangle e, with
0°<e<180°, a first excitation circuit for generating an
alternating current in the first loop, only during a first
sequence of time intervals, and a second excitation circuit
for generating an alternating current in the second loop,
only during a second sequence of time intervals interleaved
with the first sequence of time intervals.
According to still a further aspect of the
invention, there is provided an apparatus for receiving a
signal present in an interrogation zone of an electronic ,
article surveillance system, with such signal alternating at
a predetermined frequency, and the apparatus including a ,
first receiver coil for receiving the signal and providing
a first received signal that alternates at the predetermined
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frequency, a second receiver coil adjacent to the first
_ receiver coil for receiving the signal present in the
interrogation zone and providing a second received signal
which alternates at the predetermined frequency, a receive
circuit, and a switchable connection circuit interconnecting
the first and second receiver coil and the receive circuit
and including switch means for switching the connection
circuit between a first condition in which the connection
circuit supplies the first and second received signals to
the receive circuit with the first and second received
signals in phase with each other and a second condition in
which the connection circuit supplies the first and second
received signals to the receive circuit with a phase offset
of about 180° between the first and second received signals.
Further in accordance with the latter aspect of
the invention, the connection circuit may include a
summation circuit for receiving and summing the first and
second received signals to produce a sum signal and for
outputting the sum signal to the receive circuit, and a
switchable shift circuit, connected between the second
receiver coil and the summation circuit, for selectively
phase-shifting the second received signal by about 180°.
Further, the connection circuit may be maintained in the
first condition during a first sequence of time intervals
and maintained in the second condition during a second
sequence of time intervals interleaved with the first
sequence of time intervals. In addition, the first receiver
coil may include a first segment and the second receiver
coil may include a second segment arranged substantially in
parallel and in proximity with the first segment, with the
first and second receiver coils not having any other pair of
segments arranged in parallel and in proximity with each
other. In addition, the apparatus may be provided such that
it has no other receiver coils in addition to the aforesaid
first and second receiver coils.
The foregoing and other objects, features and
advantages of the invention will be further understood from
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the following detailed description of preferred embodiments
and from the drawings, wherein like reference numerals
identify like components and parts throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of an
electronic article surveillance system.
FIG. 2 schematically illustrates an antenna
configuration provided for generating an interrogation field
in accordance with a first embodiment of the invention.
FIG. 3 is a circuit diagram of an equivalent
circuit representative of the antenna configuration of Fig.
2.
FIG. 4 illustrates an antenna configuration
provided for generating an interrogation field in accordance
with a second embodiment of the invention.
FIGS. 5A, 5B and 5C are used to explain the fie-ld
distribution provided by the antenna configuration of Fig.
4, and Fig. 5C is also used to explain the field
distribution provided by the antenna configuration of Fig.
2.
FIG. 6 illustrates an antenna configuration
provided for generating an interrogation field in accordance
with a third embodiment of the invention.
FIG. 7 illustrates an antenna configuration
provided for generating an interrogation field in accordance
with a fourth embodiment of the invention.
FIG. 8 illustrates a conventional antenna
configuration.
FIG. 9 illustrates an antenna configuration
provided for generating an interrogation field in accordance
with a fifth embodiment of the invention.
FIG. 10 illustrates an antenna configuration
provided for generating an interrogation field in accordance
with a sixth embodiment of the invention.
FIG. 11 illustrates an antenna configuration
provided for generating an interrogation field in accordance
with a seventh embodiment of the invention.
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FIG. 12 illustrates an antenna configuration
provided for generating an interrogation field in accordance
with an eighth embodiment of the invention.
FIGS. 13A-13C are used to explain a field
S distribution generated by the antenna configuration of Fig.
9.
FIGS. 14A-14C are used to illustrate a field
distribution generated by the conventional antenna
configuration of Fig. 8.
FIG. 15 schematically illustrates an antenna
configuration used for receiving a marker signal in
accordance with a ninth embodiment of the invention.
FIG. 16 illustrates certain features of the
receiver antenna configuration of Fig. 15.
FIGS. 17-21 schematically illustrate various
modifications that can be made to the embodiment of Fig. 4.
FIGS. 22A and 22B respectively illustrate
alternative states of an antenna configuration provided for
generating an interrogation field in accordance with another
embodiment of the invention, and FIG. 22C a.s a timing
diagram which illustrates operation of the embodiment of
Figs. 22A and 22B.
FIG. 23 is a timing diagram which illustrates
operation of still another embodiment of the invention.
FIG. 24 illustrates an antenna configuration
provided for generating an interrogation field according to
the timing diagram of Fig. 23.
FIGS. 25-27 are illustrative of still further
antenna configurations for generating interrogation fields
in accordance with respective embodiments of the invention.
FIG. 28 schematically illustrates an antenna
configuration used for receiving a marker signal in
accordance with a further embodiment of the invention.
FIG. 29 illustrates a switchable interface circuit
that forms part of the receiver antenna configuration of
Fig. 28.
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DESCRIPTION OF PREFERRED EMBODIMENTS
An antenna configuration for generating an
interrogation field and provided in accordance with a first
embodiment of the invention will now be described with
reference to Fig. 2. In Fig. 2 reference numeral 40
generally indicates the antenna configuration, which
includes two co-planar antenna loops 42 and 44. The loops
may, for example, both be rectangular and of like shape and
size, and arranged, as shown in Fig. 2, with one loop
stacked vertically above the other. Signal generating
circuitry 46 is connected to the antenna loop 44 to directly
generate an alternating current in the loop 44.
A capacitance 48 and resistance 50 are provided in
series with the antenna loop 44 and a capacitance 52 and
resistance 54 are provided in series with the antenna loop
42.
Fig. 3 is an equivalent circuit representation of
the arrangement of Fig. 2. In addition to the elements
described in connection with Fig. 2, Fig. 3 also shows a
loop resistance 56 provided by loop 44 and a loop resistance
58 provided by loop 42.
As shown in Figs . 2 and 3 , the antenna loops 42
and 44 are arranged so that there is substantial inductive
coupling between the two loops, so that the alternating
current directly generated in loop 44 by the signal
generator 46 inductively generates an alternating current in
loop 42 that is 90 ° out of phase with the current in loop
44. For example, as shown in Fig. 2, a horizontal upper
segment 60 of the loop 44 is parallel and adjacent to the
lower horizontal segment 62 of loop 42.
Fig. 5C illustrates an interrogation signal field
distribution provided by the antenna arrangement of Fig. 2.
The wire mesh graph surface shown in Fig. 5C represents the
maximum effective signal amplitude received during an
interrogation signal cycle by a marker receiving element
that is in the above-mentioned vertical orientation. It
will be noted that the graph surface is presented as a
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function of location in both the Y and Z directions
. (referring to Fig. 1). These values are representative of
amplitudes experienced at a X-axis position that is in a
central part of the interrogation zone.
Because of the quadrature relationship between the
signals generated through the loops 42 and 44, it will be
noted that there are no substantial nulls or holes in the
field distribution.
Although this desirable field distribution can be
conveniently provided by actively driving one loop and
inductively coupling a second loop so that there is a
quadrature relationship between the respective loop signals,
it is also contemplated to provide separate signal
generators for each of the loops and to directly drive the
loops in quadrature relation.
DUAL-PLANE OUADR.ATURE ANTENNA
An antenna configuration 63 provided in accordance
with a second embodiment of the invention is illustrated in
Fig. 4. The antenna configuration 63 includes an antenna
housing 64, shown in phantom, within which are housed
antenna loops 66, 68, and 70. A signal generating circuit
72 is connected to the antenna loop 66 to generate an
alternating current in the loop 66. A signal generating
circuit 74 is connected to the loop 68 to generate in the
loop 68 an alternating current at the same frequency as the
current in loop 66, but 90° out of phase with the current in
loop 66. Also, a signal generating circuit 76 is connected
to the loop 70 to generate in the loop 70 an alternating
current at the same frequency as, but 180° out of phase
with, the alternating current in loop 68.
The antenna loop 66 is substantially rectangular
and planar, and the loops 68 and 70 are substantially co-
planar with each other. The plane of the antenna loop 66 is
substantially parallel to the common plane of loops 68 and
70. (It will be noted that, for convenience in
representation, the antenna configuration 63, has been
inflated in a direction normal to the planes of the antenna
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loops.) The respective planes of loop 66 on one hand and of
the loops 68 and 70 on the other are preferably provided
quite close to each other. Each of the loops 68 and 70 is
substantially as wide as the loop 66, but only half as high
as the loop 66. The combined area of the loops 68 and 70 is
preferably about equal to the area of loop 66. The loops 68
and 70 are preferably stacked one on top of the other in
their respective plane. The loop 66 and the combination of
loops 68 and 70 are horizontally aligned in the direction
normal to their planes so that the loop 66 substantially
overlaps with the combination of the loops 68 and 70 in the
direction normal to the planes of the antenna loops. By
overlapping in this direction, it should be understood that
lines extending in the direction normal to the planes of the
antenna loops intersect the respective plane segments
defined by the antenna loops. The loop 66 is substantially
entirely overlapping, in the direction normal to its plane,
with the combination of loops 68 and 70 in the sense that
substantially all of the area of the loop 66 overlaps in
that directionwith the combination of loops 68 and 70.
Figs. 5A and 5B are graphs similar to the above-
discussed Fig. 5C, but respectively represent field
components provided by the antenna loop 66 (Fig. 5A) and the
combination of loops 68 and 70 (Fig. 5B). The graph shown
in Fig. 5C represents the combination of the fields provided
by all three loops and, as noted before, does not have
significant nulls or holes.
An antenna configuration 63' according to a third
embodiment of the invention is illustrated in Fig. 6.
The antenna configuration 63' is the same as the
configuration 63 of Fig. 4, except that the single loop 66
of Fig. 4 is replaced by side-by-side rectangular co-planar
loops 66' and 78. The loop 66' is driven by the previously
described signal generating circuit 72, and an additional
signal generating circuit 80 is connected to loop 78 to
generate an alternating current in loop 78 that is at the
same frequency but 180° out of phase with the current -in
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loop 66'. The antenna configuration 63' of Fig. 6 provides
a relatively even field distribution in the interrogation
zone, like that provided by the antenna configuration of
Fig. 4, while providing the additional feature of far-field
cancellation by virtue of the two pairs of "bucking" loops
63' and 78, and 68 and 70.
As shown in Fig. 6, loop 68 includes a horizontal
segment 82, a vertical segment 84 extending downwardly
vertically from a right end of segment 82, a horizontal
segment 86 extending leftwardly and horizontally from a
lower end of the segment 84, and a vertical segment 88 which
extends vertically to interconnect the respective left ends
of segments 82 and 86.
Loop 70 includes a horizontal segment 90 that
extends horizontally in parallel and in proximity to the
segment 86 of loop 68. Loop 70 also includes a_segment 92
that extends downwardly vertically from a right end of
segment 90, a segment 94 which extends leftwardly and
horizontally from a lower end of segment 92, and a segment
96 which extends vertically to interconnect- the respective
left ends of segments 90 and 94.
Loop 78 includes a top horizontal segment 98, a
segment 100 that extends downwardly vertically from a right
end of the segment 98, a segment 102 that extends leftwardly
and horizontally from a lower end of the segment 100, and a
segment 104 that extends vertically to interconnect the
respective left ends of the segments 98 and 102.
Loop 66' includes a segment 106 that extends
vertically in parallel and in proximity to the segment 104
of loops 78. Loop 66' also includes a segment 108 that
extends leftwardly and horizontally from a lower end of
segment 106, a segment 110 that extends vertically upwardly
from a left end of the segment 108, and a segment 112 that
extends horizontally to interconnect the respective upper
ends of the segments 106 and 110.
Further, each of the segments 82, 86, 90 and 94
are substantially equal in length (loops 68 and 70 being
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equally wide) and each of the horizontal segments 98, 102,
108 and 112 are equal to each other inlength and have a
lengththat is substantially one-half the length of segments
82, 86, 90 and 94 (the loops 66' and 78 being equal in width
to each other and having half the width of the loops 68 and
70) .
The vertical segments 100, 104, 106, and 110 are
all equal to each other in length (the loops 66' and 78
being equal in height), and the vertical segments 84, 88, 92
and 96 are all substantially equal in length to each other
and have a length that is substantially one-half of the
length of the segments 100, 104, 106 and 110 (loops 68 and
70 being equal in height to each other and having one-half
the height of the loops 66' and 78).
Also, loop segment 92 is substantially vertically
aligned with loop segment 84, loop segment 96 is
substantially vertically aligned with loop segment 88, loop
segment 112 is substantially horizontally aligned with loop
segment 98 and loop segment 108 is substantially
horizontally aligned with loop segment 102.
DUAL-PLANE FAR-FIELD CANCELING ANTENNA
An antenna configuration 63 " provided in
accordance with a fourth embodiment of the invention is
shown in Fig. 7. The antenna configuration 63 " differs
from the configuration 63 of Fig. 4 in that the loop 66 of
Fig. 4 is replaced in the configuration of Fig. 7 with two
co-planar triangular antenna loops 114 and 116. Also, the
loops 68 and 70 of Fig. 4 are replaced in the configuration
of Fig. 7 with three stacked co-planar rectangular loops
118, 120 and 122.
A signal generating circuit 124 is connected to
loop 114 to generate an alternating current in loop 114. A
signal generating circuit 126 is connected to loop 116 to
generate an alternating current in loop 116 that is the same
in frequency as the current in loop 114 but 180° out of ,
phase. A signal generating circuit 128 is connected to loop
120 to generate in loop 120 an alternating current that is
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of the same frequency but 90° out of phase with the current
_ in loop 114. A signal generating circuit 130 is connected
to loop 118 to generate in loop 118 an alternating current
that is of the same frequency but 180° out of phase with the
current in loop 120. A signal generating circuit 132 (which
may be combined with signal generating circuit 130) is
connected to loop 122 and generates, in loop 122 an
alternating current that is the same in frequency and is in
phase with the current in loop 118.
It should also be understood that the combined
area of loops 114 and 116 is substantially equal to the
combined area of loops 118, 120 and 122.
The "bucking" pair of triangular co-planar loops
114 and 116 are of substantially equal areas. Also, the
loop 120 has substantially the same area as the combined
areas of the loops 118 and 122, which generate a signal 180°
out of phase with the signal of loop 120. As a consequence,
the antenna configuration 63 " of Fig. 7, like the
configuration of Fig. 6, provides both a relatively even
field distribution in the interrogation zone as well as far-
field cancellation.
As shown in Fig. 7, loop 118 includes a top
horizontal segment 134, a segment 136 which extends
downwardly vertically from a right end of segment 134, a
segment 138 that extends leftwardly and horizontally from a
lower end of the segment 136, and a segment 140 that extends
vertically to interconnect the respective left ends of
segments 134 and 138.
Loop 120 includes a top segment 142 that extends
horizontally in parallel and in proximity to the segment 138
of loop 118. In addition, the loop 120 includes a segment
144 that extends downwardly vertically from a right end of
the segment 142, a segment 146 that extends leftwardly and
horizontally from a lower end of the segment 144, and a
. 35 segment 148 that extends vertically to interconnect the
respective left ends of segments 142 and 146.
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Loop 122 includes a top segment 150 that extends
horizontally in parallel and in proximity to the segment 146
of loop 120. Also, loop 122 includes a segment 152 which
extends downwardly vertically from a right end of the
segment 150, a segment 154 that extends leftwardly and
horizontally from a lower end of the segment 152 and a
segment 156 that extends vertically to interconnect the
respective left ends of the segments 150 and 154. T h a
antenna loop 116 includes a segment 158 that extends
vertically, a segment 160 that extends horizontally
leftwardly from a lower end of the segment 158, and a
segment 162 that extends obliquely to interconnect a left
end of the segment 160 and an upper end of the segment 158.
The loop 114 includes a segment 164 that extends
obliquely and in parallel and in proximity to the segment
162 of loop 116. The segment 114 also includes a segment
166 that extends vertically upwardly from a lower end of the
segment 164 and a segment 168 that extends horizontally to
connect the respective upper ends of the segments 164 and
168.
Further, the horizontal segments 134, 138, 142,
146, 150 and 154 are all substantially equal in length; the
vertical segments 136, 140, 152 and 156 are all
substantially equal in length to each other; the vertical
segments 144 and 148 are substantially equal in length to
each, each being twice the length of the segments 136, 140,
152 and 156; and the vertical segments 158 and 166 are
substantially equal in length to each other, each being
twice as long as the segments 144 and 148.
Also, the segments 136, 144 and 152 are all
substantially in vertical alignment with each other; and the
segments 140, 148 and 156 are all substantially in vertical
alignment with each other.
A modification of the embodiment of Fig. 7, which
does not provide far-field cancellation, should also be
noted. In particular, an antenna configuration may be
provided which includes only the co-planar triangular loops
CA 02217459 1997-10-03
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114 and 116, but with respective signal generators, or
inductively coupled as in the embodiment of Fig. 2, such
that the respective currents in loops 114 and 116 are 90°
out of phase.
CO-PLANAR FAR-FIELD CANCELING ANTENNAS
Fig. 8 shows a known antenna configuration made up
of four stacked, rectangular co-planar loops 170, 172, 174
and 176. As indicated in Fig. 8, loop 172 transmits a
signal that is 90° out of phase with thesignal provided by
loop 170; loop 174 provides a signal that is 180° out of
phase with the signal of loop 170; and loop 176 provides a
signal that is 180° out of phase with the signal of loop
172.
It is common to employ rectangular loop antennas
disposed in a vertically oriented plane (i.e. in the
orientation referred to as "lateral" in a prior discussion
of plane orientations herein) because the vertical segments
of the rectangular loops provide horizontal and lateral
fields (i.e. fields for stimulating markers in the
horizontal and lateral orientations, respectively), while
the horizontal segments of the loops provide horizontal and
vertical fields (i.e. fields for interrogating markers in
the horizontal and vertical orientations, respectively).
It will also be noted that the arrangement of Fig.
8 tends to produce far-field cancellation. However, the
"bucking" relationship between the corresponding vertical
segments of loops 170 and 174, and between the corresponding
vertical segments of loops 172 and 176, also tends to result
in some near-field cancellation, producing holes in the
interrogation field within the desired interrogation zone.
The horizontal, vertical and lateral fields provided by the
antenna arrangement of Fig. 8 are respectively illustrated
in Figs. 14A, 14B and 14C. It will be noted that the
horizontal field (Fig. 14A) is particularly low in amplitude
for Z = 0 and Y = t20, while the lateral field (Fig. 14C) is
low in amplitude for Y = 0 and is also fairly low for Z = 0.
Fig. 9 illustrates an antenna configuration 178
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according to a fifth embodiment of the invention. As will
be seen, the configuration shown in Fig. 9 is formed
entirely of co-planar loops and provides a more uniform
field distribution than the arrangement of Fig. 8.
The antenna configuration 178 includes co-planar
triangular loops 180, 182, 184 and 186 and signal generating
circuits 188, 190, 192 and 194 respectively connected to the
loops 180, 182, 184 and 186. As shown in Fig. 9, the
alternating current generated in loop 182 is 90° out of
l0 phase with the alternating current generated in loop 180.
Also, the alternating current generated in loop 184 is 180°
out of phase with the current in loop 180, and the current
generated in loop 186 is 180° out of phase with the current
generated in loop 182.
It is to be noted that, in the arrangement of Fig.
9, there are no vertically aligned pairs of bucking vertical
segments. Rather, in each pair of vertically aligned
vertical segments, the respective signals provided by the
two segments of the pair are 90° out of phase. As a
consequence, the arrangement shown in Fig. 9 provides far-
field cancellation while also substantially improving the
evenness of the field distribution in the interrogation zone
as compared with the arrangement of Fig. 8.
The horizontal, vertical and lateral fields
provided by the arrangement of Fig. 9 are respectively
illustrated by the graphs of Figs. 13A, 13B, and 13C.
Comparing, for example, Fig. 13A with Fig. 14A, a
considerable improvement in peak amplitude for Z - 0 is
provided in the field shown in Fig. 13A.
There is an even more notable plugging of holes
with respect to the lateral field, as is seen by comparing
Fig. 13C with Fig. 14C. In particular, the field shown in
Fig. 13C exhibits a very robust improvement for Y - 0 as
compared to the field shown in Fig. 14C.
As shown in Fig. 9, loop 180 includes a top .
horizontal segment 196, a segment 198 that extends
downwardly vertically from a right end of the segment 196,
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and a segment 200 that extends obliquely to interconnect a
lower end of the segment 198 and a left end of the segment
196.
The loop 182 includes a segment 202 which extends
obliquely in parallel and in proximity to the segment 200 of
loop 180. In addition, the loop 182 includes a segment 204
that extends vertically downwardly from an upper end of the
segment 202, and a segment 206 that extends horizontally to
interconnect the respective lower ends of the segments 204
and 202.
The loop 184 includes a segment 208 which extends
horizontally in parallel and in proximity to the segment 206
of loop 182. In addition, loop 184 includes a segment 210
that is vertically aligned with the segment 204 of loop 182
and extends downwardly vertically from a left end of the
segment 208. Finally, loop 184 includes a segment 212 that
extends obliquely to interconnect a lower end of the segment
210 and a right end of the segment 208.
Loop 186 includes a segment 214 which obliquely
extends in parallel and in proximity to the segment 212 of
loop 184. Also, the loop 186 includes a segment 216 which
extends horizontally rightwardly from a lower end of the
segment 214 and a segment 218 vertically aligned with the
segment 198 of loop 180 and extending vertically to
interconnect the respective right ends of the segments 214
and 216.
Further, each of the segments 196, 206, 208 and
216 are substantially equal in length; and the segments 198,
204, 210 and 218 are all substantially equal in length to
each other. In addition, the oblique segments 200, 202, 212
and 214 are all substantially equal in length to each other.
An antenna configuration 220 provided in
accordance with a sixth embodiment of the invention is shown
in Fig. 10. The antenna configuration 220 employs four
rectangular co-planar loops 222, 224, 226 and 228. As in
Fig. 9, signal generating circuits 188, 190, 192 and 194 are
respectively connected to the loops 222, 224, 226 and 228 to
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drive the respective loops in the same phase relationship as
was described in connection with the configuration of Fig.
9. As was the case in the configuration of Fig. 9, the
configuration of Fig. 10 is arranged so that any two
vertically aligned vertical segments are driven with a 90°
phase relationship, with the result that no bucking vertical
segments are vertically aligned with each other. The
configuration of Fig. 10 provides far-field cancellation
while also avoiding significant holes in the interrogation
field provided in the interrogation zone.
As shown in Fig. 10, loop 222 includes a top
horizontal segment 230, a segment 232 which extends
downwardly vertically from a right end of the segment 230,
a segment 234 which extends leftwardly and horizontally from
a lower end of the segment 232, and a segment 238 which
extends vertically to interconnect the respective left ends
of the segments 230 and 234.
The loop 224 includes a segment 240 which extends
horizontally in parallel and in proximity to the segment 234
of loop 222. In addition, loop 224 includes a segment 242
vertically aligned with the segment 232 of loop 222 and
extending downwardly vertically from a right end of the
segment 240. Further, loop 224 includes a segment 244 which
extends leftwardly and horizontally from a lower end of the
segment 242 and a segment 246 vertically aligned with the
segment 238 of loop 222 and extending vertically to
interconnect the respective left ends of the segments 240
and 244.
Loop 226 includes a segment 248 that extends
vertically in parallel and in proximity to the segment 242
of loop 224. Loop 226 also includes a segment 250 that
extends horizontally rightwardly from a lower end of the
segment 248, a segment 252 that extends vertically upwardly .
from a right end of the segment 250, and segment 254 that
extends horizontally to interconnect the respective upper
ends of the segments 248 and 252. Segments 250 and 254 are
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respectively horizontally aligned with segments 244 and 240
of loop 224.
The loop 228 includes a segment 256 that extends
horizontally iri parallel and in proximity to the segment 254
of loop 226. The loop 228 also includes a segment 258
vertically aligned with the segment 252 of loop 226 and
- extending vertically upwardly from a right end of the
segment 256. In addition, loop 228 includes a segment 260
which extends horizontally leftwardly from an upper end of
the segment 258 and a segment 262 vertically aligned with
the segment 248 of loop 226 and extending vertically to
interconnect the respective left ends of segments 256 and
260. Segments 256 and 260 are respectively horizontally
aligned with segments 234 and 230 of loop 222.
Further, the segments 230, 234, 240, 244, 250,
254, 256 and 260 are all substantially equal in length; and
the segments 232, 238, 242, 246, 248, 252, 258 and 262 are
all substantially equal in length to each other.
It will be observed that there are a number of
pairs of vertical segments having currents that are in
bucking relationship with eachother, but in each case the
two segments making up the pair of segments are horizontally
displaced with respect to each other. For example, the
segments 222 and 248 have respective currents that are in
bucking relationship, but .the segments 222 and 248 are
displaced both horizontally and vertically with respect to
each other. Such is also the case with respect to the pair
of segments 258 and 242.
According to a seventh embodiment of the
invention, shown in Fig. 11, there is provided an antenna
configuration 264 in which the only two vertical segments
are horizontally displaced with respect to each other. The
antenna configuration 264 includes antenna loops 266, 268,
270 and 272. The loops 266-272 are all triangular and co
planar. Signal generating circuits 188, 190, 192 and 194
are respectively connected to loops 266, 268, 272 and 270.
The loops 266, 268, 272 and 270 are driven by the respective
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generating circuits according to the phase relationship
described in connection with Fig. 9 among loops 180, 182,
184 and 186.
As was the case with the embodiments of Figs . 9
and 10, the antenna configuration 264 of Fig. 11 provides
far-field cancellation while generating an interrogation
field that does not have significant holes in the
interrogation zone. Again, it is significant that there are
no vertically aligned vertical segments in bucking relation
to each other. In fact, as noted above, the only two
vertical segments are not vertically aligned with each
other.
As shown in Fig. 11, loop 266 includes a
horizontal segment 274, a segment 276 which extends
obliquely downwardly and leftwardly from a right end of the
segment 274 and has a lower end that is displaced vertically
downwardly from the midpoint of the segment 274. The loop
266 also includes a segment 278 that extends obliquely to
interconnect the lower end of the segment 276 and a left end
of the segment 274.
The loop 268 includes a segment 280 that extends
obliquely in parallel and in proximity to the segment 276,
a segment 282 that extends vertically downwardly from an
upper end of the segment 280 and a segment 284 that is
substantially aligned with segment 278 of loop 266 and
extends obliquely to interconnect the respective lower ends
of the segments 280 and 282.
Loop 270 includes a segment 286 that extends
obliquely in parallel and in proximity to the segment 284,
a segment 288 that extends horizontally leftwardly from a
lower end of the segment 286, and a segment 290 that is
substantially aligned with the segment 280 of loop 268 and
extends obliquely to interconnect the respective left ends ,
of the segments 286 and 288.
Loop 272 includes a segment 292 that is
substantially aligned with the segment 276 of loop 266 and
extends obliquely in parallel and in proximity to the
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segment 290 of loop 270. In addition, the loop 272 incudes
a segment 294 that extends vertically upwardly from a lower
end of the segment 292 and also a segment 296 that is
substantially aligned with the segment 286 of loop 270 and
extends obliquely in parallel and in proximity to the
segment 278 of loop 266 to interconnect the respective upper
ends of segments 294 and 292.
The segments 274 and 288 are substantially equal
in length, the segments 282 and 294 are substantially equal
in length to each other, and the segments 276, 278, 280,
284, 286, 290, 292 and 296 are all substantially equal in
length to each other.
An antenna configuration 264' provided in
accordance with an eighth embodiment of the invention is
shown in Fig. 12. The antenna configuration 264' is the
same as the configuration 274 of Fig. 11 except for the
phase relationship among the respective alternating currents
in the antenna loops 266, 268, 270 and 272.
In particular, in the configuration 264' of Fig.
12, the current in loop 270 is 180° out of phase with the
current in loop 266 and the current in loop 272 is 180° out
of phase with the current in loop 268. By contrast, in the
antenna configuration 264 of Fig. 11, the current in loop
270 is 180° out of phase with the current in loop 268 and
the current in loop 272 is 180° out of phase with the
current in loop 266. It should be noted that, in both
embodiments, the current in loop 268 is 90° out of phase
with the current in loop 266.
Like the embodiment of Fig. 11, the embodiment of
Fig. 12 provides a relatively even field distribution within
the interrogation zone and also provides far-field
cancellation.
QUADRATURE RECEIVER ARRANGEMENT
A receiver portion of an electronic article
surveillance system, provided according to a ninth
embodiment of the invention, will now be described with
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reference to Figs. 15 and 16. The receiver portion,
generally indicated by reference numeral 300, includes two
antenna loops 302, 304, which are preferably rectangular,
stacked, co-planar antenna loops. The respective signals
received through the antenna loops 302 and 304 are coupled
to a receiver circuit 306.
To avoid nulls in the interrogation zone, it is
desirable that the respective signals received through the
antenna loops 302 and 304 be presented to the receiver
circuit 306 in a quadrature relationship. Fig. 16
illustrates a preferred circuit arrangement for providing
such a relationship.
As shown in Fig. 16, the signals received via the
antenna loop 302 are phase shifted by +45° in a phase shift
circuit 308, and the resulting phase-shifted signal is
provided to an input of a summation circuit 310. Also, the
signal received through the antenna loop 304 is phase-
shifted by -45° in a phase shift circuit 312 and the
resulting phase-shifted signal is provided to the other
input of the summation circuit 310. The two phase-shifted
signals are summed at the summation circuit 310 and the
resulting summed signal is provided to receiver circuitry
(not shown) for further processing.
According to a preferred embodiment of the
invention, the phase shift circuit 308 may be a low-pass
filter having its 3-dB point at 58 kHz, and the phase shift
circuit 312 may be a high pass filter with its 3-dB point at
58 kHz. The phase splitting could also be performed using
appropriate LC circuitry or active filters.
It should also be noted that one of the phase
shift circuits could be arranged to provide a 90° phase
shift, in which case the other phase shift circuit would be
omitted.
The combined 90°-offset. signals provide an
interplay between the signals received by the two antenna ,
loops which is helpful in detecting marker signals. This
provides advantages as compared to a previous known
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technique in which the respective antenna signals were
analyzed in separate time slots, since the latter technique
results in nulls in the interrogation zone.
It is also contemplated to achieve the desired
quadrature relationship by providing inductive coupling
between the two antenna loops in a similar manner to that
' shown in the embodiment of Fig. 2. However, this is not
preferred because adequate inductive coupling between the
antenna loops requires that the loops be arranged with high
Q, which tends to result in excessive ringing in pulsed
magnetomechanical EAS systems. On the other hand, with the
arrangement shown in Fig. 16, the Q of the antenna loops can
be moderated so as to prevent ringing.
Although not shown in Figs. 15 and 16, it should
be understood that the quadrature receiver arrangement of
Fig. 16 can be adapted to a far-field canceling antenna
configuration.
It should further be understood that antenna
arrangements shown in this application in which respective
signal generators are provided for every antenna loop (see,
for example, Figs. 9 and 10) can be modified by arranging
two adjacent loops for inductive coupling with a 90° phase
offset, as was described in connection with Figs. 2 and 3.
Moreover, where two co-planar loops are provided with a 180°
phase offset (as in Figs. 4, 6, 9 and 10, for example) the
two loops can be provided by a single twisted loop as shown
in Fig. 3 of U.S. Patent No. 4,245,980 or in U.S. Patent No.
4,872,018.
Although no connection between signal generators
3 0 is shown in the drawings ( such as Figs . 4 and 6 ) in which
more than one signal generator is ashown, it will be
understood by those of ordinary skill in the art that
control signals or a common reference signal may be provided
to all of the signal generators in order to obtain the
synchronization required for the desired phase
relationships.
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WO 96/38877 PCT/LTS96/07442
Further variations of the preferred embodiments
already described are contemplated, including those that
will now be described with reference to Figs. 17-21.
For example, the embodiment shown in Fig. 4 can be
modified by making all three loops 66, 68 and 70 co-planar,
with the stacked pair of bucking loops 68 and 70 arranged
alongside loop 66. This arrangement is schematically
illustrated in Figs. 17 and 18, which are respectively a
perspective view and a plan view of the arrangement. It
will be noted that all of the loops 66, 68 and 70, are
vertically oriented, i.e., are arranged in a plane that is
orthogonal to a horizontal plane. Also, the loops 68 and 70
(represented by loop 68 in Fig. 18) are displaced in a
horizontal direction relative to loop 66.
The arrangement shown in Figs. 17 and 18 provides
essentially the same result as the embodiment of Fig. 4,
although with the disadvantage of having an antenna
configuration that is substantially wider (longer in the X-
axis direction -- see Fig. 1) than the embodiment of Fig. 4.
It will be understood that the respective fields (shown in
Figs. 5A and 5B) provided by loop 66 and the combination of
loops 68 and 70 are not overlaid in space to produce the
field (shown in Fig. 5C) that is provided by the embodiment
of Fig. 4. However, a marker that is in a vertical
orientation and is transported through the interrogation
zone in the X-axis direction, and with little movement in
the Y- and Z- axis directions, would sequentially experience
the field profiles shown in Fig. 5A and 5B within a short
period of time, resulting in an effective interrogation
field that is equivalent to the field shown in Fig. 5C.
It should be observed that the modification made
to the dual-plane embodiment shown in Fig. 4, which results
in the arrangement of Figs. 17 and 18, can also be made to .
the dual-plane embodiments shown in Figs. 6 and 7.
Fig. 19 schematically illustrates a further
modification which can be made to the arrangement of Figs.
17 and 18, while providing substantially the same results.
CA 02217459 1997-10-03
WO 96/38877 PCT/C1S96/07442
As seen in Fig. 19, (which is a plan view similar to Fig.
18), the pair of co-planar bucking loops 68 and 70 (again
represented in the drawing by loop 68) is shifted by a
modest amount so as not to be co-planar with the loop 66.
, Rather, the loop 66 and the combination of-loops 68 and 70
are arranged in respective planes that intersect at an angle
a , as shown in Fig . 19 . So long as a does not vary from
180° by more than about 20°, it is believed that the
arrangement in Fig. 19 would produce substantially the same
result as the arrangement of Figs. 17 and 18. Of course, as
a is reduced from 180° towards 90°, the thickness of the
antenna arrangement (i.e., its length in the Y-axis
direction) would be increased.
If the angle a is permitted to become a rather
small acute angle, as schematically illustrated in Fig. 20,
the arrangement approaches the dual-plane embodiment of Fig.
4. It is believed that, for values of a in the range of
about 15° or less, essentially the same combined field is
produced as the field shown in Fig. 5C.
Another-intersecting-plane antenna arrangement is
schematically illustrated in Fig. 21, which is a side view
of the arrangement. It will be observed that the co-planar
combination of loops 68 and 70 is arranged in a plane that
tilts relative to the plane of loop 66, with the two planes
again intersecting at an angle e. In this case, the loop 66
remains vertically oriented, but the loops 68 and 70 diverge
from a vertical orientation. It is believed that
satisfactory results can be obtained for values of a of up
to 90°, but it is contemplated to provide an arrangement
with ~ at any value in the range 0°<e<180°. Again the
intersecting plane arrangement tends to produce a somewhat
less compact antenna configuration than a dual plane
. embodiment, as shown in Fig. 4.
It will be appreciated that the modifications
illustrated in Figs. 19-21 can also be applied to the dual
plane embodiments shown in Figs. 6 and 7.
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In connection with both transmitted and received
signals, the embodiments described herein have been
concerned with signals in quadrature relationship, i.e.,
with a 90° phase offset. However, it should be noted that
satisfactory results can also be expected with a phase
relationship that deviates from a 90° offset by a modest
amount. '
Other techniques for achieving a distribution of
peak field values that is substantially equivalent to the
distribution shown in Fig. 5C will now be described,
initially with reference to Figs. 22A-22C.
In the embodiment shown in Figs . 22A and 22B, a
pair of rectangular, stacked, co-planar antenna loops 314
and 316 is provided. A horizontal segment 318 of the loop
314 is arranged in parallel and in proximity with a
horizontal segment 320 of the loop 316. It will be observed
that the antenna configuration shown in Figs. 22A and 22B
includes only two co-planar loops, and that the segments 318
and 320 are the only pair of segments which are arranged in
parallel and in proximity to each other.
Although the co-planar antenna loops shown in
Figs. 22A and 22B are rectangular, it should be noted that
other loop shapes may be provided. For example, the
embodiment shown in Figs. 22A and 22B may be modified by
replacing the loops 314 and 316 with a pair of co-planar
triangular loops like the loops 114 and 116 shown in Fig. 7.
A signal generating circuit 322 is attached to the
loop 314 to generate an alternating current in the loop 314
and a signal generating circuit 324 is connected to the loop
316 to generate an alternating current in the loop 316. A
control circuit 326 is associated with the generating
circuits 322 and 324 to establish desired timing
relationships between the respective signals generated by
the signal generating circuits.
In particular, the embodiment now being described
is alternately operated in the two conditions shown in Figs.
22A and 22B, respectively. As shown in Fig. 22A, in the
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WO 96/38877 PCT/US96/07442
first condition the antenna according to this embodiment is
_ driven with the alternating currents in the loops 314 and
316 substantially in phase, while in the other condition,
shown in Fig. 22B, the loops are driven substantially 180°
out of phase. As a result, in the condition of Fig. 22A,
the currents in the segments 318 and 320 are generated in
opposite directions, resulting in substantial cancellation
of the field components generated by the segments 318 and
320, so that the loops 314 and 316 are substantially
equivalent to a single loop transmitter. On the other hand,
in the condition shown in Fig. 22B, the antenna
configuration made up of loops 314 and 316 is equivalent to
a conventional figure-eight antenna, with the field
components generated in the segments 318 and 320 reinforcing
each other.
The timing at which the respective conditions
shown in Figs. 22A and 22B are provided is shown in the
timing chart of Fig. 22C. The condition shown in Fig. 22A
is provided during a sequence of time segments A, while the
condition shown in Fig. 22B is provided during a sequence of
time segments B, with the sequence of time segments B being
interleaved with the sequence of time segments A.
Each of the time intervals A and B may be, for
example, equivalent in duration to several cycles of the
interrogation signal. By alternately switching the antenna
configuration between a single-loop and a figure-eight
configuration, it is possible to obtain a field profile
equivalent to that shown in Fig. 5C, with the understanding
that the field amplitude shown therein would be the maximum
experienced over a time period which encompasses both an
interval A and an interval B. Thus, the embodiment
described in connection with Figs. 22A-22C again results in
a more even effective field distribution than is provided
either by a single loop or a figure-eight antenna used
alone.
Switching back and forth between a single loop and
a figure-eight antenna may be accomplished by other
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WO 96/38877 PCT/US96/07442
techniques in addition to that just described. For example,
as indicated in Fig. 23, a dual-plane antenna like that
shown in Fig. 4 may be operated so that the single loop 66
is active only during time intervals A and the figure-eight
arrangement made up of loops 68 and 70 is active only during
the sequence of time intervals B. A version of the
embodiment of Fig. 4, suitably modified to operate according '
to the "time-slices" illustrated in Fig. 23, is shown in
Fig. 24, and includes a control circuit 326' for providing
the desired on and off timing for the signal generators 72,
74 and 76. In addition, the loops 66', 68' and 70' are
respectively provided with switches 328, 230 and 332, which
are controlled by the control circuit 326' so as to open
circuit the respective antenna loop during the time
intervals in which the loop is not active. The open
circuiting of the non-active loops prevents induction
effects which would otherwise be experienced.
Other modifications of the antenna shown in Fig.
4 are illustrated in Figs. 25 and 26, respectively. In each
of Figs. 25 and 26 it will be observed that the
configuration of Fig. 4 has been made into a co-planar
configuration, by slightly increasing the width and height
of the loop 66 and arranging the loop 66 (shown as 66" or
66 " ' in Figs. 25 and 26) in the same plane with the loops
68 and 70 (68' and 70' in Fig. 26) with the loop 66" or
66 " ' circumscribing the two other loops. In the
modification shown in Fig. 25, the loops 68 and 70 are
driven in quadrature relation with loop 66" and
substantially out of phase with each other. That is, the
same phase relationship among the currents of the loops is
provided in Fig . 2 5 as in Fig . 4 . On the other hand, in
Fig. 26, the single loop 66 " ' and the figure-eight
arrangement made up of loops 68' and 70' are respectively
active in alternating sequences of time intervals, as in the
arrangement illustrated in Figs. 23 and 24.
It is to be understood that each of the quadrature
dual-plane antennas shown in Figs. 6 and 7 can be modified
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WO 96/38877 PCTIL1S96107442
for alternating time interval operation in the same manner
that the arrangement of Fig. 4 was modified to produce the
arrangement of Fig. 24. In addition, the dual-plane
antennas operated in alternating time intervals can be
modified into co-planar arrangements analogous to the
modification of Fig. ~4 illustrated in Figs. 17 and 18.
Modifications of the dual-plane alternating time interval
antennas to form intersecting-plane alternating time
interval antennas can be performed in an analogous manner to
the modifications of Fig. 4 described above with reference
to Figs. 19-21.
In addition to the co-planar antenna arrangement
of Fig. 26, in which only three loops are provided, it is
also contemplated to provide a far-field canceling co-planar
arrangement including four loops, that is, two pairs of
loops with each pair driven in a respective interleaved
sequence of time intervals. For example, the arrangement
shown in Fig. 9 can be modified to produce the arrangement
shown in Fig. 27. In Fig. 27, the triangular loops 180',
182', 184' and 186' are respectively provided with switches
334, 336, 338 and 340 and a control circuit 326" is provided
to control the signal generators 188, 190, 192 and 194 and
the switches 334, 336, 338 and 340 so that the pair of loops
180' and 184' is active during a sequence of time intervals
A (Fig. 23) and the loops 182' and 186' are open-circuited
during those intervals. In addition, during a sequence of
intervals B (again, Fig. 23), interleaved with the intervals
A, the pair of loops 182' and 186' is active and the loops
180' and 184' are open-circuited. It should be noted that
a similar modification can be made to the antenna
arrangements shown in Figs. 10-12.
The concept of switching between a single loop and
a figure-eight loop arrangement, as discussed above in
connection with Figs. 22A-22C, can also be applied to a
receive antenna arrangement like that of Fig. 15. Such a
switched receive antenna arrangement will now be described
with reference to Figs. 28 and 29.
CA 02217459 1997-10-03
WO 96!38877 PCT/LTS96/07442
The arrangement shown in Fig. 28 includes the same
receive antenna loops as in Fig. 15. Loop 302 has a
horizontal segment 334 arranged in parallel and in proximity
to a horizontal segment 336 of loop 304.- It will be
observed that the receive antenna arrangement of Fig. 28
does not include any loops in addition to the loops 302 and
304 and does not have any pair of loop segments arranged in
parallel and in proximity to each other except for the loop
segments 334 and 336.
The arrangement of Fig. 28 also includes a receive
circuit 338 connected to the antenna loops 302 and 304 by a
switchable interface circuit 340.
Details of the interface circuit 340 are shown in
Fig. 29. The interface circuit 340 includes a summation
circuit 310 which has inputs 342 and 344 and an output
connected to the receive circuit 338 for providing to the
receive circuit 338 a sum signal formed by the summation
circuit 310 from the signals respectively provided to its
inputs. The interface circuit 340 also includes a phase
shift circuit 348 which provides a phase shift of 180° to a
signal input thereto and outputs the resulting phase-shifted
signal. The interface circuit 340 also includes a switching
circuit 350.
The input 342 of the summation circuit 310 is connected
to receive the received signal provided from the antenna
loop 302. The phase shift circuit 348 is connected to
receive the received signal provided from the other antenna
loop 304, and the phase-shifted signal output from the phase
shift circuit 348 is provided to an input 352 of the
switching circuit 350. The switching circuit 350 has
another input 354 which is connected directly to receive the
received signal from loop 304 without phase shift. An
output 356 of the switching circuit 350 is connected to the ,
input 344 of the summation circuit 310.
The switching circuit 350 is switchable between a ,
position (shown in Fig. 29) in which the phase-shifted
signal output from the phase shift circuit 348 is supplied
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WO 96/38877 PCT/LTS96/07442
to the input 344 of the summation circuit 310 and an
_ alternative position in which the received signal from the
loop 304 is supplied without phase shift to the input 344 of
the summation circuit 310.
The latter condition of the switching circuit 350
is maintained during time intervals A (see Fig. 22C) so that
' the antenna arrangement of Fig. 28 operates substantially as
a single loop antenna during the time intervals A. On the
other hand, during an interleaved sequence of time intervals
B, the switch 350 is maintained in the condition shown in
Fig. 29, so that a signal from loop 304, phase shifted by
180°, is provided to the summation circuit 310. As a
result, during the intervals B the antenna arrangement of
Fig. 28 is essentially equivalent to a figure-eight
arrangement. In this way, a relatively uniform sensitivity
to signals present in the interrogation zone can be
achieved. '
Instead of providing a 180° phase shift in one of the
inputs for summation circuit 310 during the time intervals
B, phase shifts can be applied to both of the inputs for
summation circuit 310 during the time intervals B, so as to
have the inputs 180° out of phase with each other. For
example, a +90° phase shift can be applied to one input
while applying a -90° phase shift to the other input.
Although the embodiments described herein have
been presented solely as either receiving or transmitting
antennas, it is also contemplated that the antenna
configurations of the various embodiments be used both for
transmitting and receiving.
Various other changes in the foregoing antenna
configurations may be introduced without departing from the
invention. The particularly preferred embodiments are thus
intended in an illustrative and not limiting sense. The
true spirit and scope of the invention is set forth in the
following claims.
37
