Note: Descriptions are shown in the official language in which they were submitted.
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PCT/EP2010/002480
345 P 530 PCT
Magnetically coupling near-field RFID antenna
The invention relates to a magnetically coupling near-
field RFID antenna as claimed in the pre-characterizing
clause of Claim 1.
Contactless identification systems with contactless
transmission of energy and data from a data
transmission/reception device to a portable data carrier
via an electrical, magnetic or electromagnetic alternating
field are well known. In particular what is known as radio
frequency identification (RFID) offers a possibility to
contactlessly read out information located on portable
data carriers or to write data thereto. Starting
therefrom, RFID technology opens up a large number of
possible applications; for example, it opens up
possibilities for permanently checking whether for example
specific goods or products are present in warehouses
during production sequences or whether specific goods
having specific equipment features are present at specific
locations.
RFID systems have a plurality of basic components and
technical properties by which they are defined. Generally
provided is what is known as a reading apparatus, or
reader for short, which is connected to an antenna. The
reading apparatus emits a corresponding interrogation
signal via the antenna. This signal, which is received by
a tag, serves at the same time to supply energy to the
tag. The corresponding information is read out on the tag
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and returned to the transmission/reception apparatus,
known as the reader, which picks up and evaluates the
corresponding signal via the antenna. The path is in this
case a bidirectional transmission/reception path in an
identical frequency range or frequency band. For this
purpose, different frequency bands may be cleared in the
different countries for this technology.
The aforementioned tags conventionally comprise, in
addition to a substrate, for example in the form of an
optionally flexible film, a data carrier antenna and also
an associated circuit arrangement (chip) in which is
stored the corresponding information which can be read out
after reception of a signal.
In RFID technology, different types of tags and, depending
on the types of tags, to some extent also different
reception methods (some of which are also frequency-
dependent) have become known.
The corresponding transponders, referred to hereinafter
also as tags for short, differ for example in terms of the
transmission frequency, but also in terms of their purpose
of use.
For example, dipolar tags have become known, which draw
the energy irradiated by the reader, above all from the E-
field or a combination of the E and the H-field, i.e. the
electromagnetic field.
In addition, somewhat small looped tags have become known,
which are coupled primarily by the H-field, i.e. the
magnetic field.
In addition, there are also mixed forms of transponders,
i.e. tags.
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Just as the tags differ from one another, i.e. in terms of
whether the tags are oriented primarily to the reception
or the emission of E-fields, of H-fields or to the
combination, the antenna designs for RFID readers also
differ from one another.
Thus, the RFID antennas which are conventionally used are
patch antennas. Antennas of this type conventionally have
very low selectivity in their near range.
In addition, loop antennas, in particular large loop
antennas, have become known, which are suitable above all
for transmission and reception by means of magnetic
fields.
Thus, for example, according to US 2008/0048867 Al, the
use of a somewhat rectangular or circular RFID antenna has
become known, which is fitted in its circumferential
direction with one or more capacitors. Ultimately, the
capacitors can also be fitted by way of an interruption or
a plurality of interruptions in the circumferential
direction of the antenna which is, for example, in
principle somewhat circular in its configuration. An
antenna of this type is intended to be suitable, in
particular, as a UHF RFID antenna generating a magnetic
coupling to tags located in the antenna region.
Nevertheless, antennas of this type generate not
inconsiderable electromagnetic radiation perpendicularly
to the axis of the loop, as in dipoles. In addition, in
antennas of this type, a reflector has to be used in order
to achieve an improvement. For this reason too, the
antenna produces overall a comparatively large design,
partly owing to the necessary spacing between the loop
antenna and the reflector. In this case, according to this
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prior publication, the aim is to generate loop antennas of
the type in which the length of the loop portions, which
are separated from one another in each case via a
capacitor, can be longer than the wavelength of the
excitation signal.
A further segmented loop antenna has also become known
from the publication ''Segmented Magnetic Antennas for
Near-field UHF RFID'', Microwave Journal and Horizon House
Publications, Vol. 50, No. 6 June 2007. The antenna has in
principle a polygonal shape and is highly segmented. Each
individual segment is formed from a metal line comprising
a capacitor, which is connected in series, relative to the
next segment. This publication discloses as being known,
for example, an eight-polygonal antenna with six
capacitors or, for example, a sixteen-polygonal antenna
with fifteen capacitors at a magnitude of 1 pF and a
resistance of 10 0.
Furthermore, antenna designs have also become known, in
which antennas are constructed on the basis of a
microstrip line. This may be taken to be known, for
example, from US 2007/0268143 Al. Antennas of this type
possess lengths of ? A/2 and are used to implement an E-
field coupling. Typically, the length of an antenna of
this type is greater than A/2 (based on the operating
frequency) and less than A (wherein A is the wavelength in
the dielectric).
Antennas of this type can be embodied as non-radiating
antennas, for example in the form of meander line
antennas. A meander line antenna of this type is arranged
on the surface of a substrate (above a ground surface),
the meander line antenna being fed at one end and
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terminated at its opposite end using a resistor which is
connected to ground.
In an antenna construction of this type, E-field coupling
can be used to read out, for example, labels which are
5 provided with suitable tags and are moved directly
adjacently beyond the antenna in question.
A generic magnetically coupling antenna has also become
known from GB 2 431 053 A. This is a looped antenna which
is arranged parallel set apart from a ground surface. The
antenna is fed at one end and is short-circuited to ground
via an electrical connecting wire at its other end.
The length of the antenna is generally approximately 1/10
to 1/100 times the operating frequency wavelength. Such
being the case, the antenna, for example at an operating
frequency of 13.56 MHz, can have a length of approximately
220 mm to 2200 mm.
With an antenna design of this kind and dimensioning of
this kind, a standing wave is produced along the length of
the antenna, which is why in the further exemplary
embodiments of this prior printed publication it is
described that a corresponding resonant circuit has to be
added, as well as a matching circuit, in order to obtain a
corresponding matching.
In addition, an antenna arrangement is also to be
concluded as known from the prior printed publication
Xianming Oing et al: "Characteristics of a Metal-Backed
Loop Antenna and its Application to a High-Frequency RFID
Smart Shelf", IEEE Antennas and Propagation Magazine, IEEE
Service Center, Piscataway, NJ, US, Vol. 50, No. 2, 1
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April 2009 (2009-04-01), Pages 26-38, XP011263949, ISSN:
1045-9243.
A loop antenna is described which is positioned over a
metal plate. The loop antenna described forms a resonant
circuit with the capacitors of the matching circuit,
wherein the current is at a maximum in the loop antenna.
Against this background, it is the object of the present
invention to provide an improved magnetically coupling
near-field RFID antenna which has a design which is as
small as possible and causes in this case as little power
irradiation as possible in order to ensure, in this way
too, high selectivity, thus ensuring in the immediate near
field of a tag passed by the antenna that at all times
only a single tag located directly in the antenna region
can be read out.
According to the invention, the objective is achieved in
accordance with the features specified in Claim 1.
Advantageous configurations of the invention are specified
in the sub-claims.
Within the scope of the solution according to the
invention, a near-field RFID antenna is proposed, in which
power irradiation is reduced to a minimum. In other words,
it is possible to ensure within the scope of the invention
that for example less than 20 %, in particular less than
15 %, 10 % or even less than 8 %, 6 %, 4 % or even 2 % of
the power is irradiated.
The antenna according to the invention is in this case
optimized for magnetic tags, i.e. for tags which are fed
and addressed primarily via the magnetic field. In this
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case, the antenna according to the invention generates in
the near field a strong H-field.
The antenna according to the invention has in this case a
loop shape which, in terms of basic shape, can readily
vary in regions, for example can be circular, oval, square
or somewhat rectangular, n-polygonal or the like in its
configuration. The key aspect is that the antenna means,
which is referred to as a looped antenna, is returned from
a feed point via a closed path as close as possible to the
feed point and terminated there to ground via a
terminating resistor.
The near-field RFID antenna according to the invention
consists of a microstrip line which is fed at one end and
is terminated at the other end by the line impedance. As a
result, it is possible to generate a purely progressive
wave. The length L of the strip conductor of the frame-
shaped antenna is in this case less than A/2 (wherein A is
the wavelength, i.e. the operating wavelength in the
dielectric). Preferably, the length is less than A/3.
The gap between the feed point and terminating resistor is
in this case ideally as small as possible, regardless of
how the strip line frame antenna is specifically shaped,
i.e. whether it is somewhat circular, elliptical,
rectangular, etc. in its configuration. In this way, it is
possible to make the direction of flow at a specific point
in time uniform in all cases on the entire line, thus
effectively strengthening the magnetic field and reducing
the electrical field.
A further advantage of the antenna according to the
invention over large magnetic (segmented) antennas
consists in its broadband nature with respect to
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adaptation. One reason for this is the non-resonant
structure of the antenna construction according to the
invention.
The invention will be described in greater detail
hereinafter with reference to drawings, in which
specifically:
Figure 1 is a schematic spatial view of certain objects
which, set apart from one another, are moved past a near-
field RFID antenna;
Figure 2 is a schematic plan view onto a first exemplary
embodiment of a near-field RFID antenna of a reader;
Figure 3 is a schematic cross-sectional view through this
exemplary embodiment according to Figure 2; and
Figure 4 shows an exemplary embodiment differing from
Figure 2.
Figure 1 represents schematically, by way of example, a
type of conveyor belt or transport path along which it is
possible to move a plurality of objects 3 which are set
apart from one another on the conveyor belt 1.
Each of these objects 3 is to be provided with a tag
(transponder) consisting preferably of a passive
transponder, i.e. a passive tag, which receives the
required energy only from the magnetic antenna field and,
with the aid of this energy, can then read out the
information stored in the tag and send it to the RFID
antenna of the reader.
Furthermore, Figure 1 shows in this case a magnetically
coupling near-field antenna 11, which is associated with a
reader, according to an exemplary embodiment according to
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the invention, the individual objects 3 being successively
moved past the near-field antenna 11 in close proximity on
the conveyor belt/transport path 1 in order to read out
the information stored on the tag. The near-field antenna
11 defines in this case a narrowly demarcated reading
region 13, so that in all cases only an object located in
the reading region 13 can ever be identified based on the
tag located thereon.
The antenna is highly selective so that in each case only
a tag located in direct proximity to the near-field
antenna 11 can be read out, that is to say, in the reading
region 13 represented in Figure 1. The objects which are
located outside the reading region and have the separate
tags cannot be read out.
The magnetically coupling near-field antenna according to
the invention consists, in the exemplary embodiment shown,
of a looped or frame-shaped structure 15 which, in the
exemplary embodiment shown, is embodied as a strip line.
The associated strip conductor 19 is constructed on the
upper side 21a on a substrate or dielectric 21. The
dielectric can then have in this case the electricity
value Er of for example more than 2, 3, 4, 5, 6, 7, 8 or 9
or else values of less than 10, 9, 8, 7, 6, 5, 4 or 3, 2.
Preferably, the substrate 21 used is the material FR4, the
Er of which lies typically between 4.0 and 4.7.
In particular when using a substrate 21 made of FR4, it
has been found to be advantageous if the wave resistance
of the strip line is 50 ohms or approx. 50 ohms.
On the substrate or dielectric 21 thus described or in the
case of air as the dielectric using a supporting and
holding construction, there is then provided on the upper
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side of the dielectric the associated strip conductor 19
which, according to the first exemplary embodiment, is
embodied so as to extend in a precisely circular manner,
at a radius R. Ultimately, the maximum radius R of the
5 looped or frame-shaped antenna is determined by the
wavelength in the dielectric. The circumference of the
looped or frame-shaped antenna is in this case to be less
than X/2. Typically, this circumference U will be less
than or equal to X/3.
10 If the antenna is to be operated, for example, at 865 MHz
and on a substrate wherein Er = 1 (air), this would mean
that - if the circumference is to be less than A/2 - this
circumference is ultimately to have a value of less than
17.3 cm.
In the preferred range, this circumference is, as
mentioned, to be less than X/3, i.e. less than or equal to
11.6 cm. In other words, this results in a radius which is
less than or equal to 2.7 cm and is preferably less than
or equal to 1.9 cm.
Theoretically, the radius could be minimized, i.e. tend
toward 0. Nevertheless, this would excessively reduce the
size of the reading field, i.e. the reading range. A value
of about 5 mm can therefore be designated as the minimum
radius.
The exemplary embodiment shown results in the strip line
19 being fed at one end 19a to ground, for which purpose
there is provided in the substrate a hole or recess 23
which extends perpendicularly thereto and is in alignment
with a corresponding hole or recess 23' in the ground
surface 25 located on the underside of the
substrate/dielectric 21.
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As a result, a feed line 27 runs to a contact point at the
direct end 19a of the strip line 19.
The opposite second end 19b of the strip line 19 is
terminated by a terminating resistor 29, i.e. by a
resistor corresponding to the line impedance.
The gap or spacing 31 formed between the two ends 19a and
19b of the strip conductors 19 is in this case to be as
small as possible. The size of the gap or spacing 31
between the starting point and the end point 19a, 19b of
the strip line 19 is in this case to be preferably less
than 10 % and in particular less than 5 %, 4 %, 3 %, 2 %
or even less than 1 % of the total length of the strip
lines 19.
The aforementioned terminating resistor 29 is in this case
connected to the ground surface 25, which is located on
the underside 21b of the substrate 21, via one or more
through-contacts 33.
It would also be possible for a short line to be guided
from the end 19b of the strip line 19 to the underside 21b
of the substrate/dielectric 21, for example also by means
of a through contact, a sufficient (small) aperture being
provided on the underside of the substrate or dielectric
21 in the ground surface 25, wherein in this aperture in
the ground surface the resistor can then be embodied for
terminating the strip conductor 19 on the underside 21b of
the substrate/dielectric 21, which resistor is then
connected there directly to ground 25 via one or more
lines.
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In the exemplary embodiment shown, the strip line 19 is
configured in a circular manner in accordance with Figure
2.
However, the looped or frame-shaped strip conductor
antenna can also have shapes differing therefrom, for
example be configured so as to be oval, rectangular or
square, generally n-polygonal in its configuration. Figure
4 shows a differing embodiment of the invention in which,
for example, the strip conductor antenna 15 represents,
viewed from above, a strip conductor extending in a square
manner. However, in this case too, the spacing between the
starting point and the end point 19a, 19b of the strip
conductor 19 is configured so as to be as small and narrow
as possible, so that, in this case too, this spacing has
preferably less than 10 %, i.e. less than 8 %, 7 %, 6 %, 5
%, 4 %, 3 %, 2 % or even less than 1 % of the total length
of the strip conductor 19. Straight and/or curved portions
can be joined together. All that matters is that a
peripheral strip line is provided, the start 19a and end
19b of which are as close together as possible.
Ultimately, this ensures that an antenna of this type, as
is shown above all in the arrow direction 35 in Figure 1,
generates a magnetic H-field without electromagnetic
energy being irradiated in the relevant manner. It is thus
possible to ensure, within the scope of the invention,
that for example less than 20 %, in particular less than
15 %, 10 % or even less than 8 %, 6 %, 4 % or even 2 % of
the power is irradiated. This provides a highly selective
magnetically coupling near-field RFID antenna which is
active only in the region directly adjacent to the antenna
and which ensures that in each case only a closest tag can
be read out.
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Furthermore, it is in this case ensured that no or only
minimal electromagnetic energy is introduced onto the
product provided with the tag; this is desirable in many
applications including, for example, in the medical or
pharmaceutical field.
The antenna according to the invention is suitable in
particular for a frequency range of from 800 MHz to 1 GHz,
for example the range from 865 MHz to 870 MHz. It can
however also be used, for example, in the range of from
900 MHz to 930 MHz.
The exemplary embodiment has been described with reference
to a dielectric substrate. However, air can also serve as
the dielectric. In this case, the strip conductor 19 would
have to be secured and held by a suitable supporting
construction, set apart before a ground surface 25.
