Patent Application
20100230497
This invention relates to the local manipulation of electromagnetic fields, and more particularly, but not exclusively, to the use of radiation manipulating devices to allow RF (radio frequency) tags to be mounted on materials which would otherwise impede their use.
RF tags are widely used for the identification and tracking of items, particularly for articles in a shop or warehouse environment. One commonly experienced disadvantage with such tags is that if directly placed on a metal surface their read range is decreased to unacceptable levels and more typically the tag cannot be read or interrogated. This is because a propagating-wave RF tag uses an integral antenna to receive the incident radiation: the antenna's dimensions and geometry dictate the frequency at which it resonates, and hence the frequency of operation of the tag (typically 866 MHz, or 915 MHz, with 860-960 MHz being the approved range for a UHF (ultra-high frequency) range tag and 2.4-2.5 GHz or 5.8 GHz for a microwave-range tag). When the tag is placed near or in direct contact with a metallic surface, the tag's conductive antenna interacts with that surface, and hence its resonant properties are degraded or—more typically—negated. Therefore the tracking of metal articles such as cages or containers is very difficult to achieve with UHF RF tags and so other more expensive location systems have to be employed, such as GPS.
UHF RFID tags also experience similar problems when applied to any surfaces which interact with RF waves such as, certain types of glass and surfaces which possess significant water content, such as, for example, certain types of wood with a high water or sap content. Problems will also be encountered when tagging materials which contain/house water such as, for example, water bottles, drinks cans or human bodies etc.
This problem is particularly true of passive tags; that is tags which have no integrated power source and which rely on incident energy for operation. However, semi passive and active tags, which employ a power source such as an onboard battery also suffer detrimental effects on account of this problem.
One way around this problem is to place a foam spacer, or mounting between the RF tag and the surface, preventing interaction of the antenna and the surface. With currently-available systems the foam spacer needs to be at least 10-15 mm thick in order to physically distance the RF tag from the surface by a sufficient amount. Clearly, a spacer of this thickness is impractical for many applications and is prone to being accidentally knocked and damaged.
Other methods have involved providing unique patterned antennas which have been designed to impedance match a particular RF tag with a particular environment.
Accordingly, a first aspect of the invention provides apparatus comprising a resonant dielectric cavity defined between conducting surfaces, adapted to enhance an electromagnetic field at the edge of one of said conducting surfaces, wherein said dielectric cavity is non-planar.
Such apparatus provides a mounting or enabling component for an EM tag or device which is responsive to the enhanced field at a mounting site adjacent to the first conducting layer, at an open edge of the cavity.
The resonant cavity advantageously decouples or isolates the electronic device from surfaces or materials which would otherwise degrade the performance of the electronic device, such as metallic surfaces in the case of certain identification tags. This property is well documented in applicant's co-pending applications PCT/GB2006/002327 and GB0611983.8, to which reference is hereby directed. These applications describe radiation decoupling of a wide range of identification tags, particularly those that rely upon propagating wave interactions (as opposed to the inductive coupling exhibited by magnetic tags). Hence our preferred embodiment involves application to long-range system tags (e.g. UHF-range and microwave-range tags, also referred to as far-field devices)
The above referenced applications describe decouplers in which a planar dielectric layer is defined between two substantially parallel conducting layers. In certain described decouplers, the first layer does not overlie the second layer in at least one area of absence. This results in a structure which can be thought of as a sub-wavelength resonant cavity for standing waves being open at both ends of the cavity. Where the cavity length is substantially half the wavelength of incident radiation, a standing wave situation is produced, ie the mounting acts as a ½ wave decoupler as defined in the aforementioned PCT/GB2006/002327.
This structure results in the strength of the electromagnetic fields in the core being resonantly enhanced: constructive interference resulting in field strengths of 50 or 100 times greater than that of the incident radiation. Advantageously, enhancement factors of 200 or even 300 or more can be produced. In more specific applications typically involving very small devices, lower enhancement factors of 20,30 or 40 times may still result in a readable system which would not be possible without such enhancement. The field pattern is such that the electric field is strongest (has an anti-node) at the open ends of the cavity. Due to the cavity having a small thickness the field strength falls off very quickly with increasing distance away from the open end outside the cavity. This results in a region of near-zero electric field a short distance—typically 5 mm—beyond the open end in juxtaposition to the highly enhanced field region. An electronic device or EM tag placed in this area therefore will be exposed to a high field gradient and high electrical potential gradient, irrespective of the surface on which the tag and decoupler are mounted.
An EM tag placed in the region of high potential gradient will undergo differential capacitive coupling: the part of the tag exposed to a high potential from the cavity will itself be charged to a high potential as is the nature of capacitive coupling. The part of the tag exposed to a low potential will similarly be charged to a low potential. If the sections of the EM tag to either side of the chip are in regions of different electrical potential this creates a potential difference across the chip which in embodiments of the present invention is sufficient to drive it into operation. The magnitude of the potential difference will depend on the dimensions and materials of the decoupler and on the position and orientation of the EM tag.
Typical EPC Gen 2 RFID chips have a threshold voltage of 0.5V, below which they cannot be read. If the entirety of the voltage across the open end of the cavity were to appear across the chip then based on a 1 mm thick core and simple integration of the electric field across the open end, the electric field would need to have a magnitude of approximately 250V/m. If a typical incident wave amplitude at the device is 2.5V/m—consistent with a standard RFID reader system operating at a distance of approximately 5 m—then an enhancement factor of approximately 100 would be required. Embodiments in which the field enhancement is greater will afford greater read-range before the enhancement of the incident amplitude becomes insufficient to power the chip
In such a decoupler, conveniently the length of the second conductor layer is at least the same length as the first conductor layer. More preferably the second conductor layer is longer than the first conductor layer.
Preferably a tag is mounted or can be mounted on a mounting site substantially over the area of absence. The electromagnetic field may also be enhanced at certain edges of the dielectric core layer, therefore conveniently the mounting site may also be located on at least one of the edges of the dielectric core layer which exhibits increased electric field.
RF tags may be designed to operate at any frequencies, such as for example in the range of from 100 MHz up to 600 GHz. In a preferred embodiment the RF tag is a UHF (Ultra-High Frequency) tag, such as, for example, tags which have a chip and antenna and operate at 866 MHz, 915 MHz or 954 MHz, or a microwave-range tag that operates at 2.4-2.5 GHz or 5.8 GHz.
The area(s) of absence are described as being small, discrete crosses, or L-shapes but more conveniently are slits wherein the width of the slit is less than the intended wavelength of operation. A slit may be any rectilinear or curvilinear channel, groove, or void in the conductor layer material. The slit may optionally be filled with a non conducting material or further dielectric core layer material.
The described structure can therefore act as a radiation decoupling device. First and second conductor layers sandwich a dielectric core. Where the first conductor layer contains at least two islands i.e. conducting regions separated by an area of absence or a slit, preferably the one or more areas of absence is a sub-wavelength area of absence (i.e. less than λ in at least one dimension) or more preferably a sub wavelength width slit, which exposes the dielectric core to the atmosphere. Conveniently, where the area of absence occurs at the perimeter of the decoupler to form a single island or where at least one edge of the dielectric core forms the area of absence then said area of absence does not need to be sub wavelength in its width.
It is noted that the sum thickness of the dielectric core and first conductor layer of the decoupler structure may be less than a quarter-wavelength in its total thickness, and is therefore thinner and lighter compared to prior art systems. Selection of the dielectric layer can allow the decoupler to be flexible, enabling it to be applied to curved surfaces.
The length G of the first conductor layer of certain described decouplers is determined by λ≈2 nG, where n is the refractive index of the dielectric, and λ is the intended wavelength of operation of the decoupler. Clearly this is for the first harmonic (i.e. fundamental) frequency, but other resonant frequencies may be employed.
Conveniently it may be desirable to provide a decoupler with length G spacings that correspond to harmonic frequencies other than the fundamental resonant frequency. Therefore the length G may be represented by λ≈(2 nG)/N where N is an integer (N=1 indicating the fundamental). In most instances it will be desirable to use the fundamental frequency as it will typically provide the strongest response, however harmonic operation may offer advantages in terms of smaller footprint, lower profile and enhanced battery life even though it's not idealised in performance terms.
Considering the dielectric cavity of other described decouplers, the first layer and the second layer are electrically connected at one edge, locally forming a substantially “C” shaped section. This results in a structure which can be thought of as a sub-wavelength resonant cavity for standing waves being closed at one end of the cavity. Where the cavity length is substantially a quarter the wavelength of incident radiation, a standing wave situation is produced, ie the mounting acts as a ¼ wave decoupler as defined in the aforementioned GB0611983.8
In such a decoupler, the two conductor layers can be considered to form a cavity structure which comprises a conducting base portion connected to a first conducting side wall, to form a tuned conductor layer, and a second conducting side wall, the first conducting side wall and second conducting side wall being spaced apart and substantially parallel.
The conducting base portion forces the electric field to be a minimum (or a node) at the base portion and therefore at the opposite end of the cavity structure to the conducting base portion the electric field is at a maximum (antinode). An electronic device or EM tag placed in this area therefore will be located in an area of strong field, irrespective of the surface on which the tag and decoupler are mounted.
Conveniently, the first conducting side wall has a continuous length of approximately λd/4 measured from the conducting base portion, where λd is the wavelength, in the dielectric material, of EM radiation at the frequency of operation v.
Both the ½ and ¼ wave decouplers described above comprise a tuning conductor layer and a further conductor layer; preferably this further conductor layer is at least the same length as the tuning conductor layer, more preferably longer than the tuning conductor layer.
The two conductor layers are separated by a dielectric layer. They may be electrically connected at one end to create a closed cavity ¼ wave decoupler as hereinbefore defined, or contain conducting vias between the two conductor layers in regions of low electric field strength. However, there should be substantially no electrical connections between the two conductor layers in regions of high electric field strength or at the perimeter of the decoupler for open ended ½ wave versions, or at more than one end or perimeter for ¼ wave (closed end) versions.
It is noted that for a metallic body which is to be tracked by RFID, that at least one of the conductor layers of the decoupler can be part of said metallic body. RF tags generally consist of a chip electrically connected to an integral antenna of a length that is generally comparable with (e.g. ⅓rd of) their operational wavelength. The present inventors have found that tags having much smaller and untuned antennas (i.e. which would not normally be expected to operate efficiently at UHF wavelengths) can be used in conjunction with decoupling components as described herein. Usually tags with such ‘stunted’ antennas (sometimes referred to as low-Q antennas, as will be appreciated by one skilled in the art) possess only a few centimetres or even millimetres read range in open space. However, it has surprisingly been found that using such a tag with a low-Q antenna mounted on a decoupler of the present invention may be operable and exhibit useful read ranges approaching (or even exceeding) that of an optimised commercially-available EM tag operating in free space without a decoupler. Low-antennas may be cheaper to manufacture, and may occupy less surface area (i.e. the antenna length of such a tag may be shorter than is usually possible) than a conventional tuned antenna. Therefore the EM tag may be a low Q-tag, i.e. an EM tag having a small, untuned antenna. Conveniently the device will incorporate a low Q antenna, such that upon deactivation of the decoupler the read range of the low Q tag is caused to be that of a few centimetres or even millimetres.
In order to allow progressively smaller items to be tagged or monitored, it is desirable for the size of a decoupler to be reduced. Although the decouplers described in the above referenced applications can be made ‘stunted’ or low-Q tags, with the largest dimension only a half and a quarter of a wavelength respectively (at the intended frequency of operation) there is a demand to reduce this dimension further still.
In embodiments of the present invention, a standing wave is set up in the cavity as described above, but the cavity is not constrained to be monoplanar, that is, to extend only in a single plane or layer (which may be straight or curved), defined between substantially parallel upper and lower surfaces. Instead the cavity can extend beyond such surfaces, and in this way the cavity can be bent or folded at an angle. This arrangement allows a cavity having a given length or dimension, corresponding to an intended frequency of operation to occupy a smaller footprint, at the expense of increased thickness. Since the overall thickness remains small, and significantly less than arrangements employing ‘spacers’, such a device may have advantageous dimensions when absolute thickness is not critical.
Preferably the cavity comprises two or more layers, with each layer preferably being defined at least partially between a pair conducting walls, conveniently, each layer being offset. Preferably the layers are substantially parallel, and this arrangement advantageously allows the component to be built up in a laminated structure, with adjacent layers of dielectric being separated by a single conducting wall or surface.
Alternatively, the layers are not parallel, but are arranged at angles to one another. This allows for a corrugated or rippled effect.
In certain embodiments, the cavity defines a unique path length. In this way the cavity can be considered to be formed of a single plane, but bent or folded to change its physical configuration but not its topology. The cavity of such an embodiment therefore does not include any branches or junctions, and a single unique length for the cavity can be defined, which length is associated with the frequency of radiation at which enhancement occurs.
Alternatively, the cavity may be branched, and define a number of lengths, each corresponding to a frequency of enhancement.
In this specification, when referring to path lengths, the structure of a decoupler is assumed to have uniform width, unless otherwise stated. The path length is most easily understood by considering the cross section of a device, and is explained in greater detail below, with reference to the accompanying drawings.
A further aspect of the invention provides a mounting component for an electronic device comprising a first dielectric layer arranged between first and second conductor layers, and a second dielectric layer arranged between said second conductor layer and a third conductor layer, said first and third conductor layers being electrically connected at one end, thereby defining a first dielectric connecting region, joining said first and second dielectric layers, wherein said mounting component is adapted to enhance an electromagnetic field at a mounting site at an open edge of said third conductor layer.
The invention extends to methods apparatus and/or use substantially as herein described with reference to the accompanying drawings.
Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.
Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:
a & 1b illustrate two layer components
a & 5b illustrate three layer components
a illustrates a cross section of a quarter wave component with the dielectric cavity formed on two layers. The layers are defined between conducting sheets 102, 104, 106, with the bottom dielectric layer 110 between sheets 102 and 104, and the upper dielectric layer 112 between sheets 104 and 106. At the left hand end of the decoupler as viewed, conducting sheets 102 and 106 extend beyond sheet 104, and are electrically connected by an end wall 116. This arrangement results in the two dielectric layers being joined at this end.
The structure is uniform in the width direction into the plane of the paper as viewed, with the dielectric and conducting sheets exposed at the sides of the structure.
The path length 120, is an approximation of the effective length of the cavity for the purposes of the wavelength of radiation which forms a standing wave in the cavity. In
The component of
An equivalent half wave version is shown in
a shows an extension of the arrangement of
Thus for a given frequency of operation, the arrangements of
A specific implementation of the general arrangement of
Considering an approximate path length arrangement as indicated in
From the plot of
In the above described embodiments, the cavity, although folded back on itself, has a unique path length.
This structure therefore acts as a dual frequency, or broadband decoupler with the frequencies of enhancement being determined by the various effective lengths defined by the dielectric cavity.
A more complex arrangement is shown in
Sheet 1016 extends to contact end portion 1022, but not portion 1020, thereby joining the middle and upper dielectric layers only at one end. Sheet 1018 has an aperture 1032 part way along its length, thereby defining an open end, and thus a path length end.
It can be seen that three path lengths exist in this structure. Path 1040 defines a ‘C’ shape and extends part way along the upper and lower dielectric layers. Path 1042 extends at least partly along all three layers and defines an ‘S’ shape, and path 1044 extends along the upper dielectric layer only.
A tag 1050 placed over aperture 1032 will therefore experience enhancement of incident electric fields at multiple frequencies determined by the geometry of the structure described above.
In
A 3-layer dielectric cavity structure in which the cavity is folded one way then back on itself the other way, as shown in
A cross-section through the 3-layer spiral structure of
An alternative approach is employed in
Turning to
It can also be seen in
To illustrate that further number of dielectric layers are possible,
The resonance clearly visible from the plot of the electric field magnitude results in the voltage across the chip showing a resonant response as expected, as shown in
Equally the spiral structure of
Both
It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. Although the embodiment of
Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0625342.1 | Dec 2006 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2007/004877 | 12/19/2007 | WO | 00 | 5/20/2010 |
