WO2008078089A1

WO2008078089A1 - Radiation enhancement and decoupling

Info

Publication number: WO2008078089A1
Application number: PCT/GB2007/004932
Authority: WO
Inventors: Christopher Robert Lawrence; James Robert Brown
Original assignee: Omni-Id Limited
Priority date: 2006-12-22
Filing date: 2007-12-21
Publication date: 2008-07-03
Also published as: GB0625718D0

Abstract

A device adapted to enhance an incident electric filed comprises a resonant dielectric cavity formed between two conducting layers (102, 106a, 106b). The incident field is enhanced at the edge of the upper layer (106a, 106b), and this edge is suitably profiled to achieve a desired variation in enhancement along the edge. The edge is typically tapered to a point, but may include a tab or notch or other profiles.

Description

RADIATION ENHANCEMENT AND DECOUPLING

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 surfaces which would otherwise degrade their performance.

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 866MHz,or 915MHz for a UHF (ultra-high frequency) range tag and 2.4-2.5 GHz or 5.8GHz 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 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. Location of the power source or battery for active tags can also prove difficult on account of typical batteries being conductors and interacting with RF waves. 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 typically needs to be at least 10-15mm 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.

In a first aspect the present invention provides apparatus comprising a resonant dielectric cavity defined between first and second conducting layers, adapted to enhance an electric field in a region adjacent to said first layer, wherein the edge of said first layer adjacent to said region is profiled to produce a desired variation in enhancement along said edge.

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 dielectric cavity defined between the first and second conducting layers advantageously decouples or isolates the electronic device from the power source. 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 decoupling of a wide range of RF 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, trapping incident radiation. 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 5mm - 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 1mm 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 5m - 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 embodiments of the present invention, the level or distribution of electric field enhancement, or electric field strength, can be controlled along the edge of the first layer, and can be tailored to a particular electronic device at a particular position or in a particular orientation at the mounting site. Preferably the edge is profiled to produce a peaked distribution, and this can result in greater field enhancement at the peak than would be possible with a constant distribution along the edge. The field intensification can be considered to be concentrated at the peak, and preferably the peak is located at a point intermediate the length of the edge, typically at the centre of the edge.

Where the electronic device is an RFID tag, greater field intensification can result in longer read ranges.

In one preferred embodiment, the profile of the edge is tapered. This can be a straight taper, or a curved taper, resulting in a point or discrete region where maximum field intensification, and hence field strength occurs.

In an alternative embodiment, the profile is stepped, providing either a notched profile, or a profile having a 'tab' or inverted notch.

An identification device comprising an RFID tag mounted on a component as described above may be provided as a further aspect of the invention.

In one embodiment the first layer does not overlie the second layer in at least one area of absence, with the profiled edge bordering this 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 open-ended cavity length is substantially half the wavelength of incident radiation, a standing wave situation is produced, ie the mounting acts as a ^ΛA wave decoupler as defined in the aforementioned PCT/GB2006/002327. In such an embodiment, 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 mounting site is located substantially over the area of absence. The electromagnetic field may also be enhanced at certain edges of the dielectric core layer, and therefore the mounting site may conveniently 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 100MHz up to 600GHz. 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 866MHz, 915MHz or 954MHz, or a microwave-range tag that operates at 2.4-2.5 GHz or 5.8GHz.

The area of absence will typically be defined by the profile of the edge or edges of the adjacent conducting layer, which may be rectilinear or curvilinear for example. The area of absence may optionally be filled with a non conducting material or further dielectric core layer material.

The invention can therefore provide for a multi-layer structure that acts 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. 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.

The sum thickness of the dielectric core and first conductor layer of the decoupler structure may be considerably 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 non- planar or curved surfaces. Conveniently, the decoupler may not be planar and may take the form of a non-planar or curved geometry.

The length G of the first conductor layer may be determined by λ = 2nG, 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. The physical length G will vary along the width of the decoupler according to the profile of the edge, but as explained below, an effective length can be used as an approximation.

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 = (2nG)/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 is not idealised in performance terms.

In an alternative embodiment the first layer and the second layer are electrically connected at one edge, 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 1/4 wave decoupler as defined in the aforementioned GB0611983.8

In such an embodiment, 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) adjacent to 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 λ_d/4 measured from the conducting base portion, where λ_d is the wavelength, in the dielectric material, of EM radiation at the frequency of operation v. As with the half wave embodiment, an effective length can be used as an approximation, where the physical length varies along the width of the decoupler.

Both the Vz and % wave embodiments 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. According to the present invention the edge of the tuning conductor layer is profiled to achieve a desired enhancement distribution.

The two conductor layers are separated by a dielectric layer, they may be electrically connected at one end to create a closed cavity VA 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 Vz wave embodiment, or at more than one end or perimeter for VA wave (closed end) embodiment.

It may be desirable that for a metallic body which is to be tracked by RFID, that at least one of the conductor layers is part of said metallic body. Preferably, it will not be the tuned conductor layer.

RF tags generally consist of a chip electrically connected to an integral antenna of a length that is generally comparable with (e.g. 1/3^rd of) their operational wavelength. The present inventors have previously 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 a decoupler as defined in co- pending application herein defined . 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-Q 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.

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:

Figure 1 is a perspective view of a component according to the present invention.

Figures 2a - 2d show possible edge profiles.

Figure 3 is a perspective view of a quarter wave embodiment of the present invention.

Figures 4 illustrates electric field vectors.

Figure δshows modelled electric field values for different taper angles.

Figure 6 illustrates reference features on the abscissa of Figure 5.

Figure 7 illustrates a parameter for a device having a tapered upper layer. Referring to Figure 1 , a mounting component has an aluminium lower conducting layer 102, a PETG dielectric layer 104, and an aluminium upper layer consisting of two portions or islands 106a and 106b. The upper islands do not meet at the centre, and result in an area of absence 108 of the upper conducting layer, where the dielectric is exposed. The edge 110a and 110b of each island adjacent to the area of absence is tapered, having straight sides meeting at a point. The separation of the points is less than 0.5mm.

The profile of edges 110a and 110b results in enhancement of an electric field which is greater at the points than it is at either edge. It is hypothesised, although the applicant is not limited to such a hypothesis, that high electric field strengths along the edges 110a and 110b results in a build up of opposite charges thereby creating a potential difference across the area of absence, or mounting site, which can be exploited by an electronic device located there. By tapering these edges, the charge is focussed or concentrated at the points, thereby increasing the potential.

Figure 2 shows various different possible geometries for the upper conducting plane. Figure 2a shows the tapered arrangement of Figure 1 , with a symmetrical taper. The position of an RFID tag is shown at 202 in dashed line. Such a tag would typically be electrically isolated from the upper conducting layer by a thin insulating mount.

In Figure 2b the edges again have a taper, but the taper is offset or skewed away from the centre line 204. Such an arrangement could be used for asymmetrical tags for example, where an asymmetrical field enhancement is desired.

Figure 2c shows a geometry having tapered edges formed of curves. The curves may desirably be parabolic. Figure 2d shows a stepped profile, with each edge having a 'tab' 206.

It should be noted that the edge may be profiled only locally. In Figures 2c and 2d, only the central third or tenth of the edge may be affected for example. The inverse of these geometries, having a notch in each edge may also be desirable in certain applications. Furthermore, combinations of different geometries are possible. For example the edge could include two locally tapered portions. These could be arranged symmetrically either side of a centre line, as per Figure 2b for example.

Figure 3 shows a 'single island' component or decoupler, having a similar tapered edge profile to the decoupler of Figure 1. Figure 3 is arranged as a quarter wave device, with upper conducting layer 306 and lower conducting layer 302 electrically connected by end portion 310. Dielectric layer 304 is sandwiched between layers 302 and 306, and together with lower layer 302 extends beyond the profiled edge 308 of the upper layer, to define an area of absence or mounting site 312. It will be understood that the profiling of edges referred to herein is equally applicable to 'single island' embodiments having a single upper conducting layer.

As with the two island device of Figure 1 , electric field enhancement is at a maximum at the point of the upper layer, and hence an RFID tag mounted here will experience a higher electric field. This is further explained by considering Figure 4.

By definition the direction of the electric field vector is that in which a free positive charge would move. As the free charges in the metal are electrons and carry a negative charge they move in the opposite direction to that of the electric field. With the electric filed running lengthwise along the dielectric cavity, as shown in Figure 4b, because of the taper angle θ, there is a component of the electric field parallel to the edge: E sin θ. When the field from the incident wave is as shown in Figure 4b, the electrons are driven towards the point creating a region of negative charge density, half a cycle later the direction of the incident electric field has reversed and the electrons are now driven away from this point creating a region of net positive charge density.

The variation in the magnitude of the electric field in the through thickness direction along a line which passes through the vertex of the point (dashed line 314 of Figure 3) was modelled and is shown in Figure 5. The values given are at the resonant frequency of the cavity and at a phase corresponding to peak field for a range of taper angles between 0° (no taper) and 60°. Z = 150.075 mm corresponds to the surface of the lower metal layer, z = 152.075 mm corresponds to the lower surface of the upper metal layer, as shown in Figure 6. It can be seen that for all angles the field strength increases from the lower conducting layer towards the upper conducting layer and then decreases above the upper conducting layer. As the taper angle increases the peak field strength also increases: for 60° the peak field is around 3000 V/m compared to only 500 V/m for the case where there is no taper. As the taper angle increases the component of the electric field from the incident wave that lies along the edge increases thus driving more charges towards the vertex. This clearly demonstrates that the tapering of the upper conducting layer increases the peak electric field strength as expected.

The increased electric field strength at the point would be expected to give a tag mounted here a correspondingly larger read range, than a tag mounted on an equivalent device, having a straight edge.

Experimental results of the read ranges of tags do show this trend, the read range becoming greater with increasing taper angle. The experimental results however exhibit a maximum, and then decreases again as the taper becomes sharply pointed. It has been found experimentally that for embodiments having a straight edged taper, angles of approximately 20-40 degrees are advantageous, and that a maximum occurs at approximately 30 degrees. This behaviour may result from the properties of the chip used, which will typically have a maximum operating voltage, and the precise positioning and interaction with the enhanced field.

As noted above, the optimum frequency of operation, or resonant frequency of a decoupler is directly dependent on the length of the decoupler. In the case an upper layer having perpendicular edges, determination of the length is straightforward. With a profiled edge, determination of the length becomes more difficult. The effective length can be determined quite simply by experiment, by progressively reducing the length of a decoupler having a particular edge profile and determining the read range of an RFID tag mounted over the area of absence, using a reader of a particular frequency. The results will tend to produce a distribution having a maximum read range value, and the length at that vale can be taken as the effective length. For certain embodiments having a straight edged taper, it has been found that the effective length is the length from the base of the taper to the opposite edge of the device. This length G is illustrated in Figure 7. In other embodiments, the effective length may extend to a point mid way up the taper. 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.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

1. Apparatus comprising a resonant dielectric cavity defined between first and second conducting layers, adapted to enhance an electric field in a region adjacent to said first layer, wherein the edge of said first layer adjacent to said region is profiled to produce a desired variation in enhancement along said edge.

2. Apparatus according to Claim 1 , wherein the edge is profiled to produce a peaked distribution in enhancement.

3. Apparatus according to Claim 2, wherein the peak is located at a point intermediate the length of the edge.

4. Apparatus according to Claim 2 or Claim 3, wherein the peak is located at the centre of the edge.

5. Apparatus according to any preceding claim, wherein the profile of the edge is tapered.

6. Apparatus according to any preceding claim, wherein the profile of the edge is stepped.

7. Apparatus according to any preceding claim, wherein the edge is formed of straight lines.

8. Apparatus according to any preceding claim, wherein the edge of formed of curved lines.

9. Apparatus according to any preceding claim, wherein the first layer does not overlie the second layer in at least one area of absence.

10. Apparatus according to any preceding claim wherein the second conductor layer is at least the same length as the first conductor layer.

11. Apparatus according to any preceding claim wherein the effective spacing G between at least one edge of the first conductor layer and the area of absence is determined by G ~λ/2n where n is the refractive index of the dielectric, and λ is the intended wavelength of operation of the decoupler.

12. Apparatus according to Claim 1 , wherein said first layer and said second layer are electrically connected at one edge.

13. Apparatus according to Claim 12, wherein the effective length of the first conducting side wall is approximately λ/4 measured from the conducting base portion, where λ is the wavelength, in the dielectric material, of EM radiation at the frequency of operation v.

14. Apparatus according to any Claim 13 or Claim 14, wherein the second conducting side wall has a continuous length measured from the conducting base portion which is at least as long as the length of the first conducting side wall.

15. A component or apparatus according to any preceding claim, comprising an electronic device located at least partially in said region of field enhancement.

16. Apparatus according to any preceding claim, wherein said device is electrically isolated from said conductor layers.

17. Apparatus according to Claim 15 or 16, wherein said device is powered by differential capacitive coupling.

18. Apparatus according to any one of Claims 15 to 17, wherein said device is an RF tag.

19. Apparatus according to Claim 18, wherein said tag is a low Q RFID tag

20. A component or decoupler according to any one of the preceding claims wherein the total thickness of the component is less than λ/4, or λ/10, or λ/300 or λ/1000, where λ is the intended wavelength of operation of the decoupler.

21. A component or decoupler according to any preceding claim wherein the total thickness of the component or decoupler is 1 mm or less, or 500μm or less, or 100μm or less.

22. Apparatus according to any preceding claim, wherein said electric field is enhanced by a factor greater than or equal to 50, 100, or 200.