WO2013096995A1 - Improvements in rfid tags - Google Patents

Abstract

The present invention provides a radio frequency identification (RFID) tag comprising or consisting of: a first antenna/resonator and a second antenna/resonator, the first and second antennae/resonators being applied to a substrate in a partially or completely overlapping manner. In one embodiment, the first and second antennae/resonators are configured or adapted to encode data, the first antenna/resonator being rotationally polarized with respect to the second antenna/resonator, wherein in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator. It has been found that data from two rotationally polarised antennae/resonators is resolvable when the two antennae resonators at least partially overlap. This leads to an ability to encode an increased amount of data for a given area. This increase in spatial efficiency may be useful in applications where it is necessary to limit the area taken up by the tag.

Description

IMPROVEMENTS IN RFID TAGS

Cross Reference to Related Application

The present application claims priority from Australian Provisional Patent Application 2011905462, filed 29 December 2011, the contents of which is herein incorporated by reference.

Field of the Invention

The present invention is directed to the field of radio frequency identification (RFID) tags, and also devices and methods for reading such tags. RFID tags of the present invention are useful for identifying or marking diverse objects such as bank notes, supermarket items, pharmaceuticals and courier packages.

Background to the Invention

RFID is a contactless data capturing technique, which uses radio frequency (RF) waves for the identification of objects. The contact-less identification system relies on RF waves for data transmission between the data carrying device called the RFID transponder (more commonly referred to as the "tag"), and the interrogator (more commonly referred to as the "RFID reader"). RFID technology has been used for many years in a diverse range of settings. Asset management is an important application given the ability to tag and track company-owned articles. Organizations often use RFID tags combined with a mobile asset management solution to record and monitor the location of their assets, their current status, and whether they have been maintained. The technology may also be implemented in the context of inventory management, product tracking, mobile payment solutions, transportation and logistics, animal management, passport verification, toll road payments, train tickets, staff identification passes, pathology sample tracking, and theft detections systems.

Typically, a passive RFID tag includes an antenna and integrated circuit (IC). The IC performs all data processing and is powered by extracting energy from the interrogation signal transmitted by the RFID reader. An active RFID tag further includes a source of electrical energy (such as a battery) which may be used to power the transmission of RF signals from the tag to the reader.

One of the greatest potentials of RFID technology is to replace optical barcode technology. RF waves do not rely on line-of-sight communication and can achieve greater distances than optical waves. The practical implementation of RFID in place of optical barcodes has not yet been achieved due to the 100-fold price difference in an RFID tag compared to that of an optical barcode. A number of barriers exist in the art preventing the more cost effective application of RFID technology.

Application Specific Integrated Circuit (ASIC) design, testing and assembly along with the tag antenna result in a costly manufacturing process. Since silicon chips are fabricated on a wafer-by-wafer basis there is a fixed cost per wafer (around USD$1000). As the cost of the wafer is independent of the IC design, the cost of the RFID chip can be estimated based on the required silicon area for the RFID chip. Significant achievements have been made in reducing the size of the transistors allowing more transistors per wafer area. Decreasing the amount of transistors needed results in an even smaller silicon area, hence a lower RFID chip price. Although this reduces the price of the silicon chip, the miniature size imposes limitations and further handling costs. The cost of dividing the wafer, handling the die and placing the chip onto a label remains significant, even if the cost of the RFID chip was reduced to virtually zero. Hence, with highly-optimized low transistor count ASICs, implemented assembly processes and extremely large quantities of RFID chips sold per annum, a minimum cost of 5 cents is the reality for chipped RFID tags. One way to reduce the overall cost of the tag below 1 cent is to develop chipless RFID tags where the main cost of the tag is obviated. Chipless RFID tags have been developed in recent years. However, most are still early stage prototypes with few considered to be commercially viable or available. To date, the only commercially available chipless RFID tag is the SAW tag (developed by RF SAW^®). However, due to their piezoelectric nature this tag cannot be applied to banknotes, postage stamps or other paper/plastic based items. A further problem for chipless RFID tags is the limited amount of data that can be encoded. The prior art provides two main chipless data encoding techniques: time domain reflectometry (TDR) based and frequency signature based encoding chipless RFID.

TDR-based chipless RFID tags are interrogated by sending a signal from the reader in the form of a pulse and listening to the echoes of the pulse sent by the tag. A train of pulses is thereby created, which can be used to encode data.

Spectral signature-based chipless tags encode data into the spectrum using resonant structures. Each data bit is usually associated with the presence or absence of a resonant peak at a predetermined frequency in the spectrum.

The challenge for both chipless RFID data encoding techniques is data capacity. While spectral signature tags exhibit more data capacity then TDR tags the prior art has failed to provide the ability to encode data in the range of 64, 96 or 128 data bits.

A further problem is the physical size of current RFID tags. Where the product to be marked or identified is small, or the product has a characteristic which causes a difficulty in applying a standard size RFID tag, a smaller tag is desirable. While a tag may be made physical smaller, the downside is that less data may be encodable.

The reading of data from RFID tags can be adversely affected by environmental parameters, such as extremes in temperature (and especially high temperatures) and also magnetic fields. This can be a significant shortcoming for certain applications of RFID technology. It is an aspect of the present invention to provide a passive, chipless RFID tag that is low cost, and or of reduced size, and/or applicable to a large range of substrates, and/or having improve data capacity, and/or the ability to reliably function in the presence of magnetic fields, and/or the ability to reliably function within a broad range of temperatures.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Summary of the Invention

In a first aspect, the present invention provides a radio frequency identification (RFID) tag comprising or consisting of: a first antenna/resonator and a second antenna/resonator, the first and second antennae/resonators being applied to a substrate in a partially or completely overlapping manner. The first and second antennae/resonators may be configured or adapted to encode data, the first a ntenna/resonator being rotationally polarized with respect to the second antenna/resonator, wherein in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator. The first antenna/resonator may be applied to a first surface of the substrate and the second antenna/resonator applied to a second surface of the substrate. In some embodiments the first antenna/resonator and second antenna/resonator are on opposing sides of the substrate. Preferably the first antenna/resonator is rotationally polarized approximately 90 degrees with respect to the second antenna resonator.

The substrate may be a non-conductive material, and/or may be a dielectric material. It may be comprised substantially or completely of paper, cardboard, rubber, a synthetic polymer, a plastic, a tape or glass.

In certain embodiments, the substrate is the object to be marked or identified, or is part of the object to be marked or identified. In some embodiments the substrate is substantially planar. In some embodiments, the first and/or second antenna/resonator is/are a linearly polarised antenna, such as a dipole antenna, a patch antenna, a folded dipole antenna, a squiggle dipole antenna, a monopole antenna, or a wire-based antenna. The one or more linearly polarised antenna may have been cut or the length otherwise altered in order to encode data.

In certain embodiments of the tag the first and/or second antenna/resonator is/are a circularly polarised antenna.

The first and/or second antenna/resonator may be an L-shaped resonator, a spiral resonator, a split ring resonator, a hair-pin resonator, and a materials-based resonator. In some embodiments, the first and/or second antenna/resonator is/are comprised substantially or completely of a material having a moderate to high conductivity, such as a metallic material. Preferably, the first and/or second antenna/resonator is/are comprised substantially of a conductive ink. The conductive ink may applied to the substrate by a printing technique, the printing technique optionally selected form the group consisting of a letterpress technique, a digital technique (including electrophotography, inkjet, xerography, laser), a gravure printing technique, a screen printing technique, a vacuum deposition technique, a 3D technique, a lithography technique, a thermography technique, a reprographic technique, a flexography technique, an electrostatic technique.

In other embodiments, the first and/or second antenna/resonator is/are comprised substantially of a conductive epoxy or conductive nanoparticles.

The RFID tag may comprise or consist of a third, fourth, fifth, sixth, seventh, eighth, ninth, tenth or more separate antennae/resonators.

In a second aspect there is provided an RFID reader capable of reading the data encoded by an RFID tag as described herein. The reader may comprise at least one transmitting antenna and at least one receiving antenna, and/or may be adapted or configured to interrogate by continuous wave sweeping frequency or pulse interrogation. In other embodiments the reader may be adapted or configured to read the spectral signature of one or more tags by reference to the amplitude of the received signal and/or phase of the received signal. A third aspect of the present invention provides an RFID system comprising an RFI D tag as described herein, and an RFID reader as described herein.

I n a fourth aspect there is provided a kit of parts comprising an RFID tag as described herein, and an RFID reader as described herein. A fifth aspect of the present invention provides a method for fabricating an RFI D tag, the method comprising the steps of: providing a substrate, applying a first antenna/resonator to a first surface of the substrate, applying a second antenna/resonator to a second surface of the substrate, securing the position of the first antenna/resonator with respect to the position of the second antenna/resonator, wherein the second antenna/resonator is rotated relative to the second antenna resonator to provide sufficient rotational polarization such that in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator. Where the first and/or second antenna/resonator is/are comprised substantially of a conductive ink, and wherein the step(s) of applying the first and/or second antenna/resonator is a printing technique, the printing technique is optionally selected form the group consisting of a letterpress technique, a digital technique (including electrophotography, inkjet, xerography, laser), a gravure printing technique, a screen printing technique, a vacuum deposition technique, a 3D technique, a lithography technique, a thermography technique, a reprographic technique, a flexography technique, an electrostatic technique. Preferably the printing technique is an inkjet technique or a 3D technique.

A sixth aspect provides a method for marking or identifying an item, the method comprising the steps of: providing an item and applying a first antenna/resonator to a first surface of the item, applying a second antenna/resonator to a second surface of the item, securing the position of the first antenna/resonator with respect to the position of the second antenna/resonator, wherein the second antenna/resonator is rotated relative to the second antenna resonator to provide sufficient rotational polarization such that in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator. Brief Description of the Figures

Fig. 1 is a photograph of two multi resonant structures applied to a single film, the two structures having a rotational polarisation of 90 degrees. Left Panel, upper surface; Right Panel, lower surface.

Fig. 2 is a schematic illustration of a chipless RFID tag with vertically polarized dipole antennae on the upper layer (blue) and horizontally polarized dipole antennae on the lower layer (red). This is in effect the result of overlaying the structures of the left and right panels as shown in Fig. 1.

Fig. 3 is a schematic representation of dipoles alternately encoding a "0" (left) and "1" (right). Note the "cut" in the dipole on the left.

Fig. 4 is a graph showing the change in resonant frequency of the dipole element when cut (or "shortened"). Fig. 5 is a schematic representation of two multiresonant structures which can be overlayed to provide for 12-bit chipless tag data encoding.

Fig. 6 is a schematic representation of (plan view) of the layers of a "stacked" tag having 3 dielectrics and 4 metal layers.

Fig. 7 is a schematic representation of a cross-sectional view of the tag shown in Fig.6. Fig. 8 is a schematic representation of a bistatic radar approach for tag interrogation by a reader.

Fig. 9 is a block diagram for an RFID reader device potentially useful in the context of the present invention.

Fig. 10 is a schematic representation of a low gain monopole antenna potentially useful in the context of the present invention. Fig. 11 is a schematic representation of a high gain log periodic dipole antenna array potentially useful in the context of the present invention.

Fig. 12 is a schematic representation of a high gain horn antenna potentially useful in the context of the present invention. Fig. 13 is a schematic representation of a monostatic radar approach for tag interrogation by reader.

Fig. 14 is a block diagram of an RFID reader device potentially useful in the context of the present invention.

Fig. 15 shows schematic representations of various antenna and resonator types potentially useful in the context of the present invention.

Fig. 16 is a schematic representation of a bistatic radar setup for testing a chipless tag of the present invention placed on a plastic jig.

Fig. 17 is a schematic representation of a monostatic radar setup for testing a chipless tag of the present invention placed on a plastic jig. Fig. 18 is a schematic representation of a bistatic reader antenna setup for chipless tag interrogation using a vector network analyser as the reader.

Fig. 19 is the graphical output of a vector network analyser as applied to a chipless RFID tag copper printed response (ID = 00000) with Tx power of -10 dBm.

Fig. 20 is the graphical output of a vector network analyser as applied to a chipless RFID tag copper printed response (ID = 00000) with Tx power of -40 dBm.

Fig. 21 is the graphical output of a vector network analyser as applied to a chipless RFID tag copper response in magnitude (upper panel) and phase (lower panel).

Fig. 22 is a schematic representation of a single strip 1 bit tag.

Fig. 23 is a schematic representation of a dual strip 1 bit tag. Fig. 24 is the graphical output of a vector network analyser as applied to the dual strip tag of Fig. 23 (heavy line) and the single strip tag of Fig. 22 (light line) 1 bit tag. Fig. 25 is the graphical output of a vector network analyser showing dipole fundamental mode resonances prior to being cut (heavy line) and after being cut in half (light line).

Fig. 26 is the graphical output of a vector network analyser showing the spectrum magnitude response of 3 bit tag with ID 000 (upper panel) and no tag present between the reader antennae (lower panel).

Fig. 27 is the graphical output of a vector network analyser showing a tag having ID 010 (upper panel), and no tag present between the reader antennae (lower panel) demonstrating the feasibility of encoding different IDs for a tag.

Fig.28 is a schematic diagram of a tag having 3 dipoles in the vertical polarization and 1 dipole in the horizontal polarization.

Fig. 29 is the graphical output of a vector network analyser showing response of a 6 bit tag with polarization diversity.

Fig. 30 is a block diagram of a custom built RFID reader potentially useful in the context of the present invention.

Fig. 31 is a schematic representation of an experimental setup using custom RFID reader.

Fig. 32 is the graphical output of a vector network analyser showing the 3 resonances (Ml, M2 and M3) of the tag.

Fig. 33 is a graph showing a received digitized spectral signature of the chipless tag using the custom RFID reader.

Fig. 34 is the graphical output of a vector network analyser showing the spectrum magnitude response of 3 bit tag with ID 000, pointing out the rise in the sll due to tag resonances.

Detailed Description of the Invention

After considering this description it will be apparent to one skilled in the art how the invention is implemented in various alternative embodiments and alternative applications. However, although various embodiments of the present invention will be described herein, it is understood that these embodiments are presented by way of example only, and not limitation. As such, this detailed description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention. Furthermore, statements of advantages or other aspects apply to specific exemplary embodiments, and not necessarily to all embodiments covered by the claims.

Unless the contrary intention is expressed, the features presented as preferred or alternative forms of the invention can be present in any of the inventions disclosed as alone or in any combination with each other. Throughout the description and the claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises" is not intended to exclude other additives, components, integers or steps.

In a first aspect the present invention provides a radio frequency identification (RFID) tag comprising or consisting of: a first antenna/resonator and a second antenna/resonator, the first and second antennae/resonators being applied to a substrate in a partially or completely overlapping manner. In one embodiment, the first and second antennae/resonators are configured or adapted to encode data, the first antenna/resonator being rotationally polarized with respect to the second antenna/resonator, wherein in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator.

The present invention is predicated at least in part on the finding that it is possible to resolve data from two rotationally polarised antennae/resonators when the two antennae resonators at least partially overlap. This leads to an ability to encode an increased amount of data for a given area by spatially overlapping two separate antennae/resonators, and rotationally polarizing the first with respect to the second. This increase in spatial efficiency provides advantages in many RFID applications but is particularly important in the ma rking or identification of small items such bank notes, or in circumstances where it is necessary to limit the area taken up by the tag for aesthetics or reasons relating to the proper functioning of the item. In the context of the present invention, the term "overlapping" is intended to mean that at least one part of the first antenna/resonator overlies at least one part of the second antenna resonator. Where there are more than two antennae/resonators, it will be understood that the term does not require at least one part of each antenna/resonator to overlie at least one part of all other antenna/resonators: it is sufficient that there is overlap between two antennae/resonators. From the embodiments disclosed herein it will be clear that the term "overlapping" is not intended to mean that the first and antennae/resonators must be in physical or electrical contact.

As used herein, the term "resonator/antenna" encompasses a range of devices that are responsive to suitable RF excitation. In particular, the scope of the invention encompasses passive circuits having multiple frequency resonances, antennae having similar resonance properties, and combinations of resonators and antennae, such as a multi-resonant structure coupled to a suitable wideband antenna. The term "radio frequency" or "RF", as used herein, encompasses frequencies commonly utilised in the propagation of electromagnetic radiation for communications and other purposes, and includes at least those frequencies in the range of 3 kHz to 300 GHz. Of particular interest, in relation to resonant structures formed in accordance with embodiments of the present invention, are those frequencies in the microwave and millimetre wave ranges, for example RF frequencies exceeding 1 GHz. While the RFID tags of the present invention may be of the active type and/or include a chip, preferred embodiments are of the passive chipless type. These passive forms are more amenable to very low cost production.

It is contemplated that an RFID tag of the present invention may be a discrete entity that is capable of independent fabrication, with the intention of being subsequent fastened to an item to be marked or identified. In this embodiment, the tag may be fabricated from three dedicated components (i) a first antenna/resonator, (ii) a second antenna/resonator and (iii) a substrate disposed between the first and second antennae/resonators to provide a self- contained tag. The tag may comprise a pressure sensitive adhesive on one outer surface to facilitate fastening. While the substrate will generally be a discrete layer (or layers) disposed between the two antennae/resonators, it is also contemplated that a coating on one or both of the antennae/resonators will be operable. Alternatively, the two antennae/resonators may be maintained apart by some means such that air is the substrate. Alternatively, the tag may be formed in the process of manufacturing an item to be marked or identified, and become integral to the item. Accordingly, in certain embodiments of the RFID tag the substrate is the object to be marked or identified, or is part of the object to be marked or identified. For example, where the tag is to incorporated into paper packaging, the first antenna/resonator may be applied to the outside surface of the package and the second antenna/resonator applied to the inside surface of the packaging. In that embodiment, the substrate is a sheet of paper of the packaging.

As another example, where the tag is incorporated into a television set, the first antenna/resonator may be applied to the outside of the appliance casing with the second antenna/resonator applied to the inside of the casing. In that embodiment, the substrate is the appliance casing.

A further example is where the first antenna/resonator is applied to one surface of a polymer bank note and the second antenna/resonator applied to the other side. In that embodiment, the substrate is the bank note.

The present RFID tags are divergent from approaches of the prior art, and in particular embodiments wherein the first antenna/resonator is applied to a first surface of the substrate and the second antenna/resonator is applied to a second surface of the substrate.

In certain embodiments, the first antenna/resonator and second antenna/resonator are on opposing sides of the substrate.

The present RFID tags are distinguished from the prior art in other aspects. For example, in some embodiments tags of the present invention do not require inductors or capacitors to tune the resonant frequency of the antennae. Thus, in certain embodiments the RFID tag is devoid of an inductor or a capacitor.

Certain embodiments of the present invention do not require a multiresonating circuit, as if found in some prior art devices often disposed between the transmitting and receiving antenna to provide spectral signature encoding. While the transmitting and receiving antenna on these existing tags may utilization polarization diversity, it is not used to encode data but only to isolate the transmitting and receiving signal from and to the tag. Thus, in certain embodiments of the invention the RFID tag is devoid of a multiresonating circuit. I n some embodiments, the RFID tag is devoid of a metallic patch, or a metallic patch having etched slots. I n prior art tags, these slots resonate at different frequencies to encode data where each resonant frequency has a 1 to 1 correspondence with a data bit. Use of such patches in tags of the present invention is less preferred because of the potential for one patch to block, reflect or otherwise interfere with the signal of another overlapping patch. Furthermore, these metallic patches are typically implemented by printing copper (or any other high cost conductive material) onto a substrate. Given that these patches use slots and rely on gap coupling to resonate, inferior results would be expected when printed using a low cost conductive ink.

Thus, preferred embodiments of the present tags use a dipole antenna as the first and/or second antenna/resonator. The skilled artisan understands that the dimensions of the dipoles are dependent on the operating frequency and the dielectric properties of the substrate. The length, thickness and spacing between the dipoles may require optimization using a electromagnetic simulator for optimal performance, this being within the ability of the skilled person.

The use of a dipole antennae may allow for the inclusion of more antennae within a given frequency band. This could effectively mea n that a much smaller frequency band may be able to encode the same number of bits. For example, some prior art tags require around 5 GHz to encode 8 bits of data, while the tags of the present invention may require less than about 4 GHz, 3 GHz, 2 GHz, 1 GHz or 500 M Hz.

I n some embodiments of the RFID tag one or more of the linearly polarised antennae has been cut or the length otherwise altered in order to encode data. As demonstrated in the Examples herein, a data bit ("0" or "1") can be encoded in a dipole by cutting or altering the length of the dipole such that the signal detected by a reader is altered, or is not even detectable by the reader. An advantage of these embodiments is that data can be easily encoded in a tag by alternatively laser ablating (or not laser ablating) the various dipoles in a tag.

In certain embodiments, the RFID tags of the present invention encode data using the spectral signature technique. Each antenna/resonator operates and/or resonates at a different frequency. Typically, each antenna/resonator has a 1:1 correspondence with a data bit (for example: 6 resonators = 6 data bits).

Generally speaking, M (a real integer greater than 0) antennae/resonators may operate between frequencies Fl and F2, where Fl is the resonant frequency of resonator 1 and F2 is the resonant frequency of resonator M, and where resonators 2 to M-1 operate at frequencies between Fl and F2.

Polarization diversity enables the reuse of the frequency spectrum by separate antennae/resonators applied to an item. Thus, the present invention provides for a certain number (N; a real integer greater than 0) of antennae/resonators applied to the item in an overlapping manner, the antennae/resonators having the same physical dimensions (and therefore the same resonant frequencies) but rotated relative to each other.

Accordingly, when the RFID reader device interrogates the overlapping antennae/resonators it will read at a polarization appropriate for the first antenna/resonator thereby effectively preventing the reception of any signal from the second antenna/resonator. In this way the number of bits that may be encoded may be increased two-fold for a given area, where the first and second antennae/resonators have complete spatial overlap.

Advantageously, embodiments of the invention are able to include information encoded within a multiresonant structure, which may be retrieved by applying an appropriate RF excitation (such as an impulse, a wideband signal, or a swept narrowband signal), wherein the information may be recovered by analysing the corresponding frequency response. More particularly, in preferred embodiments, the presence and/or absence of resonant responses in amplitude and/or phase at corresponding characteristic frequencies is used to encode digital information. Through the use of a multiresonant structu re which comprises a plurality of substructures, with which the plurality of resonances are associated, the resonant response, and hence the encoded information, may be modified by suitable formation of the individual substructures.

Certain embodiments of the invention provide for RFID tags comprising or consisting of a third, fourth, fifth, sixth, seventh, eighth, ninth, tenth or more separate antennae/resonators. These embodiments provide for even further data to be encoded for a given area. There may be a necessity for different frequency bands to be implemented to avoid interaction between signals emitted by separate antennae/resonators. Thus, where there are K (K is a real integer greater than 1) paired antennae/resonators (each of the pair rotationally polarized with respect to each other), each pair disposed on top of other pairs, and for pair 1 the antennae/resonators operate between frequencies Fl and F2 (F2 > Fl), then for pair 2 the antennae/resonators need to operate between frequencies F3 and F4 (where F3> F2, F4 > F3), then for pair 3 the antennae/resonators need to operate between frequencies F5 and F6 (where F5>F4, F6 >F5), et cetera.

Given the advantage of the inventive disclosure herein, the skilled artisan will understand that it is theoretically possible to resolve signals emitted by the RFID where the rotational polarization is any angle greater than 0 degrees but less than 180 degrees. This rotation can be of any angle from greater than 0 to less than 180 degrees. Since polarization diversity is the highest at 90 degrees rotation, a preferred embodiment of the invention provides that the first antenna/resonator is rotationally polarized approximately 90 degrees with respect to the second antenna resonator.

It is contemplated that the use of multiple rotations within a single tag may be operable. Thus, the first antenna/resonator may be disposed horizontally, the second antenna/resonator disposed vertically, and a third antenna/resonator disposed at 45 degrees. It is further contemplated that the tag may comprise antennae/resonators that are in the same orientation (i.e. not rotationally polarized), as long as they are adequately separated (spatially or functionally) to allow their respective signals to be resolved separately. Thus, while the rotational polarization of two antennae/resonators allows for the potential to increase the amount of data stored in a given area, further data can be encoded by the tag by adding further antennae/resonators in the same orientation to others in the tag. These embodiments apply in circumstances where the product is the substrate such that multiple antennae/resonators are disposed about the product, such that at least two are in the same orientation.

The substrate may be a non-conductive material, and/or may be a dielectric material. The term "dielectric", as used herein, encompasses a broad range of substantially nonconducting materials providing suitable substrates for conductive structures formed in accordance with embodiments of the invention. These include materials specifically designed for use as substrates for electrical circuits (such as FR4 and other materials commonly used in printed circuit board manufacture), as well as other materials, e.g. polymers and paper, from which articles such as security documents, bank notes, and other negotiable instruments may be fabricated.

In certain embodiments the substrate is comprised substantially or completely of paper, cardboard, rubber, a synthetic polymer, a plastic, a tape or glass. In other embodiments of the invention the substrate is substantially planar. The antenna/resonator may be any suitable contrivance known to the skilled person. In some embodiments, the first and/or second antenna/resonator is/are a linearly polarised antenna. The linearly polarized antenna may be a dipole antenna, a patch antenna, a folded dipole antenna, a squiggle dipole antenna, a monopole antenna, or a wire-based antenna.

The first and/or second antenna/resonator may be a circularly polarised antenna in some embodiments. In the context of the present invention, the use of oppositely circularly polarised antennae/resonators (i.e. left hand compared with right hand polarisation) is a special form of rotational polarisation.

In other embodiments of the RFID tag, the first and/or second antenna/resonator is/are an L-shaped resonator, a spiral resonator, a split ring resonator, a hair-pin resonator, or a materials-based resonator.

The first and/or second antenna/resonator of the RFID tag may be comprised substa ntially or completely of a material having a moderate to high conductivity, and may be a metallic material. Other forms of the invention provide that the first and/or second antenna/resonator is/are comprised substantially of a conductive ink, or a conductive epoxy, or conductive nanoparticles. These forms of the invention obviate the need for costly metals, and provide for very low cost solutions for applications where RFID applications were previously considered economically unviable.

In some embodiments the conductive ink is applied to the substrate by a suitable printing technique, many of which are known to the skilled person. The printing technique may be a letterpress technique, a digital technique (including electrophotography, inkjet, xerography, laser), a gravure printing technique, a screen printing technique, a vacuum deposition technique, a 3D technique, a lithography technique, a thermography technique, a reprographic technique, a flexography technique, or electrostatic technique. Inkjet and 3D printing techniques are particularly suitable for use in the present invention.

In another aspect, the present invention provides an RFID reader capable of reading the data encoded by an RFID tag described herein. The RFID reader may comprise a single antenna that acts both as a transmitting and receiving antenna in a monostatic radar configuration. In this embodiment, the single antenna is in fact a transmitting/receiving antenna which is dually polarized. The RFID reader may be adapted or configured for a bistatic approach, having separate transmitting and receiving antennae. In some embodiments, the reader may comprise at least two transmitting antennae and/or at least two receiving antennae. This allows for the simultaneous interrogation of at least the first and second antennae/resonators of the RFID tag. Multiple transmitting and receiving antennae may further allow for the location of a tag within a three-dimensional space using triangulation and time arrival of the tag signals.

For situations where spatial diversity is implemented (for example, where two tags are affixed to the same side of the substrate, the tags polarized in the same plane) it is possible for the reader to comprise further transmitting and receiving antennae (in bistatic mode), or in monostatic mode the reader may comprise further combination transmitter/receiver antennae to accommodate spatial encoding. The may reader may be adapted or configured to interrogate by continuous wave sweeping frequency or pulse interrogation in some embodiments. This allows for the reading of antennae/resonators having multiple resonant frequencies.

In some embodiments, an RFID reader of the present invention may be distinguished by one or more of the following features. Conventional RFID readers typically operate mostly at HF, UHF and 2.45 GHz bands while the present readers may operate outside these bands. Conventional readers use amplitude shift keying (ASK) and binary phase shift keying (BPSK) time-domain based demodulating algorithms while the present readers may decode the tag by sweeping the spectrum and acquiring its spectral signature. Furthermore, the present readers may interrogate the tag without the requirement for any handshaking algorithms between tag and reader, this enabling faster read rates.

It is contemplated that the reader may be adapted or configured to interrogate the tag with a very short RF pulse in time (for example, of 1 nanosecond or several hundred picoseconds duration). Short pulses have a very wide frequency spectrum. Therefore, instead of using a frequency sweeping technique from Fl to F2 where the frequency of a continuous wave is changed (within a very narrow spectrum) across a broad frequency range in a long duration in time (from microseconds to milliseconds) it may be possible to use a short pulse with a wide spectrum.

The reader may detect one or both of amplitude and phase of the received signal. Accordingly, in certain embodiments, the reader is adapted or configured to read the spectral signature of one or more tags by reference to the amplitude of the received signal and/or phase of the received signal. Each antenna/resonator (having a certain polarization and spatial placement) will add a resonance to the frequency signature in both amplitude and phase, with this information being capable of being transformed into information. A particular advantage for readers that detect both amplitude and phase is that one signal may be used to validate the other. This provides for a higher level of accuracy in the information obtained from the tag.

A reader of the present invention may further comprise means for reading other types of information (such as an optical barcode, an active RFID tag, or text via an OCR method) to provide for a multifunctional reading device. As will be understood by the skilled person, the reader may be adapted or configured to be handheld. The reader may also be adapted or configured to be semi-permanently or permanently mounted in a desired position, such as about a conveyor belt, a book shelf, an ATM, a table, or a vehicle for example. I n a further aspect the present invention provides an RFID system comprising an RFI D tag as described herein, and an RFID reader as described herein.

Yet a further aspect provides a kit of parts comprising an RFID tag as described herein, and an RFID reader as described herein. The kit may further comprise instructions as to how the tags are to encoded or applied to an item, and how to use the reader to read the tags. The instructions may in the form of text, audio, video or graphics.

A further aspect provides a method for fabricating an RFI D tag, the method comprising the steps of: providing a substrate, applying a first antenna/resonator to a first surface of the substrate, applying a second antenna/resonator to a second surface of the substrate, securing the position of the first antenna/resonator with respect to the position of the second antenna/resonator, wherein the second antenna/resonator is rotated relative to the second antenna resonator to provide sufficient rotational polarization such that in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator. As mentioned supra, conductive inks are a preferred material and therefore in certain embodiments of the method the step(s) of applying the first and/or second antenna/resonator is a printing technique. Exemplary printing techniques include a letterpress technique, a digital technique (including electrophotography, inkjet, xerography, laser), a gravure printing technique, a screen printing technique, a vacuum deposition technique, a 3D technique, a lithography technique, a thermography technique, a reprographic technique, a flexography technique, or an electrostatic technique. Particular preferred printing techniques in the context of the method are inkjet printing and 3D printing.

Yet a further aspect of the invention provides a method for marking or identifying an item, the method comprising the steps of: providing an item applying a first antenna/resonator to a first surface of the item, applying a second antenna/resonator to a second surface of the item, securing the position of the first antenna/resonator with respect to the position of the second antenna/resonator, wherein the second antenna/resonator is rotated relative to the second antenna resonator to provide sufficient rotational polarization such that in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator. It will be understood that the various RFID tags, RFID readers, systems and methods described herein may be implemented in any application currently exploiting RFID technology and also other applications, including but not limited to marking or identifying the following items: postage stamps, identification cards, passports, documents, envelopes, printed circuit boards, bank notes, asset management, inventory management, et cetera.

The present invention will now be more fully described by reference to the following non-limiting examples, and in which the following numerical descriptors are referenced:

I. Open Dipole representing logic "0"

2. Shorted Dipole representing logic "1"

3. Response of shorted dipole

4. Response of open dipole

5. Frequency shift

6. Top Side with code 101010

7. Bottom side witch code 001010

8 Dielectric 1 Top Side

9. Metal 1

10. Dielectric 1 Bottom Side

II. Metal 2

12. Dielectric 2 Top Side

13. Dielectric 2 Bottom Side

14. Metal 3

15. Dielectric 3 Top Side

16. Dielectric 3 Bottom Side

17. Metal 4

18. Metal 1

19. Dielectric 1

20. Metal 2

21. Dielectric 2 22. Metal 3

23. Dielectric 3

24. Metal 4

25. FID Reader

26. Transmitter antenna

27. EM signal to tag

28. EM signal from tag

29. Tag

30. Receiver antenna

31. PC with Middleware and GUI

32. Data link

33. Comms interface

34. Chipless RFID reader

35. Digital to analog converter 36. Voltage controlled oscillator

37. Power amplifier

38. Low noise amplifier

39. Transmitter antenna

40. EM signal to tag

41. Receiver antenna

42. EM signal from tag

43 Low Noise Amplifier

44. Bandpass filter

45. RF to DC converter

46. Analog to digital converter

47. microprocessor

48. RFID reader

49. Transmitter receiver antenna

50. EM signal to tag

51. EM signal from tag

52. Tag

53. PC with middleware

54. Data link

55. Chipless RFID reader 56. Comms interface

57. Digital to analog converter

58. Voltage controlled oscillator

59. Power amplifier

60. Low noise power amplifier

61. Antenna

62. EM signal

63. Circulator

64. Low noise amplifier

65. Bandpass filter

66. RF to DC converter

67. Analog to digital converter

68. Microprocessor

69. Inset fed patch/resonator 70. Rectangular patch/resonator

71. Diamond patch/resonator

72. Triangular patch/resonator

73. Squiggle dipole/resonator

74. Folded dipole/resonator 75. Bent dipole/resonator

76. L-shpaed dipole/resonator

77. Circualr dipole/resonator

78. C-shaped dipole//resonator

79. Split ring resonator

80. Spiral resonator

81. Equally folded dipole/resonator

82. Network analyzer

83. Transmitter antenna stand

84. Transmitter antenna

85. Tag

86. Receiver Antenna

87. Receiver antenna stand

88. Tag stand

89. Acrylic jig/base 90. Network analyzer

91. Antenna stand

92. Reader antenna

93. Tag

94. Tag stand

95. Acrylic jig/base

96. Network analyser as reader

97. Transmitter antenna

98. Acrylic stand/jig

99. Receiver antenna

100. Tag

101 . single strip tag

102 . dual strip tag

103. Shorted dipole fundamental resonance frequency 104. Shorted dipole harmonic

105. Open (cut) dipole fundamental resonance frequency

106. RS-232 port

107. Display

108. Digital to analog converter

109. Oscillator

110. Amplifier

111. Filter

112. Connector

113. Connector

114. Low Noise Amplifier

115. Amplifier

116. Filter

117. Rectifier

118. Analog to digital converter

119. Microcontroller

120. Power supply circuit

121. Reader power supply

122. Reader circuit

123. Reader antenna 124. Tag placed on acrylic jig

125. Operating frequency band of FID reader

126. Second resonance

127. Third resonance

128. First resonance

129. Second resonance

130. Third resonance

EXAMPLE 1: Horizontally polarized and vertically polarized dipole antennae placed on opposing sides of a thin flexible Teflon laminate

Vertically polarized dipole antennae were printed on the upper surface of a laminate while the horizontally polarized dipole antennae were placed on the lower side of the laminate. The dipole antennae were printed using copper. The laminate used was Taconic TLX-8, which is 0.17 mm thin. Photograph of the tag's bottom and top side are presented in Fig. 1 a) and b).

Fig. 2 shows the layout of the tag where the red colour represents the upper layer conductor, the blue color represents the lower layer conductor while the black color represents the substrate.

This chipless tag was designed so that spatial and frequency reuse is efficient since the size and data encoding capacity of the chipless RFID tag should be optimal for efficient performance. The antennae on opposite sides of the substrate are cross-polarized in order to increase isolation between them when read by the RFID reader.

This chipless RFID tag was further designed to operate between 2 and 4 GHz and encode 32 data bits (16 bits encoded with vertically polarized antennae and 16 bits encoded with horizontally polarized antennae). This RFID tag is scalable to operate at any frequency by altering the size of the dipoles.

The physical dimensions of each dipole element are presented in Table I, infra. > I. Dipole antenna element dimensions

Resonant

Name Data Bit(2^Ax) Length (mm) Frequency (GHz)

I⁵¹ 15 64.5 2.11

14 60.5 2.253

13 57.5 2.372

4ⁱⁿ 12 55.1 2.473

11 53 2.569

10 51 2.666

49 2.773

46.8 2.895

44.6 3.033

10ⁱⁿ 6 42.5 3.176

1 " 5 40.5 3.315

12ⁱⁿ 4 38.5 3.475

13ⁱⁿ 3 36.8 3.617

14ⁱⁿ 2 35.3 3.748

15^th 1 34 3.874

16^th 0 32.8 4 In order to encode data in the tag the dipole antenna was cut resulting in a change in its resonant frequency. Data encoding is performed by not having a resonance present at a particular frequency. When the resonance is present this is a considered to be a logic "0", when it is absent it is considered to be logic 1. In prior art tags, the presence or absence of a data bit is controlled by adding and removing a resonator/antenna. This is not efficient, and it changes the layout of the tag. By cutting the dipole and changing its effective length (and in this case creating multiple dipoles operating at frequencies outside the desired frequency band of the tag), data encoding can be performed using laser ablation with minimum layout modifications. This is shown in Figs 3 and 4. The tag is intended to work in this example from 2 to 3 GHz. The open dipole is cut in half and technically the dipole antenna is therefore transformed into 2 dipole antennae operating at 2 times higher frequency outside the 2-3 GHz frequency band and therefore invisible. In Figs 3 and 4 the shorted dipole is operating at 2 GHz while the open dipoles are operating at 4 GHz. The frequency shift can be controlled and the dipole can be cut in 3 equal pieces which will mean that it is operating at a frequency around 6 GHz. This is important for other embodiments of the invention.

Fig.5 shows the top and bottom side dipole printed on the dielectric. The top side tag has data encoded 101010 while the bottom side has 010100 which in total gives a 12 bit tag ID of 010100101010.

Example 2: "Stacked" tag produced by layering multiple dielectrics.

This embodiment is fabricated to comprise several dielectrics with printed antennae/resonators on them stacked one on top of the other. The number of dielectrics that can be stacked is only limited by the requirements of the application and frequency spectrum bandwidth required for data encoding.

An example of 3 dielectrics stacked on each other to increase the data encoding capacity of the tag is presented in Fig. 6. In this embodiment, the dielectrics 1, 2 and 3 are of the same material, but this need not be so. From Fig. 6 it is clear that metall antennae/resonators are printed on the top side of dielectricl with vertical polarization while metal2 antennae/resonators are printed on the bottom side of dielectricl with horizontal polarization. Resonators printed on dielectricl (metall and metal 2) operate in the same frequency bandwidth from frequencies Fl to F2 (for example let Fl = 2 GHz and F2 = 3 GHz) where frequency reuse is achieved using polarization diversity. For dielectric2 the top side is left with no printed antenna/resonator elements since they are present on the bottom side of dielectricl and direct contact of metal would render the printed antennae/resonators non operational. The bottom side of dielectric2 has a vertically polarized antennae/resonators printed in metal 3 layer. For dielectric3 the top side has no metal printed because metal3 is printed on the bottom side of dielectric2. However the bottom side of dielectric3 is printed with horizontally polarized antennae/resonators on metal 4. It is important to note that metal 3 and metal 4 antennae/resonators operate in the same frequency band between frequencies F3 and F4 (which are for example F3 = 3.1 GHz and F4 = 4 GHz). Frequencies Fl and F2 are in the relationship F2 > Fl. Frequencies F3 and F4 are in the relationship F4 > F3. Frequency F3 must be greater than F2. A more general explanation would be that there cannot be a sharing of frequencies amongst elements in the stacked architecture in order to ensure correct operation of the tag. A layer stack of the tag presented in Fig.6 is shown in Fig.7.

EXAMPLE 3: RFID Reader #1.

The RFID Reader#l comprises 2 reader antennae, a transmitting and receiving antenna. The arrangement of the reader antennae and the tag is presented in Fig. 8.

This chipless RFID reader is an electronic device which can detect the ID of the chipless tag when it is within the reader's interrogation zone. The block diagram of the chipless RFID reader and its basic components are shown in Fig. 9. The RFID reader has transmitting and receiving antennae to send the interrogation signal to the chipless tags and receive the encoded signal from the chipless tags. The RFID reader transmitter comprises a voltage controlled oscillator (VCO), low noise amplifier (LNA) and power amplifier (PA). Tuning of the VCO's output frequency is done by the microcontroller through the digital-to-analog (ADC) converter. The reader transmitter generates the interrogation signal which is sent to the chipless tag. The interrogation signal can either be a continuous wave with a change in frequency induced in time domain so that it covers the entire spectrum or it can be a short pulse which will have a large spectrum. The chipless transponder encodes its spectral signature into the reader's interrogation signal which propagates to the reader's receiver antenna.

The signal processing of the received tag signal is performed at the receiver end of the RFID reader and results in a digital signal being sent to the microprocessor of the RFID reader. The receiver comprises a LNA, a band-pass filter (BPF), a demodulating circuit which converts the RF signal to baseband and an analog-to-digital converter (DAC). The microprocessor uses tag detection and decoding algorithms to determine the ID of the chipless tag, which is sent to an application or software enterprise on a personal computer (PC) which provides the graphical user interface (GUI) between the RFID system and the user. The interface between the RFID reader and the PC can be (but not limited too)through a RS-232 connection, LAN interface or a wireless transmission such as WLAN, Bluetooth, Zigbee etc.

The RFID reader antenna can be of two types: low gain and high gain antenna. Low gain antennae are used for short range readings and have a less directional radiation pattern. High gain antennae in general improve reading range and are more directional beam pattern but at the cost of having greater dimensions than low gain antennae and are more complex to design. They are usually antenna arrays. Both high gain and low gain antennae have to operate over the entire band in which the chipless tag is encoding data in the frequency spectrum. An example of a low gain reader antenna is a monopole antenna shown in Fig. 10. An example of a high gain antenna is shown in Figs. 11 and 12.

Since the tags use polarization diversity the reader antennae need to be capable of handling this feature of the tag in order to interrogate the tag and decode the spectral signature entirely. This can be done by either using antennae which exhibit dual polarization operation or having 2 antennae for the transmitter antenna with each antenna operating in a different polarization and 2 antennae for the receiver antennae as well operating in both polarizations.

EXAMPLE 4: RFID Reader Device #2 The RFID Reader#2 comprises 1 reader antenna which acts as a transmitting and receiving antenna. The arrangement of the reader antenna and the tag is presented in Fig.13.

The block diagram of the chipless RFID reader #2 and its basic components are shown in Fig. 14.

The main difference between the Reader#2 and Reader#l is that there is one antenna and therefore there must be an interface to handle to connection of the transmitter (Tx) and receiver (Rx) to the same antenna. This is done using a circulator. Other circuits that could be used are a bidirectional coupler which is less bulky than a circulator but has greater leakage between the Tx and Rx circuits. The other option is to have a high speed switch which switches between the Tx and Rx. The rest of the circuit can be based on a similar concept. Also the reader antenna can be high or low gain depending on the application and required reading range.

EXAMPLE 5: Bistatic and Monostatic Radar Systems

RFID tags were tested according to the following: 1. Bistatic radar setup - tag placed between 2 reader antennae

2. Monostatic radar setup - tag placed in front of 1 reader antenna

The bistatic radar setup is shown in Fig.16. The tag is placed between the reader antennae. One reader antenna transmits the signal while the other reader antenna receives it. The transmitted signal is the interrogation signal which reaches the tag, the tag encodes its spectral signature in the interrogation signal which is then received by the readers receiver antenna. The instrument used to detect the spectral signature was a Rohde&Schwarz Vector Network Analyzer ZVH8 which displays the transmission loss (S21) parameter. This experiment used tags printed with copper and also with low cost conductive ink printed on low cost polymer. Successful operation of the tag and its wireless detection by the ZVH8 are the primary goals. Other goals are investigation of maximum reading range, multiple tag readings and minimum reader transmit power required to detect the tag.

The monostatic radar setup is shown in Fig.17. The tag is placed in front of the reader antenna. One reader antenna is used to transmit the interrogation signal to receive the interrogation signal backscatter by the tag. The backscattered signal was used to decode the spectral signature of the tag. Successful operation of the tag and its wireless detection by the ZVH8 are the primary goals. Other goals are investigation of maximum reading range, multiple tag readings and minimum reader transmit power required to detect the tag.

The bistatic and monostatic radar setups may be altered by substituting the Rohde & Schwarz ZVH8 vector network analyser by a prototype RFID reader.

EXAMPLE 6: Demonstration of Bistatic and Monostatic Systems

Bistatic setup details:

- Reader antennae are 2 monopole antennae (disc loaded on Taconic TLXO)

- Tag is either copper printed on Taconic TLX-8 or conductive ink printed on plastic substrate

- Reader antennae and tag are placed on a custom built acrylic stand

- Vector Network Analyzer Rohde&Schwarz ZVH8 (0.1 to 8 GHz)

- Reader Antennae are separated by 2.5 cm

- Tag placed 1cm from each reader antenna

Tag used in Figs. 18 and 19 is a copper printed single polarization, no spatial diversity, 5 bits

The spectral signature of the tag is also encoded in the phase therefore there is a duality of the spectral signature in the magnitude and the phase. The reader therefore should have the ability to interrogate magnitude and phase and compare them to confirm the I D of the interrogated tag. Phase information is known to have more resilience to noise than amplitude and may be an alternative signature used, especially for long range readings in the future.

The copper tags operate correctly as seen in Fig. 20, however they do not provide a low cost solution which is based on conductive ink printed tags. Accordingly tags have been printed with low cost conductive ink on low cost plastic substrate. The conductive ink has a small conductivity compared to copper (conductive ink has 200000 S/m while Cu has 58000000 S/m conductivity). Tag used in Figs 21 and 22 is conductive ink printed single polarization no spatial diversity 1 bit with single and dual stripes (Figs. 21 and 22 respectively). The dual strip is used to increase the resonance of the bit so that is more easily detected. This is presented in Fig. 23. Some frequency shift is also present due to modification in layout (dipole repetition).

The single dipole is then cut in the middle to show that the resonance shifts higher and therefore can encode a logic "0" (dipole not cut) and a logic "1" (dipole cut). The presence and absence of the resonance of the dipole is presented in Fig. 7.

From Fig. 24 it can be seen that the shorted dipole (not cut dipole) has a resonance around 2.5 GHz shown in the yellow trace and its harmonic resonance is around 3.8 GHz. After the dipole being cut the primary resonance shifts up due to the change in the length of the dipole (effectively two shorter dipoles are created resonating at 3.9 GHz). The effect of cutting the dipole is equivalent of removing the dipole out of the reader's interrogation zone since the resonance is absent in the desired frequency band The new dipole resonance could be changed even higher by cutting the dipole into 3 smaller dipoles. This proves the concept of controlled dipole length variation for data encoding. From now the presence of the resonance will be taken as logic "0" and the absence as logic "1" for a particular data bit. Each data bit has a reserved resonance frequency for a dipole, therefore each dipole is a data bit (n dipoles = n data bits).

From the single bit tag, a version was fabricated printed with conductive ink with 3 dipoles encoding data in the vertical polarization. With this experiment it was desired to show that it is possible to encode more than 1 bit in the same polarization. Fig. 25 shows the spectral signature response of a tag with 3 dipole (yellow trace) vs no tag present between the reader antennae (white trace). The spectral ID shown in Fig. 25 is ID 000.

1 dipole was then placed in the vertical polarization in order to test the polarization diversity scheme. The dipole was placed on the other side of the plastic sheet in order to test the response and see whether adequate polarization isolation occurs so that frequency re-use can be achieved and therefore increase the number of bits by a factor of 2. The tag is shown in Fig. 28. This can be seen as a tag ID of 000011 where the upper 3 bits are in vertical polarization and the lower 3 are in horizontal. The last 2 bits are 1 because there will be no resonance present since only 1 dipole is placed in the horizontal polarization. The spectral signature in both polarizations is presented in Fig. 29.

In Fig. 29 the vertical polarization response is show by the white trace while the horizontal polarization response is shown in the yellow trace. It is clear that the white trace shows 3 distinct dips while the yellow trace shows 1. It is also clear that the isolation between the polarizations is sufficient since the lower to bits in the vertical polarization are not showing the horizontal polarization response as expected, only the single dip due to the single dipole is shown in the yellow trace. This proves the concept of the polarization diversity used on opposite sides of a dielectric which increases the number of bits by 2 and makes the tag more spatially efficient since the dipole can be placed on two sides over each other.

The next experiment was to replace the ZVH8 vector network analyser by a custom built RFID reader. The custom built reader is shown in Fig. 30.

The custom built RFID reader is connected to the transmitter and receiver antennae on the experimental setup stand and is used to interrogate the tag and receive the spectral signature, digitize it and then send the digital data to a PC through the RS232 port where it is displayed using MS Excel. The RFID reader has a VCO in its transmitter side and a power amplifier. The receiver end comprises 2 stages of power amplifiers and filters and a diode rectifier used for conversion from RF to DC. The DC signal is actually the envelope of the received RF signal and is sent to the analog to digital converter which converts the analog data to digital data. The digital data is then sent to the PC via the onboard microprocessor using the RS232 connection. Fig. 31 shows the setup with the RFID reader. It is important to note that the RFID reader is a prototype and operates between 1.95 and 2.49 GHz, and therefore all 3 resonances in the spectral signature cannot be detected. A fully optimized reader can be designed to cover the entire frequency band from 1.7 to 2.5 GHz but for experimental purposes this prototype reader is sufficient. Only one resonance can be detected clearly (the second one M2 at 2.14 GHz) while the 3-rd (M3 at 2.52 GHz) is half out of the reader's band of operation. The first resonance (M l at 1.8 GHz) is at 1.8 GHz which is below the reader's capability to interrogate. For proof of concept this is good enough though. Fig.32 shows the response using the vector network analyser with markers which point out the resonant frequencies as described above. Fig. 33 shows the detect response using the custom RFI D reader. From Fig. 34 it is clear that two different responses are observed : 1) Tag is placed for interrogation (blue trace) and tag is removed from interrogation zone (pink trace). From these two readings it can be seen that as per Fig. 16 the reader can detect the 2nd resonance at 2.14 GHz and partially the 3rd resonance around 2.5 GHz. This confirms that the chipless RFI D tag can be detected using custom electronics for the RFID reader which makes the entire system practically viable.

Monostatic setup details:

- Reader antenna is 1 monopole antenna (disc loaded on Taconic TLXO)

- Tag is either copper printed on Taconic TLX-8 or conductive ink printed on plastic substrate

- Reader antenna and tag are placed on a custom built acrylic stand

- Vector Network Analyzer Rohde&Schwarz ZVH8 (0.1 to 8 GHz)

- Tag placed 1cm from reader antenna.

The monostatic reader approach will detect not dips but jumps in the sll of the reader antenna since the dipoles will backscatter power at their resonance frequencies back to the RFI D reader antenna. The response of the 3 bit conductive ink printed tag is presented in Fig. 34.

From Fig. 34 it can be seen that there are 3 distinct jumps in the measured sll due to the power getting reflected back to the reader antenna at the tag's resonance frequencies. This proves that a single reader antenna in monostatic radar mode can be used also to detect the chipless RFI D tag.

While the invention has been described in the foregoing with reference to a number of prototype chipless RFID tag devices, it will be appreciated that embodiments of the invention may be employed in a variety of other applications. For example, antenna/resonator structures embodying the invention may be fabricated on, printed on, or incorporated into, a variety of different articles, including, but not limited to, RFI D tags, security documents, and negotiable instruments, such as bank notes. They may accordingly be used for security and/or authentication purposes, as well as for the identification, detection and/or tracking of various items or articles of interest. It will therefore be understood that the invention is not limited to the specific embodiments described herein, which are provided by way of example only.

Claims

CLAIMS:

1. A radio frequency identification (RFID) tag comprising or consisting of: a first antenna/resonator and a second antenna/resonator, the first and second antennae/resonators being applied to a substrate in a partially or completely overlapping manner.

2. An RFID tag according to claim 1 wherein the first and second antennae/resonators configured or adapted to encode data, the first antenna/resonator being rotationally polarized with respect to the second antenna/resonator, wherein in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator.

3. An RFID tag according to claim 1 or claim 2 wherein the first antenna/resonator is applied to a first surface of the substrate and the second antenna/resonator is applied to a second surface of the substrate.

4. An RFID tag according to any one of claims 1 to 3 wherein the first antenna/resonator and second antenna/resonator are on opposing sides of the substrate.

5. An RFID tag according to any one of claims 1 to 4 wherein the first antenna/resonator is rotationally polarized approximately 90 degrees with respect to the second antenna resonator.

6. An RFID tag according to any one of claims 1 to 5 wherein the substrate is a non- conductive material.

7. An RFID tag according to any one of claims 1 to 6 wherein the substrate is a dielectric material.

8. An RFID tag according to any one of claims 1 to 7 wherein the substrate is comprised substantially or completely of paper, cardboard, rubber, a synthetic polymer, a plastic, a tape or glass.

9. An RFID tag according to any one of claims 1 to 8 wherein the substrate is the object to be marked or identified, or is part of the object to be marked or identified.

31

10. An RFID tag according to any one of claims 1 to 9 wherein the substrate is substantially planar.

11. An RFID tag according to any one of claims 1 to 10 wherein the first and/or second antenna/resonator is/are a linearly polarised antenna.

12. An RFID tag according to claim 11 wherein the linearly polarized antenna is selected from the group consisting of a dipole antenna, a patch antenna, a folded dipole antenna, a squiggle dipole antenna, a circular-shaped dipole antenna, a c-shaped antenna, a monopole antenna, and a wire-based antenna.

13. An RFID tag according to claim 12 wherein one or more linearly polarised antenna has been cut or the length otherwise altered in order to encode data.

14. An RFID tag according to any one of claims 1 to 13 wherein the first and/or second antenna/resonator is/are a circularly polarised antenna.

15. An RFID tag according to any one of claims 1 to 14 wherein the first and/or second antenna/resonator is/are selected from the group consisting of an L-shaped resonator, a spiral resonator, a split ring resonator, a hair-pin resonator, and a materials-based resonator.

16. An RFID tag according to any one of claims 1 to 15 wherein the first and/or second antenna/resonator is/are comprised substantially or completely of a material having a moderate to high conductivity.

17. An RFID tag according to any one of claims 1 to 16 wherein the first and/or second antenna/resonator is/are comprised substantially or completely of a metallic material.

18. An RFID tag according to any one of claims 1 to 17 wherein the first and/or second antenna/resonator is/are comprised substantially of a conductive ink.

19. An RFID tag according to claim 18 wherein the conductive ink is applied to the substrate by a printing technique, the printing technique optionally selected form the group consisting of a letterpress technique, a digital technique (including electrophotography, inkjet, xerography, laser), a gravure printing technique, a screen printing technique, a vacuum deposition

32 technique, a 3D technique, a lithography technique, a thermography technique, a reprographic technique, a flexography technique, an electrostatic technique.

20. An RFID tag according to any one of claims 1 to 19 wherein the first and/or second antenna/resonator is/are comprised substantially of a conductive epoxy or conductive nanoparticles.

21. An RFID tag according to any one of claims 1 to 20 comprising or consisting of a third, fourth, fifth, sixth, seventh, eighth, ninth, tenth or more separate antennae/resonators.

22. An RFID reader capable of reading the data encoded by an RFID tag according to any one of claim 1 to 21.

23. An RFID reader according to claim 22 comprising at least one transmitting antenna and at least one receiving antenna.

24. An RFID reader according to claim 22 or claim 23 adapted or configured to interrogate by continuous wave sweeping frequency or pulse interrogation.

25. An RFID reader according to any one of claims 22 to 24 adapted or configured to read the spectral signature of one or more tags by reference to the amplitude of the received signal and/or phase of the received signal.

26. An RFID system comprising an RFID tag according to any one of claim 1 to 21, and an RFID reader according to any one of claims 22 to 25.

27. A kit of parts comprising an RFID tag according to any one of claim 1 to 21, and an RFID reader according to any one of claims 22 to 25.

28. A method for fabricating an RFID tag, the method comprising the steps of:

providing a substrate,

applying a first antenna/resonator to a first surface of the substrate, applying a second antenna/resonator to a second surface of the substrate, securing the position of the first antenna/resonator with respect to the

position of the second antenna/resonator,

wherein the second antenna/resonator is rotated relative to the second antenna resonator to provide sufficient rotational polarization such that in use the data encoded by the first

33 antenna/resonator is readable separately to the data encoded by the second antenna/resonator.

29. A method according to claim 28 wherein the first and/or second antenna/resonator is/are comprised substantially of a conductive ink, and wherein the step(s) of applying the first and/or second antenna/resonator is a printing technique, the printing technique optionally selected form the group consisting of a letterpress technique, a digital technique (including electrophotography, inkjet, xerography, laser), a gravure printing technique, a screen printing technique, a vacuum deposition technique, a 3D technique, a lithography technique, a thermography technique, a reprographic technique, a flexography technique, an electrostatic technique.

30. A method for marking or identifying an item, the method comprising the steps of:

providing an item and applying a first antenna/resonator to a first surface of the item,

applying a second antenna/resonator to a second surface of the item, securing the position of the first antenna/resonator with respect to the

position of the second antenna/resonator,

wherein the second antenna/resonator is rotated relative to the second antenna resonator to provide sufficient rotational polarization such that in use the data encoded by the first antenna/resonator is readable separately to the data encoded by the second antenna/resonator.

34