WO2025108542A1

WO2025108542A1 - Rfid tag

Info

Publication number: WO2025108542A1
Application number: PCT/EP2023/082722
Authority: WO
Inventors: Charles Vilner; Mohammad ABEDIAN
Original assignee: HID Global Corp
Current assignee: HID Global Corp
Priority date: 2023-11-22
Filing date: 2023-11-22
Publication date: 2025-05-30
Anticipated expiration: 2026-05-22

Abstract

An RFID tag (10) is provided. The RFID tag (10) can be used in applications where it is exposed to high-power electromagnetic fields, for example, inside a microwave oven. A metallic strip (22A, 22B) is arranged on a substrate (12) adjacent to an RFID antenna (14) of the RFID tag (10) in order to control the current density in the RFID antenna (14). The presence of the metallic strip (22 A, 22B) allows for obtaining a simple and cost-effective RFID tag (10), because a commonly used substrate (12) such as a PET inlay can be used.

Description

RFID TAG

Technical Field

[01] The present disclosure generally relates to radio frequency identification (RFID) devices, in particular, to an RFID tag that is resistant to high-power electromagnetic fields.

Background

[02] Generally, RFID devices such as, for example, RFID tags, etc. include an RFID antenna and an integrated circuit connected to the RFID antenna. Upon presence of an electromagnetic field emitted by a reader device, the RFID antenna supplies energy from the electromagnetic field to the integrated circuit, which integrated circuit may communicate with the reader device using radio frequency (RF) communication protocols. In this manner, for example, data can be read from a memory associated with the integrated circuit, and can also be written into said memory, if desired.

[03] Real-time, secure RFID technology is extensively used in logistics monitoring, retail, military, and health care applications. In some applications, the RFID tags are exposed to high-power, in particular, high-frequency electromagnetic fields during operation, and the RFID tags should be able to withstand such high-power electromagnetic fields. For example, RFID tags may be provided on items that are placed in a microwave oven, and such RFID tags should be able to withstand the high-power electromagnetic field inside the micro wave oven.

[04] US 11,308,379 B2 discloses RFID tags with a shielding structure for incorporation into microwaveable food packaging. A shielding structure is electrically coupled to the RFID antenna across a gap and overlays the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a high-frequency electromagnetic field.

[05] The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems.

Summary of the Disclosure

[06] According to one aspect of the present disclosure, an RFID tag has a substrate and an RFID antenna arranged on the substrate. The RFID antenna includes a chip connection portion, and an intermediate portion extending from the chip connection portion to an end portion of the RFID antenna in a longitudinal direction of the RFID antenna. The RFID tag further has an RFID chip electrically connected to the chip connection portion and configured to perform RFID communications via the RFID antenna at a first frequency, and a metallic strip arranged on the substrate separate from the RFID antenna. The metallic strip extends along at least part of the intermediate portion.

[07] Other features and aspects of the present disclosure will become apparent from the following description and the accompanying drawings.

Brief Description of the Drawings

[08] Fig. 1 is a schematic plan view of an exemplary RFID tag in accordance with the present disclosure,

Fig. 2 is a schematic plan view of another exemplary RFID tag in accordance with the present disclosure, and

Fig. 3 is a schematic plan view of yet another exemplary RFID tag in accordance with the present disclosure.

Detailed Description

[09] The following is a detailed description of exemplary embodiments of the present disclosure. The exemplary embodiments described herein are intended to teach the principles of the present disclosure, enabling those of ordinary skill in the art to implement and use the present disclosure in many different environments and for many different applications. Therefore, the exemplary embodiments are not intended to be, and should not be considered as, a limiting description of the scope of protection. Rather, the scope of protection shall be defined by the appended claims.

[10] The present disclosure is based at least in part on the realization that, generally, when an RFID tag including an RFID antenna designed to operate at a first frequency is exposed to a high-power electromagnetic field, for example, having a frequency that is considerably higher than the first frequency for which the RFID antenna is designed, the strong electromagnetic field may produce a charge accumulation on the surface of the RFID antenna, which may result in the generation of heat and even sparks. Known solutions to this problem involve, for example, covering the RFID antenna with high permittivity materials to enhance the capacitance, or applying a shielded conductor structure to disperse the charge accumulation. However, it has been realized that these conventional approaches suffer from complex fabrication procedures, result in an increase in size, and are associated with high processing costs. This makes said approaches unattractive for mass production and large-scale applications.

[11] It has been realized that the above disadvantages can be overcome by including parasitic metallic strips on the substrate on which the RFID antenna is provided in order to control the current distribution on the RFID antenna traces. In particular, it has been found that the metallic strips can absorb the heat at high frequencies that are used, for example, in microwave applications. In contrast to the above-mentioned previous approaches, the RFID tags according to the present disclosure can be of smaller size, and also can be more durable. For example, the RFID tags can be used several times in a microwave oven. This can be achieved while using a simple inlay structure, for example, a PET inlay, on which the RFID antenna and the RFID chip are provided.

[12] Fig. 1 shows a schematic plan view of an exemplary RFID tag 10 in accordance with the present disclosure. As shown in Fig. 1, RFID tag 10 includes a substrate 12 and an RFID antenna, in particular, a dipole antenna 14 arranged on substrate 12. Substrate 12 may be substantially rectangular, and may have a size that allows attachment of the same to different items, depending on the application. For example, substrate 12 may have a size of about 60 mm by 20 mm. A material of substrate 12 may be any appropriate material that can be used for providing RFID antenna 14 on the same, while at the same time being able to be attached to different items or articles. For example, substrate 12 may be formed from PET or other materials such as PVC, PC, PE, PETE, Teslin®, PDMS, paper, cotton, and the like. An adhesive tape (not shown) for attaching substrate 12 to the respective articles or items, or the like may be provided. While Fig. 1 shows RFID antenna 14 as being exposed, it will be appreciated that RFID antenna may be covered by an additional layer to embed the same. The additional layer may be formed from the same material as substrate 12, or from a different material. In some embodiments, substrate 12 may be formed from a material having a relative permittivity a of between 2 and 4, preferably between 2.5 and 3.5.

[13] RFID antenna 14 includes opposite end portions 14 A, 14B in a longitudinal direction x of RFID antenna 14, a chip connection portion 16, and a pair of intermediate portions 18 A, 18B respectively connecting chip connection portion 16 to opposite end portions 14 A, 14B. It will be appreciated that RFID antenna 14 may have any appropriate shape or configuration that allows for performing RFID communications at a desired frequency. For example, a frequency for such RFID communications may be a frequency in the 860-960 MHz frequency band. However, it will be appreciated that, depending on the application, other frequencies can be used. Generally, RFID antenna 14 will have a longitudinal shape, and longitudinal direction x corresponds to the direction along which an extension of RFID antenna 14 is greatest. RFID antenna 14 may be formed on substrate in a known manner, for example, by coating substrate 12 with a metallic material, gluing RFID antenna 14 to substrate 12, or the like. Etching may be performed on a metallic layer on substrate 14 to obtain a desired shape of RFID antenna 14. [14] The pair of opposite end portions 14 A, 14B can have any appropriate shape, provided that the desired properties of RFID antenna 14 can be obtained. It will be appreciated that, generally, in case of RIFD antenna 14 being a dipole antenna, RFID antenna 14 will be symmetric, i.e., the shape of opposite end portions 14A, 14B will also be symmetric.

[15] In accordance with the present disclosure, an RFID chip 20 is electrically connected to chip connection portion 16, which is a central connection portion and arranged close to the center of substrate 12 in longitudinal direction x. RFID chip 20 is configured to perform RFID communications via RFID antenna 14 at a first frequency in a known manner, with connection portion 16 serving as a feeding point or loop for RFID chip 20 in a known manner.

[16] In the example shown in Fig. 1, each of opposite end portions 14A, 14B forms an extended area portion having a thickness wl in a direction y perpendicular to longitudinal direction x that is greater than a thickness w2 of the intermediate portion 18 A, 18B connected to the same in the direction y perpendicular to longitudinal direction x. However, the shape of end portions 14A, 14B is not limited to the shape shown in Fig. 1, and may simply be a linear (non-extended) end portion of RFID antenna 14.

[17] Intermediate portions 18 A, 18B may connect opposite end portions 14 A, 14B to central chip connection portion 16 in any appropriate manner. In the example shown in Fig. 1, each intermediate portion 18 A, 18B is essentially Z-shaped, with two portions extending along longitudinal direction x being joined by a third portion extending along the direction y orthogonal to longitudinal direction x. However, it will be appreciated that a shape of each intermediate portion 18A, 18B may also be a linear shape, i.e., a point of connection between each end portion 14 A, 14B and the associated intermediate portion 18A, 18B may be on the opposite side of substrate 12 in the direction y, or any other appropriate shape, for example, including one or more loops or zigzag patterns. [18] Central chip connection portion 16 is, as previously mentioned, arranged essentially at the center of substrate 12 in longitudinal direction x. For example, central chip connection portion 16 may include a linear portion connecting both intermediate portions 18 A, 18B and extending along longitudinal direction x, for example, with a width that is greater than that of intermediate portions 18 A, 18B, and includes a pair of opposing arms 16 A, 16B including opposing end portions 19A, 19B. In the example shown in Fig. 1, the pair of opposing arms 16 A, 16b extends from the linear portion of chip connection portion 16 on one side of substrate 12 towards an opposite side of the same in the direction y.

[19] A slot S is formed between opposing end portions 19A, 19B to separate the same, and RFID chip 20 is electrically connected to the pair of opposing end portions 19 A, 19B across slot S in a known manner, for example, by soldering or the like. The configuration of end portions 19 A, 19B and slot S will be described in more detail in the following.

[20] In accordance with the present disclosure, a pair of metallic strips 22A, 22B is arranged on substrate 12 separate from RFID antenna 14, (i.e., spaced apart and without being electrically connected to the same). Each of the pair of metallic strips 22A, 22B extends along at least part of an associated one of the pair of intermediate portions 18 A, 18B, as shown in Fig. 1. In the examples shown in the figures, metallic strips 22A, 22B are arranged on the same surface of substrate 12 (i.e., in a same plane) as RFID antenna 14. In other embodiments, however, metallic strips 22A, 22B may also be arranged in a different plane, for example, on an opposite surface of substrate 12, and may overlap RFID antenna 12 at least in part in a plan view of substrate 12.

[21] For example, as shown in Fig. 1, the pair of intermediate portions 18 A, 18B may each include a linear portion extending along longitudinal direction x, and the pair of metallic strips 22A, 22B may extend parallel to the linear portions of the pair of intermediate portions 18A, 18B. Metallic strips 22A, 22B may be formed, for example, from Al or any other appropriate conductive metallic material. As initially described, the presence of metallic strips 22 A, 22B allows to control the current distribution on the respective antenna portions of RFID antenna 14, due to absorbing at least a portion of the heat that is generated when RFID tag 12 is exposed to high-power electromagnetic fields such as microwave radiation. In particular, metallic strips 22A, 22B introduce extra traces to flow the currents that are generated, and serve to keep the current density low on the adjacent portions of RFID antenna 14.

[22] Here, it will be appreciated that metallic strips 22A, 22B may have any desired shape or length, as long as they extend along the portions of RFID antenna 14 that need to be protected the most. In the example shown in Fig. 1, said portions are the relatively thin portions of intermediate portions 18 A, 18B, as well as the portions of arms 16 A, 16B of chip connection portion 16 adjacent to metallic strips 22 A, 22B.

[23] In some embodiments, the pair of metallic strips 22 A, 22B may extend up to and along opposite end portions 14 A, 14B. In other words, as shown in Fig. 1, metallic strips 22A, 22B may extend up to or beyond the distal ends of RFID antenna 14. In the example shown in Fig. 1, where the opposite end portions 14 A, 14B form an extended area portion, the pair of metallic strips 22 A, 22B may extend adjacent to said extended area portions at least in longitudinal direction x. In other embodiments, metallic strips 22A, 22B may also extend at least partially around end portions 14 A, 14B, for example, extended area portions of the same. In other words, although not shown in Fig. 1, each metallic strip

22 A, 22B may have an additional portion that extends adjacent to the associated end portion 14A, 14B also, for example, in the direction y perpendicular to longitudinal direction x. In other words, each metallic strip 22A, 22B may extend at least partially around the associated end portion 14 A, 14B.

[24] Regardless of whether or not metallic strips 22A, 22B extend around extended area portions of end portions 14A, 14B, as shown in Fig. 1, each extended area portion may include a rounded comer portion having a radius r adjacent to the associated one of the pair of metallic strips 22A, 22B. Radius r may be predefined, and result in that a distance between metallic strip 22A, 22B and the associated end portion 14A, 14B increases towards a distal end of RFID antenna 14. A radius of curvature of the extended area portions may be, for example, between 4 and 8. A maximum distance between metallic strips 22A, 22B and the end portions 14 A, 14B facing the same may be between 2 mm and 4 mm. Radius r may be between 4 mm and 8 mm, for example, around 6 mm.

[25] In the example shown in Fig. 1, each of the pair of metallic strips 22 A, 22B extends at least in part between chip connection portion 16 and the associated one of the pair of intermediate portions 18A, 18B. For example, each of the pair of metallic strips 18A, 18B may be substantially L-shaped, with a first (e.g. short) leg arranged between the respective intermediate portion 18 A, 18B and the arm 16 A, 16B of chip connection portion 16 adjacent to the same. A second (e.g. long) leg of each metallic strip 22A, 22B may extend in longitudinal direction x towards a distal end of RFID antenna 14 and substrate 12. In this manner, as previously described, all the important portions of RFID antenna 14, in which the high-power electromagnetic field may produce a high current density, can be protected in an appropriate manner.

[26] Returning to the description of chip connection portion 16 and slot S, in the example shown in Fig. 1, the pair of opposing end portions 19A, 19B of arms 16A, 16B forms a pair of disc-shaped pad portions separated by slot S. Centers of the pair of pad portions are offset from each other in the direction y perpendicular to longitudinal direction x, as shown in Fig. 1. RFID chip 20 is connected to the disc-shaped pad portions of end portions 19A, 19B in an appropriate manner, as indicated by the dashed line in Fig. 1. In the example shown in Fig. 1, each pad portion is connected to a portion of arms 16A, 16B extending in the direction y perpendicular to longitudinal direction x by a curved portion, where the curved portions of the two arms 16 A, 16B are point-symmetric with respect to a center of slot S. With this structure of chip connection portion 16, the current density at the feed point, i.e., the point at which RFID chip 20 is connected to chip connection portion 16 can be further reduced. [27] With the configuration of end portions 19 A, 19B as shown in Fig. 1, a minimum distance defined by slot S between the disc-shaped pad portions is, for example, about 0.1 mm, and said minimum distance increases up to a maximum distance of, for example, 0.2 mm on the outer sides of slot S. This configuration serves to additionally reduce the current density at end portions

19 A, 19B to which RFID chip 20 is connected, and to improve the durability of the connection.

[28] Fig. 2 shows another embodiment of an exemplary RFID tag 10 in accordance with the present disclosure. As shown in Fig. 2, the exemplary RFID tag 10 differs from the RFID tag 10 shown in Fig. 1 in the configuration of the pair of opposing end portions 19A, 19B. In the example shown in Fig. 2, the pair of opposing end portions 19 A, 19B includes a pair of interlocking fingers 21 A, 21B defining slot S. A total length of slot S is between 10 and 30 mm, preferably between 15 and 20 mm. A width of slot S may be between 0.1 and 0.2 mm.

[29] In the example shown in Fig. 2, each end portion 19 A, 19B includes two interlocking fingers 21 A, 21B. However, it will be appreciated that in other embodiments, the number may be more than two. As shown in Fig. 2, at least some of interlocking fingers 21 A, 21B may have a plurality of protrusions and depressions interlocking with corresponding depressions and protrusions provided in the adjacent finger of the other end portion 19A, 19B. This serves to further increase the length of slot S at the position where RFID chip 20 (not shown in Fig. 2) is connected.

[30] As also shown in Fig. 2, in some embodiments, a width of a first leg of metallic strip 22A, 22B extending along intermediate portion 18 A, 18B may be smaller than a width of a second leg of metallic strip 22A, 22B extending between chip connection portion 16 and intermediate portion 18A, 18B. This can further reduce the current density in the vicinity of connection portion 16, in particular, the point at which RFID chip 20 is connected to the same.

[31] Fig. 3 shows another embodiment of an exemplary RFID tag 10 in accordance with the present disclosure. The embodiment shown in Fig. 3 differs from the embodiment shown in Fig. 2 in the configuration of end portion 19 A, 19B, as well as the arrangement of the same on substrate 12. As shown in Fig. 3, in the present example, each end portion 19 A, 19B includes three interlocking fingers, which, however, do not include the protrusions and depressions shown in Fig. 2.

[32] Additionally, in the example shown in Fig. 3, the pair of opposing end portions 19A, 19B is arranged in a central portion of substrate 12, i.e., further towards a center of the same in the direction y perpendicular to the longitudinal direction x. This results in that there is a distance d of at least 3 mm, preferably more than 4 mm from edges of substrate 12 in the direction y perpendicular to longitudinal direction x to end portions 19A, 19B. With this structure, a length of slot S may be greater than in the example shown in Fig. 2, for example, between 20 and 40 mm, preferably between 20 and 30 mm.

[33] In the previously described examples, RFID antenna 14 has been described as a dipole antenna. However, it will be appreciated that, in other embodiments, RFID antenna may be a different antenna, for example, a monopole antenna. In this case, one of end portions 14A, 14B is omitted, and the same applies to one of intermediate portions 18A, 18B. Accordingly, only a single metallic strip 22A, 22B is provided and extends along at least part of intermediate portion 18 A, 18B in the above-described manner. The remaining configuration described above may be the same. A dashed line in Fig. 3 indicates an exemplary configuration in case a monopole antenna is used, with the portion left of the dashed line being omitted, and the portion on the right side forming RFID tag 10.

[34] As previously described, RFID chip 20 is configured to perform RFID communications via RFID antenna 14 at a first frequency, for example, in the 860-960 MHz frequency band. Depending on the application, it will be appreciated that the high-power electromagnetic field to which RFID tag 10 may be exposed may have a second, different frequency, in particular, a frequency that is considerably higher. Accordingly, it will be appreciated that it is advantageous to adapt a length of metallic strips 22A, 22B to a (resonance) frequency corresponding to the second frequency. In particular, when the second frequency is a frequency that is typically used in microwave applications, such as 2.45 GHz, it may be advantageous when a length of each of the pair of metallic strips 22A, 22B is between 20 mm and 40 mm, preferably between 30 mm and 35 mm.

[35] An exemplary width of each of the pair of metallic strips 22A, 22B may be between 1 mm and 4 mm, preferably between 1 mm and 3 mm. Further, an exemplary distance between metallic strips 22A, 22B and the portions of RFID antenna 14 adjacent to the same, for example, the pair of intermediate portions 18 A, 18B, may be between 0.5 mm and 2 mm, preferably about 1 mm.

[36] It should be appreciated that, although in the foregoing the example of microwave applications was described, the present disclosure can also be used in combination with other applications, i.e., any applications in which an RFID tag is subjected to electromagnetic radiation at high power with a frequency that is different from the frequency that is used for the RFID communications to be performed. As previously mentioned, in particular the length of metallic strips 22A, 22B may be adjusted accordingly.

Industrial applicability

[37] As described above, with the technique disclosed herein, an RFID tag 10 can be achieved in a simple and cost-effective manner, which RFID tag 10 may be used in high-power, in particular, high-frequency applications such as in microwave ovens.

[38] An exemplary design in accordance with the present disclosure has been practically tested in a microwave oven at 1000 W, using a PET inlay. The RFID tag was heated ten times in total in the microwave oven, for durations between five minutes and thirty minutes. It could be verified that the RFID tag was still functional after being heated several times for such extended periods of time. In particular, the RFID tag disclosed herein was also successfully tested and verified on ceramic material such as a bowl or the like. Accordingly, with the present techniques, it becomes possible to provide a reusable RFID tag 10 to be attached to a food container such as a bowl or a glass, where the tag can be read by an associated reader even after several uses, for example, inside a microwave oven.

[39] It will be appreciated that the foregoing description provides examples of the disclosed systems and methods. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the general disclosure.

[40] Recitation of ranges of values herein are merely intended to serve as a shorthand method for referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All method steps described herein can be performed in any suitable order, unless otherwise indicated or clearly contradicted by the context.

[41] Although the preferred embodiments of the present disclosure have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.

Claims

1. An RFID tag (10) comprising: a substrate (12); an RFID antenna (14) arranged on the substrate (12), the RFID antenna including a chip connection portion (16), and an intermediate portion (18A, 18B) extending from the chip connection portion (16) to an end portion (14A, 14B) of the RFID antenna (14) in a longitudinal direction (x) of the RFID antenna (14); an RFID chip (20) electrically connected to the chip connection portion (16) and configured to perform RFID communications via the RFID antenna (14) at a first frequency; and a metallic strip (22A, 22B) arranged on the substrate (12) separate from the RFID antenna (14), the metallic strip extending along at least part of the intermediate portion (18A, 18B).

2. The RFID device of claim 1, wherein the intermediate portion (18A, 18B) includes a linear portion extending along the longitudinal direction (x), and the metallic strip (22A, 22B) extends parallel to the linear portion of the intermediate portion (18A, 18B).

3. The RFID device of claim 1 or 2, wherein the metallic strip (22A, 22B) extends up to and along the end portion (14 A, 14B).

4. The RFID device of claim 3, wherein the end portion (14A, 14B) forms an extended area portion having a thickness (wl) in a direction (y) perpendicular to the longitudinal direction (x) that is greater than a thickness (w2) of the intermediate portion (18 A, 18B) in the direction (y) perpendicular to the longitudinal direction (x), and the metallic strip (22 A, 22B) extends adjacent to the extended area portion in the longitudinal direction (x).

5. The RFID device of claim 4, wherein the extended area portion includes a rounded corner portion having a radius (r) adjacent to the metallic strip (22A, 22B).

6. The RFID device of claim 4 or 5, wherein the metallic strip (22A, 22B) extends at least in part around the extended area portion, for example, in the direction (y) perpendicular to the longitudinal direction (x).

7. The RFID device of any one of claims 1 to 6, wherein the metallic strip (22A, 22B) extends at least in part between the chip connection portion (16) and the intermediate portion (18A, 18B).

8. The RFID device of any one of claims 1 to 7, wherein the metallic strip (22A, 22B) is substantially L-shaped, preferably, wherein a width of a first leg of the metallic strip extending along the intermediate portion (18A, 18B) is smaller than a width of a second leg of the metallic strip extending between the chip connection portion (16) and the intermediate portion (18A, 18B).

9. The RFID device of any one of claims 1 to 8, wherein the RFID antenna (14) is a dipole antenna and includes a pair of the intermediate portions (18A, 18B) respectively connected to a pair of opposite end portions (14A, 14B), and the RFID device includes a pair of the metallic strips (22 A, 22B) respectively extending along at least part of an associated one of the pair of the intermediate portions (18A, 18B).

10. The RFID device of any one of claims 1 to 9, wherein the chip connection portion (16) includes a pair of opposing arms (16A, 16B) including a pair of opposing end portions (19A, 19B), respectively, a slot (S) being formed between the opposing end portions (19A, 19B) to separate the same, and the RFID chip (20) being electrically connected to the pair of opposing end portions (19A, 19B) across the slot (S).

11. The RFID device of claim 10, wherein the pair of opposing end portions (19A, 19B) each include a plurality of interlocking fingers (21 A,

2 IB) defining the slot (S), a length of the slot (S) being between 10 and 40 mm, preferably between 15 and 30 mm.

12. The RFID device of claim 10, wherein the pair of opposing end portions (19A, 19B) forms a pair of disc-shaped pad portions separated by the slot (S), centers of the pair of pad portions being offset from each other in a direction (y) perpendicular to the longitudinal direction (x).

13. The RFID device of any one of claims 10 to 12, wherein the pair of opposing end portions (19A, 19B) is arranged on the substrate (12) with a distance (d) of at least 3 mm, preferably more than 4 mm, from edges of the substrate (12) in a direction (y) perpendicular to the longitudinal direction (x).

14. The RFID device of any one of claims 1 to 13, wherein a length of the metallic strip (22A, 22B) is between 20 mm and 40 mm, preferably between 30 mm and 35 mm.

15. The RFID device of any one of claims 1 to 14, wherein the substrate (12) is formed from a material having a relative permittivity a of between 2 and 4, preferably between 2.5 and 3.5, for example, PET.