WO2009011599A1

WO2009011599A1 - An rfid tag

Info

Publication number: WO2009011599A1
Application number: PCT/NZ2008/000169
Authority: WO
Inventors: Mathias Martin Ernest Ehlen
Original assignee: Times-7 Holdings Limited
Priority date: 2007-07-18
Filing date: 2008-07-17
Publication date: 2009-01-22

Abstract

A radio frequency identification (RFID) tag including a radio frequency transponder driving an antenna and a reactive element which form resonant circuit tuned to a desired frequency band. The antenna may be notch antenna or a slot antenna and may include a reflector and/or one o more director. The reactive element may be a capacitive element in the form of a conductive strip spaced apart from the antenna or capacitively coupled to the antenna by a further conductive strip. The RFID tag may be incorporated into a credit card which may also be provided with a magnetic stripe. The RFID tag provides a compact design with high gain suitable fo long range passive RFID applications such as vehicle identification or sports timing.

Description

AN RFID TAG

FIELD OF THE INVENTION

This invention relates to a radio frequency identification tag (RFID tag) suitable for use in a wide range of RFID tag applications including vehicle identification and sports timing applications.-

BACKGROUND OF THE INVENTION

RFID tags are generally classified into "passive" tags which utilise the energy of received RF radiation to transmit RF radiation including an identification number of the RFID tag and "active" tags which utilise a power source, such as an internal battery, to power RF transmission from the tag.

Active tags generally have superior range and performance but are more expensive and, for extended use, require an external power supply or periodic power supply replacement. Passive tags require no power supply but do not have suitable performance at the range required in some more demanding applications such as vehicle identification and sports timing.

A range of antenna designs have proposed to enhance the operating range of RFID antennas. Such antennas must both effectively receive incident RF transmissions and propagate outgoing RF transmissions. Whilst many antenna designs have been proposed they generally do not provide a sufficient operating range to be effective in applications such as vehicle identification and sports timing, whilst also achieving a small form factor. It would be desirable to provide a compact passive RFID tag operable over a sufficient range to make it useful in vehicle identification and sports timing applications or to at least provide the public with a useful choice.

EXEMPLARY EMBODIMENTS

According to one exemplary embodiment there is provided a radio frequency identification tag for operation in a desired frequency band 'comprising: a. an antenna; b. a reactive element; and c. a radio frequency transponder driving the antenna and reactive element, wherein the antenna and reactive element are dimensioned and arranged to form a resonant circuit tuned to the desired frequency band.

According to another exemplary embodiment there is provided a radio frequency identification tag for operation in a desired frequency band comprising: > a. a notch antenna; b. a radio frequency transponder driving the notch antenna; and c. a director spaced apart from the notch antenna in the primary direction of propagation of the beam of the notch antenna in use.

According to a further exemplary embodiment there is provided a radio frequency identification tag for operation in a desired frequency band comprising: a. a slot antenna; b. a radio frequency transponder driving the slot antenna; and c. a director spaced apart from and in front of the slot antenna in the primary direction of propagation of the beam of the slot antenna in use. According to another exemplary embodiment there is provided a radio frequency identification tag for operation in a desired frequency band comprising: a. a notch antenna; b. a radio frequency transponder driving the notch antenna; and c. a reflector spaced apart from and behind the notch antenna in the primary direction of propagation of the beam of the notch antenna in use.

According to a still further exemplary embodiment there is provided a radio frequency identification tag for operation in a desired frequency band comprising: a. a slot antenna; b. a radio frequency transponder driving the slot antenna; and c. a reflector spaced apart from and behind the slot antenna in the primary direction of propagation of the beam of the slot antenna in use.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description of embodiments given below, serve to explain the principles of the invention.

Figure 1 shows an RFID tag having a folded notch antenna according to one embodiment;

Figure 2 shows an RFID tag having a folded notch antenna according to another embodiment; Figure 3 shows an RFID tag having a slot antenna according to one embodiment;

Figure 4 shows an RFID tag having a straight notch antenna according to another embodiment;

Figure 5 shows an RFID tag having a stepped notch antenna according to one embodiment; Figure 6 shows an RFID tag having a straight notch antenna with capacitively coupled limbs according to one embodiment;

Figure 7 shows an RFID tag having a folded notch antenna according to another embodiment;

Figure 8 shows an RFID tag having a flared notch antenna according to one embodiment;

Figure 9 shows an RFID tag in which the antenna includes a director;

Figure 10 shows an RFID tag in which the antenna includes a reflector;

Figure 11 shows an RFID tag incorporated into a credit card; and Figure 12 shows an RFID tag having a notch antenna according to another embodiment. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In one aspect of the present invention a reactive element is provided in conjunction with an antenna of an RFID tag to form a resonant circuit tuned to a desired frequency band. In the embodiments described below the reactive element is a capacitive element. However, the reactive element could be an inductive element where a predominantly capacitive antenna such as a patch is employed. The term "capacitive elenient" refers to the predominant reactive component of the element and it will be appreciated that such elements will have some inductive component. Likewise, whilst notch and slot antennas have a capacitive element they are predominantly inductive.

By providing an antenna and reactive element as a tuned resonant circuit, received RF radiation may be more effectively captured and transmitted radiation more effectively propagated. Whilst the antenna will be tuned for operation in a desired frequency band it may need to receive RF radiation at a first frequency and transmit it a second frequency. Depending upon the application the resonant circuit may be tuned to the first or second frequency or an intermediate frequency. If the efficiency of energy capture is most important for the application the resonant circuit may be tuned to the frequency of incident radiation whereas if transmission range is most important the resonant circuit may be tuned to the frequency of transmission.

Typically efficient energy capture is most important and so the resonant circuit will typically be tuned at the frequency of incident RF radiation.

Figure 1 shows an RFID tag 1 including a folded notch antenna 2 and a reactive element in the form of a capacitive element 3 driven by an RF transponder 4. The notch 5 tapers from , its proximal to its distal end to broaden the bandwidth of operation of the antenna. Capacitive element 3 is dimensioned and arranged so as to provide a resonant circuit with notch antenna 2 at the desired frequency of operation. Capacitive element 3 preferably runs substantially parallel to the body of notch antenna 2 and maintains a substantially constant spacing. RFID tag 1 will typically be provided mounted on a dielectric material which may have a conductive sheet forming a ground plane on the underside thereof, although in many applications a ground plane will not be provided to minimize unit cost. The conductive components 2 and 3 may be conveniently formed by etching a metal coated dielectric board or other methods known in the art. Where a ground plane is employed this may be used to form reactive elements also.

Straight, folded, tapered, flared or stepped notches may be used.

In this embodiment the circuit components have the following approximate values at an operating frequency of 866 MHz: Chip impedance 50-J400

Antenna Inductance +J1200 Antenna Capacitance -j1020 Total impedance 50-J220.

The tag is capacitively tuned in this embodiment and resonates at approximately 940MHz.

Figure 2 shows a variant of the antenna shown in figure 1 in which capacitive element 6 is shorter to tune antenna 7 for use with a different dielectric material. The length of the notch is also .shorter to tune for use with a different dielectric material.

Figure 3 shows a slot antenna embodiment including a slot antenna 8 having a slot 9 therein. RF transponder 10 drives slot antenna 8 and capacitive element 11. A slot antenna has the advantage of providing twice the gain of a notch antenna but it is also twice the length of a notch antenna (The preferred slot length is about a half wavelength). Again, capacitive element 11 forms a resonant circuit with the predominantly inductive slot antenna 8. Straight, folded, tapered, flared or stepped slots may be used.

Figure 4 shows an embodiment employing a straight notch antenna 12 having a straight notch 13. RF transponder 14 drives notch antenna 12 and capacitive element 15 to form a resonant circuit. This embodiment avoids circular polarization and provides high gain in one direction and so is useful in applications where the orientation of the RFID tag is controlled.

Figure 5 shows a stepped notch antenna embodiment. In this embodiment notch antenna 16 has a first notch section 17 and a second notch section 18 which enables the notch to be tuned to resonate at two frequencies whilst the resonant circuit of antenna 16, RF transponder 19 and capacitive element

20 resonates at a third frequency which may or may not be equal to either of the resonant frequencies of the notch. This allows the bandwidth of the antenna to be broadened.

Figure 6 shows a straight notch variant in which the notch antenna is formed of conductive limbs 21 and 22 capacitively coupled by a conductive sheet 23. Conductive sheet 23 is separated from limbs at 21 and 22 by a dielectric layer so that the coupling is capacitive. This capacitive element may be adjusted by selection of the dielectric material, spacing and dimensions to tune the resonant circuit of limbs 21 and 22, capacitive coupling 23 and

RFlD transponder 24 to resonate at the desired frequency.

Figure 7 shows a folded notch antenna utilizing this enhanced capacitive coupling technique. In this embodiment RF transponder 25 drives notch antenna 26 and capacitive elements 27 and 28. Capacitive element 28 is coupled to antenna 26 and capacitive element 27 in the form of a conductive sheet having a dielectric material interposed between element 28, and elements 26 and 27. By providing this enhanced capacitive coupling a smaller capacitive element 28 may be employed.

Figure 8 shows a flared notch antenna 29 having curvilinear walls to notch 30. Either linear or curvilinear notch side walls may be employed. RF transponder 31 drives capacitive element 32 and notch 'antenna 29 as in previous embodiments. Use of a flared notch is appropriate where a broad bandwidth of operation is required.

Figure 9 shows an embodiment employing a notch antenna 33 and a director 34. RF transponder 35 drives antenna 33 and capacitive element 36 as described above. Adding a director provides improved directionality (i.e. increased gain in the primary direction of propagation). Director 34 is preferably spaced a quarter wavelength (at the desired operating frequency) away from antenna 33 and is smaller than or equal to the size of antenna 33. Additional directors such as director 37 may be provided for improved antenna directivity. Such additional directors should diminish in size with distance from antenna 33 and are preferably spaced about 3λ/4, 5λ/4 or 7λ/4 respectively from the antenna where λ is the wavelength at the frequency of operation.

Figure 10 shows an embodiment in which a reflector 41 is employed in combination with a notch antenna 38. As above transponder 39, antenna 38 and capacitive element 40 form a resonant circuit. Reflector 41 enhances the gain of antenna 38 in the direction propagation (to the right in Figure 10). Reflector 41 must be of greater length than antenna 38 and is preferably spaced about a quarter wavelength away from antenna 38. Both a reflector and one or more director may be employed together to further enhance gain in one direction.

Figure 1 1 shows an embodiment in which an antenna 42, transponder 43, capacitive element 44 and director 45 are packaged within a credit card 46.

The conductive portions 42, 44 and 45 may be formed by etching a metal sheet mounted to a dielectric. The transponder may then be electrically connected to elements 42 and 44 and the assembly encased in a credit card. The director 45 may be replaced by a reflector, or both a reflector and director may be utilised if space permits at the wavelength of the operating frequency. Multiple directors could also be added. Such a credit card 46 may include a magnetic stripe 47 as well to enable the card to be used for other transactions, including use as a loyalty card. Such a card may be mounted to an item to be timed, such as a vehicle or bicycle etc. The magnetic stripe 47 may serve the additional function of being a reflector or director.

Figure 12 shows a final variant similar to that of, figure 1 but including an extended portion 48 extending from notch antenna 49. This portion 48 has been found to enhance gain in a direction orthogonal to the notch antenna 49. This portion can extend up to a quarter wavelength in any desired direction away from the closed end of the notch antenna 49 depending upon the application.

The above RFID tags are preferably operated at UHF (typically in the 840 to 960 MHz range) but could be adapted for us.e in other frequency bands. For the notch antenna embodiments the length of the notch is preferably a quarter wavelength. The folded notch tag design has achieved consistent reading distances of 5 meters making it suitable for use in sports timing and vehicle identification. Whilst the invention has been described in relation to RFID transponders that both receive and transmit RF signals the term "transponder" is used in this specification to include devices that only receive or only transmit RF signals also.

There is thus provided an inexpensive, compact, high gain RFID tag that efficiently captures received RF radiation to provide consistent read ranges allowing passive tags to be deployed in applications' such as vehicle identification and sports timing.

While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant's general inventive concept.

Claims

1. A radio frequency identification tag for operation in a desired frequency band comprising: a. an antenna; b. a reactive element; and c. a radio frequency transponder driving the antenna and reactive element, wherein the antenna and reactive element are dimensioned and arranged to form a resonant circuit tuned to the desired frequency band.

2. A radio frequency identification tag as claimed in claim 1 wherein the reactive element is a capacitive element.

3. A radio frequency identification tag as claimed in claim 2 wherein the capacitive element is in the form of a conductive strip spaced apart from and capacitively coupled to the antenna.

4. A radio frequency identification tag as claimed in claim 3 wherein the capacitive coupling is provided via a conductive bridge having an intervening dielectric layer.

5. A radio frequency identification tag as claimed in any one of claims 2 to 4 wherein the antenna is a notch antenna.

6. A radio frequency identification tag as claimed in claim 5 wherein the capacitive element is positioned adjacent a portion of the notch antenna remote from the point at which it is driven.

7. A radio frequency identification tag as claimed in any one of claims 3 to 6 wherein the antenna has a straight notch.

8. A radio frequency identification tag as claimed in any one of claims 3 to 7 wherein the antenna has a folded notch.

9. A radio frequency identification tag as claimed in 7 or claim 8 wherein the notch tapers outwardly towards the notch opening.

10. A radio frequency identification tag as claimed in claim 9 wherein the taper in notch is linear or curvilinear.

1 1. A radio frequency identification tag as claimed in 7 or claim 8 wherein the notch includes step transitions in its width.

12. A radio frequency identification tag as claimed any one of claims 3 to 1 1 wherein the length of the notch is about a quarter of the wavelength at the desired operating frequency.

13. A radio frequency identification tag as claimed in any preceding claim including a director.

14. A radio frequency identification tag as claimed in claim 13 wherein the director is equal to or smaller in length than the antenna.

1 5. A radio frequency identification tag as claimed in claim 13 or claim 14 wherein the director is spaced about a quarter of a wavelength of the desired operating frequency away from the antenna.

16. A radio frequency identification tag as claimed in any one of claims 13 to 15 including one or more additional director spaced away from the antenna at a distance of about 3λ/4, 5λ/4 or 7λ/4 respectively from the antenna where λ is the wavelength at the frequency of operation.

1 7. A radio frequency identification tag as claimed in claim 16 wherein each director decreases in size with its distance from the antenna.

18. A radio frequency identification tag as claimed in any one of the preceding claims including a reflector spaced apart from the antenna.

19. A radio frequency identification tag as claimed in claim 18 wherein the director is of greater length than the antenna.

20. A radio frequency identification tag as claimed in claim 18 or claim 19 wherein the reflector is spaced about a quarter wavelength away from the antenna.

21. A radio frequency identification tag as claimed in any one of claims 2 to 4 wherein the antenna is a slot antenna.

22.A radio frequency identification tag as claimed in any preceding claim including a ground plane spaced apart from the antenna.

23. A radio frequency identification tag as claimed in any preceding claim incorporated into a credit card.

24. A radio frequency identification tag as claimed in claim 23 wherein the credit card includes a magnetic stripe.

25. A radio frequency identification tag as¹ claimed in claim 24 wherein the magnetic stripe functions as a reflector or a director.

26.A radio frequency identification tag for. operation in a desired frequency band comprising: a. a notch antenna; b. a radio frequency transponder driving the notch antenna; and c. a director spaced apart from the notch antenna in the primary direction of propagation of the beam of the notch antenna in use.

27. A radio frequency identification tag for operation in a desired frequency band comprising: a. a slot antenna; b. a radio frequency transponder driving the slot antenna; and c. a director spaced apart from and in front of the slot antenna in the primary direction of propagation of the beam of the slot antenna in use.

28.A radio frequency identification tag for operation in a desired frequency band comprising: a. a notch antenna; b. a radio frequency transponder driving the notch antenna; and c. a reflector spaced apart from and behind the notch antenna in the primary direction of propagation of the beam of the notch antenna in use.

29. A radio frequency identification tag for operation in a desired frequency band comprising: a. a slot antenna; b. a radio frequency transponder driving the slot antenna; and c. a reflector spaced apart from and behind the slot antenna in the primary direction of propagation of the beam of the slot antenna in use.

30. A radio frequency identification tag as claimed in any one of the preceding claims including an antenna dimensioned for operation at ultra high frequency.

31. A radio frequency identification tag as claimed in any one of the preceding claims including a conductive portion extending from the antenna.