WO2011141860A1

WO2011141860A1 - Wideband uhf rfid tag

Info

Publication number: WO2011141860A1
Application number: PCT/IB2011/052017
Authority: WO
Inventors: Elies Ennabli
Original assignee: Assa Abloy AB
Current assignee: Assa Abloy AB
Priority date: 2010-05-14
Filing date: 2011-05-06
Publication date: 2011-11-17
Anticipated expiration: 2012-11-14

Abstract

The tag comprises essentially a patch antenna folded around a dielectric support whereby the patch antenna is formed essentially by an elongated continuous conductive band which is folded in order to define at least a top elongated part (2), a back part (4) and a bottom part (5), the top part comprises two elongated slots (6,7) which are positioned side by side and parallel to the length of said top part, said slots defining a middle zone of the conductive band between them, said middle zone comprising a first part (8) and a second part (8') of different lengths, which are separated from each other by a non conductive gap, and a RFID chip is connected over the said gap to each of said first and second parts (8,8').

Description

WIDEBAND UHF RFID TAG

TECHNICAL FIELD

The present invention concerns the field of RFID tags, more specifically the field of compact UHF wideband tags showing a patch antenna.

Such tags are used for identifying or marking humans, animals or objects, for example beer kegs and other similar products such that they may be read while going through a portal at a distance of about 4 to 5 meters.

BACKGROUND ART UHF passive (battery free) tags can have multiple form factors and electrical properties, depending essentially on the choice of the antenna type. In our case, we were willing to combine the smallest form factor possible and with a broad frequency band to be operational in the different markets Europe, USA and Japan each having specific ranges of frequencies (Europe: 865-869 MHz, USA: 902 - 928 MHz, Japan: 952-954 MHz), all in between 860 MHz and 960 MHz.

To get good radiation characteristics (or antenna gain), the antenna should be in harmony with the wavelength and therefore its size should be comparable to this wavelength. At usual operating frequencies of UHF RFID, this means that the antenna should be quite large and not practical. Several approaches have been introduced to reduce the antenna dimensions and maintain good radiation properties. PI FA antennas and shorted patch antennas are the most popular in the wireless and communication fields. Antennas with some similar properties are known in the prior art and disclosed in the following publications (all incorporated by reference in the present application). WO 96/27219 shows a planar meandering inverted-F antenna, which is in one embodiment a broadband omnidirectional radiator and in another embodiment a narrow band omnidirectional radiator. The meandering inverted-F is a planar radiating structure having alternating cutouts along a longitudinal dimension of a planar radiating element or patch which is parallel to a nearly coextensive ground plane. In all cases the antenna structure as a whole has the advantage of an efficient omnidirectional radiation pattern from a structure which has a maximum dimension of less than 1/5 of the wavelength of the operating frequency and preferably as small as 1/10 of the wavelength of the operating frequency. Factors which impact frequency and bandwidth include: meandering patch length, vertical element (also called "post") height and width, and permittivity of the dielectric spacer between the meandering patch and the ground plane. The structure is easily manufactured due to its simple design and absence of requirements of exotic materials or multidimensional shaping processes.

US 2002/180650 shows an electrically tunable multiband planar antenna. More specifically, this prior art describes an antenna arrangement for mobile telecommunication devices comprising a planar radiating element, a ground plane parallel to the planar radiating element and means for providing a grounding connection between the planar radiating element and the ground plane. The means for providing a grounding connection between the planar radiating element and the ground plane comprise a number of grounding connections. A number of them is independently controllable between certain states and the state of each independently controllable grounding connection has an effect on certain operational frequency bands of the antenna arrangement. US 2006/164303 illustrates a multiband planar broadband patch antenna, in particular for transmitting and/or receiving digital and/or analogue terrestrial television UHF/SHF signals, having a bandwidth low frequency tuned reflector and a radiator connected to a specific power supply and radiating in a frequency F1 . The radiator also has a slot tuned to a frequency F2. The antenna is characterized in that the radiator has another slot tuned to a frequency F3 different from frequencies F1 and F2, the slots being connected through a linking slot designed to constitute a coupling line to provide a substantially identical electromagnetic current at each of the slots of frequency F2, F3, respectively.

US 2005/259007 shows a surface-mounted antenna and portable wireless device incorporating the same with a ground electrode provided on a first surface of a dielectric body. A radiation electrode has a first end which is left open and a second end which is connected to the ground electrode. A feeding terminal is provided on the first surface. A feeding electrode has a first end which is connected to the feeding terminal and a second end which is connected to the ground electrode. At least a part of the feeding electrode is extended in parallel with an elongated direction of the radiation electrode, so as to excite the radiation electrode with an induction coupling in a non-contact manner.

US 2006/032926 discloses a Radio frequency identification (RFID) tag and manufacturing method thereof wherein an RFID tag includes a dielectric member, an antenna pattern formed on and around a surface of the dielectric member, and an IC chip that is electrically connected to the antenna pattern by means of two chip pads.

US 2009/046019 discloses an antenna apparatus which enables switching directivity suitable for a plurality of usage patterns of a wireless terminal, such as that achieved during voice conversation and that achieved during data communication, and is easily slimmed down, as well as providing a wireless terminal using the antenna apparatus. An antenna apparatus includes a linear radiating element placed on a first, plane; a first parasitic element placed on the first plane in parallel with the radiating element; a first ground conductor placed on the first plane; a first switch which connects both ends of the first parasitic element to the first ground conductor; and a second ground conductor placed on a second plane opposing the first plane, wherein a part of the first ground conductor is placed in parallel with the radiating element and on a side opposite the first parasitic element with the radiating element sandwiched therebetween; and the second ground conductor is placed opposite the radiating element, and ends of the second ground conductor oppose an area sandwiched between the radiating element and the first parasitic element.

US 2006/145927 discloses a planar inverted-F antenna (PIFA) which has a Co- Planar Waveguide (CPW) feeding structure and can be attached to a metal surface, and an RFID tag using the same. The PIFA includes a radiation patch layer; a Co-Planar Waveguide (CPW) feeding layer; a feeding probe; and a short-circuit. The CPW feeding layer includes a feeding means and a ground surface. The feeding probe electrically connects the radiation patch layer and the feeding means and provides a Radio Frequency (RF) signal to be radiated to the radiation patch layer. The short-circuiting means short-circuits the radiation patch layer and the ground surface through the dielectric layer. The PIFA can be applied to a passive RFID tag. Impedance matching between the antenna and the RFID chip is possible. Also, the PIFA can easily control resonant frequency of the antenna and reactance

US 2007/229276 discloses a RFID tag having a loop antenna and comprises: a flat plate shaped dielectric member; first and second loop antenna patterns that are formed on a first and second surface of the dielectric member so that they are separated from each other by a specified space, and so that each is continuous from the first surface to the second surface of the dielectric member; and an IC chip that electrically connects the first and second loop antenna patterns on one of the surfaces. WO 2008/006947 discloses a radio frequency identification tag which is adapted to work at a frequency between 850 MHz and 950 MHz. The tag may comprise a planar inverted F antenna, a loop antenna, or a dual patch antenna.

In addition to these patent prior art documents, there is also non-patent literature in the field of the present invention. These articles include:

-) G. Marrocco et al., Body matched slot Antennas for radiofrequency identification, found on the internet Oct. 28 2009, 4p.

-) G. Marrocco, The art of UHF RFID antenna design : impedance matching and size reduction techniques, IEEE Antennas and Propagation Magazine, vo.50, N.1 , Jan. 2008, 21 p.

SUMMARY OF THE INVENTION

An aim of the present invention is to improve the known antennas and tags of the prior art.

More specifically, an aim of the present invention is to propose a tag that is operational in the different markets of Europe, USA and Japan each having specific ranges of frequencies (Europe: 865-869 MHz, USA: 902 - 928 MHz, Japan: 952 - 954 MHz), and that has an antenna with a relatively small form factor compared to the antennas proposed by competitors. The antenna and the tag according to the present invention are defined by the content of the claims and comprise among others the following features:

- A slot in H-shape is cut on the top part of the patch antenna in order to create double resonance frequencies;

- The width of each H-arm is very narrow compared to its length to lower the frequency ratio and obtain a broadband performing antenna); - A chip bridge is positioned asymmetrically on the centre of H-Slot to also lower the frequency ratio

- A folded part of the antenna allows a substantial form factor reduction while good electrical performances are still possible. The back part of the patch antenna creates a so called "short-circuited patch antenna effect" which allows a substantial antenna gain increase thanks to the bottom conductive part additional radiation. The antenna overall length can be reduced while keeping simultaneously good radiation performances thanks to this technique;

- The patch is fixed on / wrapped around an ABS dielectric support, which is hollow.

More specifically, the RFID tag according to the present invention comprises essentially a patch antenna folded around a dielectric support , where the patch antenna is formed essentially by an elongated continuous conductive band which is folded in order to define at least a top elongated part, a back part and a bottom part, said top part comprising two elongated slots which are positioned side by side and parallel to the length of said top part, said slots defining a middle zone of the conductive band between them, said middle zone comprising a first part and a second part of different lengths, which are separated from each other by a non conductive gap, and a RFID chip is connected over the said gap to each of said first and second parts.

A whole tag is relatively simple to manufacture and assemble, since the entire antenna comes in a single inlay wrapped around the inner dielectric support. The support may be in any dielectric material, for example ABS, Polycarbonate, PTFE or another equivalent material.

According to the present invention, the folded patch H-slot antenna (with H arms being long compared to the arms width), succeeds to achieve the folwowing requirement simultaneously: it is very cheap and simple to manufacture, it performs well over a large frequency range (wideband characteristic) and it has acceptably small form factor. The different dimensions of the folded patch H-slot antenna should be determined according to where the tag is used. Whether the tag is placed on metal, plastic or other material, the antenna dimensions should be adapted in order to have the best performance. By performance one mean: maximum read range at a certain operating frequency.

The folded patch H-Slot antenna creates double resonance frequencies in the UHF region (one resonance frequency only in this region if no H-Slot). The intrinsic shape characteristic of H-slot helps in creating two resonances with a predetermined frequency ratio between both.

The H-slot has also this characteristic that the chip (bridge) is positioned asymmetrically on the centre of the H-slot. This is important as if chip is centred the wideband property of the tag disappears (frequency ratio too high). Additionally, the position of the chip (bridge) influences the radiation pattern and as such allows focussing radiation energy more in a direction than in another.

Furthermore, due to the patch folding, the overall length of the antenna is substantially reduced (halved), while the performance is kept excellent at these two resonance frequencies. This is because the patch folding increases the antenna gain. In fact the bottom conductive layer contributes substantially to the overall antenna radiation, so the loss in gain due to the shortening of the top part (without folding) is compensated by the folded bottom part. The patch folding is a variant of the well-known technique used for size-reducing the half- wave patch antennas to quarter-wave patch antennas. This technique consists in short-circuiting the top part to bottom part of the standard patch antenna ("shorted patch"). By lengthening the bottom conductive area of the patch antenna, both resonance frequencies are adjusted simultaneously, however the delta frequency is kept less or more constant. Assuming the form factor is fixed (tag not to exceed certain height, certain length and certain width), the most important parameters for impedance (performance) optimization are the dimensions of the slots, the bottom conductive length/width, the gap between bottom and top conductive parts, material of the dielectric material (the antenna support) .

All these parameters allow determining precisely the position and the form of the two desired resonances. As said above, we want one of the resonance to be around 860 MHz (first resonance frequency) and the other at 960 MHz (second resonance frequency). The combination of both resonance peaks should result in a total frequency response higher than a minimal required (operational) response in the entire frequency band between these two frequencies. This is the desired wideband effect and figure 10 schematically illustrates the two resonant frequencies.

Following is the method describing how one achieves this goal.

Size of the slot: because modifying the dimensions of the slots modifies the impedance of antenna seen by chip, the lengthening of the slot lowers the higher (second) resonance frequency. Widening the slot also lowers the higher resonance frequency.

Size of the bottom conductive part: lengthening/widening the lower conductive area of the antenna, results in higher resonance for first and second frequencies. As mentioned before, by lengthening the bottom conductive part of the patch antenna, both resonance frequencies are adjusted simultaneously, however the delta frequency is kept more or less constant.

Bottom part - top part gap distance: the gain increase/decrease depending on the height. The gap distance has mostly influence on the first resonance frequency.

Dielectric material (the antenna support): any material with higher dielectric constant than air positioned in the gap between the top part and the bottom part of the patch antenna down-shifts the resonance frequencies

• Top part: the total surface area has influence on the resonance frequencies.

In summary, by lengthening, widening the slots and/or by making the bottom conductive area shorter or longer, one can control both resonance frequencies and/or frequency ratio for optimization of the read range performance of the UHF tag.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a top view of the antenna according to the present invention; Figure 2 illustrates a side view of an antenna according to the present invention;

Figure 3 illustrates a side perspective view of an antenna according to the present invention; Figure 4 illustrates a bottom view of an antenna according to the present invention;

Figure 5 illustrates a top view of an antenna mounted on a support according to the present invention;

Figure 6 illustrates a side view of an antenna mounted on a support according to the present invention;

Figure 7 illustrates a side perspective view of an antenna mounted on a support according to the present invention; Figure 8 illustrates a bottom view of an antenna mounted on a support according to the present invention;

Figure 9 illustrates an embodiment of a tag comprising an antenna according to the present invention;

Figure 10 illustrates schematically the two resonant frequencies of the tag according to the present invention. DETAILED DESCRIPTION

An embodiment of the antenna and of the tag according to the present invention is described by reference to the figures 1-4 where the same parts are identified with the same references.

The antenna 1 has accordingly a first elongated top part 2 which is in principle flat. This top part 2 has at its front end a front curved end 3. At its back end, the top part also comprises back curved end 4, said back curved end extending then in a bottom part 5 of the antenna 1. Preferably, said bottom 5 is substantially flat and aligned with the top part.

The top part 2 in addition comprises two elongated slots 6, 7 which extend substantially parallel to each other along the top part 2. The slots are preferably dimensioned such that their length is far larger than their width. This is a result of the optimization of the antenna to meet the desired requirements (broadband, good read range, small form factor). H-slot antennas have usually less elongated slots, where the length and width of the slots are more or less equivalent in dimension, but rarely over-proportionally different in dimensions such as in the present invention.

Also preferably, the slots extend only on the top part 2 of the antenna 1 and neither on the first curved end 3 nor on the second curved end 4. Also, along the length of the slots, the middle zone of the antenna created by the slots 6, 7 is interrupted to form two distinct first and second parts 8 and 8'. Bridging these two antenna parts 8, 8', a chip 9 is then placed to form the tag as is usually done in the RFID field. The chip may be connected to the first and second parts 8, 8' by any known techniques, for example flip-chip bonding process technology or another equivalent manner.

Figures 5 to 8 illustrates the antenna of figures 1-4 mounted on a dielectric support 10 and the same parts/elements are identified with the same reference numbers as in figures 1-4, the description of which apply correspondingly to figures 5-8.

More specifically, the support 10 has an external surface 1 1 on which the antenna 1 is folded and then fixed, for example by gluing. The support is typically made of a synthetic material, for example ABS or another equivalent dielectric material and is fabricated (for example) by injection. The extremities of the support are rounded in order that the patch antenna can be smoothly wrapped around it without creating sharp edges or folds. This increases the mechanical stability of the patch antenna and eases its application on the support.

The support 0 comprises four cavities 2, 13 in its middle of which only two are illustrated in figures 6 and 8 (the two others are symmetrically placed on the other side, shown in figure 9, references 12', 13'). This reduce the material costs and the weight, without significantly degrading the positive effect of the dielectric material and the mechanical stability of the tag.

Figure 9 illustrates an example of a tag using the antenna and support as described above in the present specification in relation to figures 1-8. With reference to these figures and corresponding description, the same parts are identified with the same reference numbers. More specifically, in figure 9, the antenna 1 on a support 10 a placed in a container 20 having a corresponding shape. Once in the container 20, said container 20 is closed with a lid 21 which ultra-sonic welded (or attached by other equivalent means). The container comprises in addition a flange 22 which is used in cooperation with a corresponding ring 23 for final fixation on the object to be marked. Typically, the ring may be made in metal for on-metal application. Typical sizes (only given as illustrative examples) are the following for an on- metal tag (tag affixed on metal surface):

- total length of the antenna of about 20 mm;

- total width of the antenna: about 13 mm

- length of the slots 6, 7: about 52 mm

- width of a slot 6, 7: about 1 mm

- length of the middle part 8: about 37 mm

- width of middle part 8, 8': about 2 mm

- bottom part - top part gap distance: about 10 mm

- length of the dielectric support: about 65 mm

- width of the support: about 5 mm

- total thickness of the support: about 10 mm

- thickness of the walls of the support: about 2 mm

- curving radius of the extremities of the support: about 4.5 mm This antenna design can be used for any on-metal application (tag affixed on metal) and test results were excellent in term of read range and bandwidth at UHF frequencies. The bottom part of the antenna design is positioned in proximity to the metal surface, wherein a capacitive effect is created between the both surfaces. This capacitance is used to modify the impedance of the antenna and shift the desired resonances. In some applications, it is required that the tag is applied on non-conductive materials. In this case, the antenna bottom conductive part is no longer in proximity to any metal surface and one needs to optimize and adapt the antenna design to this new application. Accordingly, one way doing this is to enlarge the bottom conductive part to decrease resonance frequencies (in this case, 15 mm longer). Additionally, to have a better frequency ratio, one has to reduce the width of the slot by 0.25 mm.

Typical sizes for non-metal applications (only given as illustrative examples, however one can compare with the on-metal sizes counterpart) are the following:

- total length of the antenna of about 135 mm;

- total width of the antenna: about 13 mm

- length of the slots 6, 7: about 52 mm

- width of a slot 6, 7: about 0.75 mm

- length of the middle part 8: about 37 mm

- width of middle part 8, 8': about 2 mm

In specific application, a requirement is that the tag is to be mounted on both metal (for ex. beer kegs tagging) or on non-conductive materials (for ex. wood for pallet tagging). A solution is to integrate into the tag a metal strip/plate underneath the bottom part (in the packaging, for ex. in the lid 21 ). This would make the antenna as if it was used all the time on metal. The tag performance is therefore independent on where it is mounted.

Typically, the antenna is made by an etching process for example, but other equivalent processes may be used as well. The metal used for the antenna may be Al, Cu, Ag or any other equivalent material. The metal is applied on a substrate, for example of synthetic material such as PET. The thickness of the antenna (metal + substrate) is typically in the range from about 60 ym to about 100 pm. Of course other sizes may be used as well and the range given is only a non-limiting example. For example, if the metal used is Al, the thickness of the metal layer may be about 10 Mm and the thickness of the substrate may be about 50 pm. Other sizes are of course possible in the frame of the present invention.

It is also possible to add adhesive layers on the substrate to attach in a better way the metal layer.

As said above, the different sizes and relative shapes all have an influence on the characteristics of the antenna so that the dimensions mentioned in the present specification are only indicative and other values may be determined or chosen depending on the sought characteristics of the antenna and device.

Of course, all the values and embodiments are given by way of examples and variations are possible.

For example, the antenna may not comprise a first curved end 3 and may be shorter than illustrated.

Claims

1. A wideband RFID tag comprising essentially a patch antenna folded around a dielectric support and characterized in that:

- the patch antenna is formed essentially by an elongated continuous conductive band which is folded in order to define at least a top elongated part (2), a back part (4) and a bottom part (5),

- said top part comprises two elongated slots (6,7) which are positioned side by side and parallel to the length of said top part, said slots defining a middle zone of the conductive band between them, said middle zone comprising a first part (8) and a second part (8') of different lengths, which are separated from each other by a non conductive gap,

- a RFID chip is connected over the said gap to each of said first and second parts (8,8').

2. A tag as defined in claim 1 , showing a continuous operative frequency band between 860 MHz and 960 MHz.

3. A tag as defined in one of the preceding claims, wherein the length of said top part is at least twice its width.

4. A tag as defined in one of the preceding claims, wherein said bottom part is shorter than said top part.

5. A tag as defined in one of the preceding claims, wherein the slots extend on the major part of the length of the top part.

6. A tag as defined in one of the preceding claims, wherein the length of the said slots is far greater than their width, and in particular at least 10 time greater..

7. A tag as defined in one of the preceding claims wherein the gap distance between the top part and the bottom part defined by the thickness of the dielectric support is of at least 4 mm.

8. A tag as defined in one of the preceding claims wherein the dielectric support shows a hollow structure and/or cavities.

9. A tag as defined in one of the preceding claims wherein the dielectric support shows rounded extremities, such that the said folded patch antenna shows rounded transitions portions between respectively the top, the back and the bottom part .

10. A product comprising a tag as defined in one of the preceding claims.