RFID Tag with Narrow Gap for Microwaveable Food Packaging

Abstract

An antenna (1) for an RFID (Radio Frequency Identification) tag (100), comprising: two antenna parts (11a, 11b), arranged in opposite end regions of the antenna; and at least one intermediate part (12) forming a bridge between the antenna parts. One of the at least one intermediate part includes a power feed region (13a; 13b) to be connected to an integrated circuit (3). Further, a first gap (14; 14b) is arranged in one of the at least one intermediate part and has a gap length of less than 80 μm, the first gap forming a low impedance path for currents at microwave frequencies.

Description

Technical Field

[0001] The present invention relates to an antenna for a radio frequency identification (RFID) tag and a corresponding RFID tag. The RFID tag is arranged to be usable in a microwave oven and can be arranged, for example, on or incorporated in a microwaveable food package or food item. The present invention also relates to a package for a microwaveable food item including such an RFID tag. Background Art

[0002] RFID tags are now being used more and more frequently and for various applications (such as smart labels / tags). RFID tags are typically arranged as transponders in a planar configuration, for example, arranged under a conventional printed decoding label, and include a chip and an antenna. The label / tag is typically made of paper, fabric, or plastic and is usually prepared together with an RFID inlay laminated between a carrier and a marking medium, for example, for a specially designed printer unit. Smart tags offer advantages over conventional barcode tags, such as higher data capacity, the possibility of reading and / or writing outside of direct line of sight, and the ability to read multiple tags / labels at once.

[0003] It is also known to directly incorporate RFID tags in packaging materials to form so-called smart packaging products.

[0004] An increasingly interesting application of RFID tags is for packaging, such as for food, that is intended to be microwave-heated in a microwave oven. Thus, RFID tags can be used, for example, for purposes such as logistics tracking. However, typically, food for microwave heating, etc., is cooked or heated in a microwave oven without removing the food container packaging. The packaging can even be part of the cooking process.

[0005] During heating or cooking in a microwave oven, the RFID functionality is no longer needed and used, and the RFID tag can be removed before placing the RFID tag in the microwave oven. However, removing the RFID tag can be cumbersome and difficult and can also be easily forgotten.

[0006] However, exposing an RFID tag to microwaves in a microwave oven may cause concentrated heating, which may pose a safety risk. The RFID tag has a gap across which the RFID chip is placed. The power received by the RFID tag from a conventional reader device is typically low, around a few microwatts, while a microwave oven can typically operate at a power level of over 800 watts, which creates a very high voltage across the gap. Additionally, RFID antennas are typically designed to operate at UHF frequencies, for example in the range of approximately 860 MHz to 930 MHz, where the antenna receives incident power from an RFID reader and converts it into a voltage across the RFID chip to allow it to operate. On the other hand, microwave ovens typically operate at a higher frequency, usually around 2450 MHz. When exposed to microwaves in a microwave oven, the microwaves will also be incident on the antenna of the RFID tag. The very high power level and frequency of these microwaves will create a high voltage on the antenna, especially across the gap bridged by the RFID chip, as the gap must be relatively small, typically in the range of 100 - 200 μm. This high voltage may cause breakdown and generate an arc, and may result in package deformation, arcing, and flashing, and the package may even catch fire. This is a safety risk and may also damage the microwave oven.

[0007] US 2018 / 0189623 proposes a solution to this problem. Here, a shielding layer is provided that is electrically coupled to the antenna across the gap and covers the RFID chip to limit the voltage across the gap when the antenna is exposed to microwaves from a microwave oven. However, even though this alleviates the above - mentioned safety issues, it makes the production of RFID tags complex, cumbersome, and expensive.

[0008] Therefore, there is still a need for an improved RFID tag that can be microwaved without the safety risks and other problems discussed above. Summary of the Invention

[0009] Accordingly, an object of the present invention is to provide an antenna for an RFID tag and a corresponding RFID tag, as well as a package for microwaveable food products including such an RFID tag, which alleviates at least in part the above - mentioned problems and at least in part addresses one or more of the above - mentioned needs.

[0010] This object is achieved by an antenna, an RFID tag, and a package according to the appended claims.

[0011] According to a first aspect of the present invention, there is provided an antenna for an RFID (Radio Frequency Identification) tag, comprising:

[0012] two antenna components, which are arranged in opposite end regions of the antenna;

[0013] At least one intermediate member forming a bridge between the antenna portions, one of the at least one intermediate members including a power feed region to be connected to an integrated circuit; and

[0014] A first gap disposed in one of the at least one intermediate members, the first gap having a gap length of less than 80 μm.

[0015] The present invention is based on the recognition that by providing a very narrow gap, either as a gap between power feed regions (i.e., the gap under the RFID chip) or in a bypass path in the vicinity thereof, the voltage build-up under microwaves in a microwave oven can be greatly reduced. Since the sides forming the gap are very close, a high capacitance is generated, which effectively forms a low-impedance path for the frequencies used in a microwave oven (e.g., approximately 2.45 GHz). At the same time, since the frequencies for operating the RFID tag (e.g., frequencies within the UHF band) are in the range of approximately 860 - 960 MHz, the RF current flow at these frequencies is provided with a low-impedance path across the gap and is not short-circuited and is still prevented from propagating across the gap. Thus, by using such a narrow gap, the normal operation of the RFID tag at UHF frequencies is not affected at all, while the voltage build-up problem at microwave oven frequencies is also greatly alleviated.

[0016] Without being bound by any theory, it is believed that the IC gap and the antenna (especially the matching portion of the antenna, such as the conductor loop in the middle of the antenna) form an LC resonant circuit. The typical dimensions of a UHF antenna including such a matching portion and a conventional IC gap have a resonant frequency close to the microwave band, e.g., 2.45 GHz. The resonant effect amplifies the voltage build-up across the gap and the RF current in the loop. The narrow gap provides a low-impedance path for the microwave current and shifts the resonant frequency of the circuit to a lower frequency away from the microwave band. This effectively reduces the voltage build-up and the current amplitude.

[0017] It has been found that by providing a first gap having a gap length of less than 80 μm disposed in one of the at least one intermediate members, the voltage build-up problem at microwave oven frequencies is greatly reduced. When a smaller gap length is provided, such voltage build-up is further reduced. Thus, in one embodiment, the gap length is less than 50 μm, preferably less than 30 μm, and most preferably less than 25 μm. In one embodiment, a gap length of approximately 20 μm is provided.

[0018] More preferably, the width of the first gap is relatively large, whereby the voltage established across the gap at microwave frequencies is even further reduced. In one embodiment, the width of the first gap exceeds the length of the gap, preferably being greater than 2 times the length of the gap, more preferably greater than 3 times the length of the gap, and most preferably greater than 5 times the length of the gap. In one embodiment, the width of the first gap exceeds 150 μm, preferably exceeds 200 μm, and most preferably exceeds 250 μm.

[0019] The first gap is preferably arranged to form a low impedance path for radio waves having a frequency of more than 2 GHz (e.g., at a frequency of about 2.45 GHz, which corresponds to the frequency conventionally used for microwave ovens).

[0020] The antenna can be of many different types, such as a dipole antenna, a monopole antenna, a loop antenna, or a slot antenna.

[0021] In one embodiment, the antenna can be a dipole antenna having two dipole antenna elements arranged in opposite end regions. The two dipole antenna elements arranged in the opposite end regions of the antenna can have various shapes and sizes, as is known per se in the art. For example, these elements can extend in a generally linear direction, or can extend in a non-linear manner (e.g., in a meandering form, etc.). These elements can also be folded or bent so as to extend in two or more directions. In one embodiment, the dipole antenna portion can be terminated by end elements which can have an enlarged width at least in some positions. The end elements can be, for example, of a generally circular or generally rectangular shape.

[0022] At least one intermediate element forms a bridge between the antenna elements. At least one of the at least one intermediate element includes a power feed region to be connected to an integrated circuit. The power feed regions are preferably separated by a gap, which can be the above-mentioned narrow first gap or an additional gap.

[0023] The intermediate element is preferably electrically connected to the antenna portion. However, this connection can also be non-electrically conductive, whereby coupling can be obtained by electromagnetic field coupling (e.g., by inductive coupling).

[0024] The power feed regions are arranged to be electrically coupled to an integrated circuit, an RFID chip, so as to bridge the gap between the power feed regions. Thus, each power feed region is arranged to transmit current between a connector of the RFID chip and one of the antenna elements.

[0025] In one embodiment, a narrow first gap is provided between the power feed regions. Thus, the narrow first gap serves two purposes: 1) to provide an IC gap to be bridged by the RFID chip, and 2) to reduce voltage build-up at high frequencies. In this series of embodiments, it facilitates the production of the antenna, making the production of the antenna more cost-effective.

[0026] In another embodiment, a second gap is provided between the power feed regions, and the narrow first gap is arranged in a bypass connection, which is arranged at a distance from the power feed regions. In such an embodiment, the second gap may have a greater gap length than the narrow first gap, for example, a gap length in the range of 100 - 200 μm. However, alternatively, the size of the second gap can also be designed to reduce voltage build-up at high frequencies in the above-mentioned manner.

[0027] In this series of embodiments, the antenna becomes more complex and the production cost is higher. On the other hand, in this type of embodiment, the placement and attachment of the RFID chip to the antenna can generally be simpler, thus reducing the overall production cost of the RFID tag.

[0028] The antenna is preferably configured to operate in the UHF band (e.g., at one or more frequencies in the range of 860 - 960 MHz).

[0029] According to another aspect of the present invention, there is provided an antenna for an RFID (Radio Frequency Identification) tag, comprising:

[0030] Two antenna components, which are arranged in the opposite end regions of the antenna;

[0031] At least one intermediate component, which forms a bridge between the antenna components, and one of the at least one intermediate components includes a power feed region to be connected to an integrated circuit, wherein the power feed region is separated by an IC gap; and

[0032] A bypass gap, which is arranged in a bypass connection, which is arranged at a distance from the power feed region, and the bypass gap is arranged to provide a low impedance bypass path at microwave frequencies.

[0033] The bypass gap can be arranged within the same intermediate component including the power feed region, or alternatively, it can be arranged in a separate intermediate component.

[0034] The bypass gap can be arranged to provide a low impedance bypass path at microwave frequencies in various ways. As an alternative, the gap length can be very short, as described above, and preferably less than 80 μm. However, as another option, a longer gap length can be used in combination with a very wide gap width (e.g., much greater than the gap length).

[0035] To obtain a very wide gap width, the gap of the bypass gap can have a meandering extension. Thus, a very wide gap width can be obtained within a relatively limited area (e.g., between two intermediate portions of the antenna).

[0036] The gap width of such a wide gap may, for example, exceed 10 mm, preferably exceed 20 mm, more preferably exceed 30 mm (e.g., exceed 40 mm, or exceed 50 mm). In an exemplary embodiment, the gap has a width of approximately 60 mm. In such an embodiment, the gap length can be much higher than 80 μm (e.g., in the range of 100 - 750 μm), and preferably in the range of 200 - 600 μm, and still provide an effective low-impedance bypass path at microwave frequencies (e.g., approximately 2.45 GHz).

[0037] According to another aspect of the present invention, there is provided an RFID tag including an antenna as described above, an integrated circuit connected to a power feeding component of the antenna, and a dielectric substrate on which the antenna is disposed.

[0038] The dielectric substrate can basically be any non-conductive material. In one embodiment, the dielectric substrate material is made of at least one of the following materials: paper, board, polymer film, fabric, and non-woven material. In particular, the substrate can be made of paper.

[0039] The RFID tag can be passive (i.e., powered by the electromagnetic field of the reader) or active (i.e., powered by an on-board battery).

[0040] The antenna can be made of any material as long as the material is conductive. For example, the antenna can be formed of aluminum, but other metals such as silver and alloys can also be used. The antenna can be formed on the substrate in various ways known per se in the art, such as by printing with a conductive ink (e.g., silver ink), first providing a conductive layer on the substrate, and then removing this conductive layer by methods such as grinding, cutting, etching, etc. to form the desired antenna shape or forming the conductive layer into the desired antenna shape.

[0041] According to yet another aspect of the present invention, there is provided a packaging for microwaveable food products, the packaging including a casing and an RFID tag as described above. The RFID tag can be attached to the casing (e.g., by an adhesive), but can also be formed as an integral part of the casing, in which case the dielectric substrate of the RFID tag can be formed, for example, of the material forming the casing of the packaging. Thus, for example, for the production of smart packaging products, the antenna of the RFID tag can be directly provided on the packaging material, for example, in the form of a sheet or a mesh.

[0042] It should be understood that the above-described detailed structure and advantages of the first aspect of the present invention are also applicable to other aspects of the present invention.

[0043] These and other aspects of the present invention will become apparent from the embodiments described hereinafter and will be elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] For purposes of illustration, the present invention will be described in more detail hereinafter with reference to the embodiments shown in the accompanying drawings, in which:

[0045] Figure 1 is a schematic top view of an antenna according to a first embodiment;

[0046] Figure 2 is using Figure 1 a schematic top view of an RFID tag with the antenna;

[0047] Figure 3 is a schematic top view of an antenna and an RFID tag using the antenna according to a second embodiment.

[0048] Figure 4 is a more detailed schematic top view of a narrow gap formed in an antenna according to an embodiment;

[0049] Figure 5 is a more detailed schematic top view of a narrow gap formed in an antenna according to another embodiment;

[0050] Figure 6 is a schematic perspective view of a microwaveable food packaging including an attached RFID tag according to an embodiment;

[0051] Figure 7 is a schematic perspective view of a microwaveable food packaging including an RFID tag integrated in the packaging material according to another embodiment;

[0052] Figures 8 - 13 is a field map of a simulation of an antenna according to an embodiment of the present invention and a comparative example, where Figures 8 - 10 illustrates the field map of a comparative example with a gap length of 200 μm, and illustrates the temperature, surface current, and electric field strength, where Figures 11 - 13 shows the field map of an embodiment of the present invention with a gap length of 20 μm, and also shows the temperature, surface current, and electric field strength respectively;

[0053] Figure 14 is a schematic top view of another antenna according to an embodiment, which is similar to Figure 1 the antenna design of, but is provided with smoother corners and transitions; and

[0054] Figure 15 Schematic top view of another antenna with a meandering gap according to another embodiment. Detailed Description of the Invention

[0055] In the following detailed description, preferred embodiments of the present invention will be described. However, it should be understood that the features of different embodiments are interchangeable between the embodiments and can be combined in different ways, unless otherwise specifically stated. It should also be noted that for clarity, the dimensions of some components shown in the drawings may be different from the corresponding dimensions in the actual implementation of the present invention, such as the thickness of each layer, the relative dimensions of the gap length and the gap width, etc. Although the embodiments discussed below include dipole antennas, the antennas can also be other types, such as monopole antennas, loop antennas, or slot antennas.

[0056] Figure 1 Antenna 1 according to an embodiment of the present invention is shown. The antenna is a dipole antenna arranged for an RFID tag and is preferably arranged to operate in the UHF band.

[0057] Antenna 1 includes two dipole antenna parts 11a and 11b, which are arranged in the opposite end regions of the antenna. The dipole antenna parts are located at one of their ends, the ends being closest to each other, and are connected to a power feeding device. Here, the power feeding device is provided in the form of an intermediate part 12, which forms a bridge between the dipole antenna parts and is provided with two power feeding regions 13a and 13b separated by a gap 14. The first power feeding region 13a is connected to the first dipole antenna part 11a, while the second power feeding region 13b is connected to the second dipole antenna part 11b.

[0058] As discussed in more detail below, the power feeding regions will be connected to the connectors of an integrated circuit, an RFID chip, which will thus be arranged to cover and bridge the gap 14.

[0059] At the other ends of the dipole antenna parts that are not connected to the power feeding regions, end parts 15a and 15b can be provided. The end parts are preferably provided with smooth rounded corners and can be arranged, for example, as generally circular regions. Avoiding sharp ends prevents voltage build-up.

[0060] The two dipole antenna parts 11a and 11b are preferably generally equal in size and shape and are preferably symmetric with respect to each other.

[0061] In the illustrated embodiment, the shapes of the dipole antenna parts 11a and 11b are elongated wires. However, other shapes are also feasible. For example, the parts can extend in a meandering shape at least on one part. The parts can also have an overall folded or bent shape. Many other shapes are also feasible, as is well known in the art.

[0062] In addition, the antenna may also be asymmetric. For example, antenna components 11a and 11b do not have to be symmetric and have the same size and shape.

[0063] In addition, end components 15a and 15b may have the same width as the rest of the dipole antenna components. However, preferably, the end components are slightly enlarged and at least partially have a greater width. End components 15a and 15b are in the form of a circle in the illustrated example, but other shapes are also feasible, such as a rectangle.

[0064] In the illustrative example, the dipole antenna components are also connected by another intermediate component 16 for impedance matching. However, in other antenna designs, such an additional intermediate component may have other shapes or may even be omitted.

[0065] To avoid power build-up on gap 14 when exposed to microwaves in a microwave oven (whose frequency is typically much greater than the frequency of the UHF band, such as 2.45 GHz), gap 14 extends with a very short gap length g. The gap length g is at least less than 80 μm, but preferably even smaller (e.g., less than 50 μm, more preferably less than 30 μm), and most preferably less than 25 μm. In one embodiment, a gap length g of about 20 μm is provided.

[0066] Since the gap length g is very short, the capacitance generated is very high, which effectively forms a low-impedance path for radio frequency current at the frequency used in a microwave oven (such as about 2.45 GHz). At the same time, since the frequency for RFID tag operation (such as the frequency within the UHF band), that is, approximately in the range of 860 - 960 MHz, the RF current at this frequency does not provide a low-impedance path and is not short-circuited, and is still blocked from propagating across the gap.

[0067] It is generally preferred to avoid sharp edges and corners in the antenna to avoid possible power build-up. Therefore, the antenna preferably has an overall smooth design with rounded or chamfered corners and transitions between different components. Figure 14 An example of such a smooth antenna design is shown. This antenna design is similar to Figure 1 the antenna design, but in which all corners and transitions have been smoothed.

[0068] In Figure 2 is shown an RFID tag using Figure 1 the antenna. RFID tag 100 herein includes the above-described antenna 1 disposed on a substrate 2, and an integrated circuit, RFID chip 3, which is disposed on the antenna and connected to power feed regions 13a and 13b such that the RFID chip bridges gap 14.

[0069] The dielectric substrate can essentially be any non-conductive material, such as paper, board, polymer film, fabric, and non-woven material. In particular, the substrate can be made of paper.

[0070] The antenna can be made of any material as long as the material is conductive. For example, the antenna can be formed of aluminum, but other metals such as silver and alloys can also be used. For example, an alloy with a relatively low melting temperature can be used, such as an alloy including tin and bismuth. Forming the antenna on the substrate can be carried out in various ways, as is known per se in the art, for example by printing with a conductive ink (such as silver ink), first by providing a conductive layer on the substrate and subsequently removing the conductive layer, for example by grinding, cutting, etching, etc. to form the desired antenna shape or forming the conductive layer into the desired antenna shape.

[0071] The narrow gap can be formed by removing conductive material from the gap, for example, by laser or other fine cutting techniques.

[0072] The RFID chip 3 can take any of a variety of forms (including the types commonly referred to by those of ordinary skill in the art as "chips" or "tag strips"), include any of a variety of possible components, and be configured to perform any of a variety of possible functions. Preferably, the RFID chip includes an integrated circuit for controlling the RF communication and other functions of the RFID tag.

[0073] In the embodiments discussed with respect to Figure 1 and 2 the narrow gap also forms a so-called IC gap, i.e., the gap bridged by the RFID chip. Thus, in this embodiment, one and the same gap serves both as an IC gap and as a gap for reducing voltage build-up at high frequencies.

[0074] However, it is also feasible to provide the IC gap and the gap for avoiding voltage build-up as two separate gaps. Such an embodiment will now be discussed in more detail in connection with Figure 3 More detailed discussion of such an embodiment will now be provided in connection with

[0075] In Figure 3 the embodiment, the intermediate component is different from the first-discussed embodiment. The remaining components of the antenna (such as the dipole antenna components) are the same as or similar to those of the first-discussed embodiment and will not be discussed in detail again. Except for the specific details discussed below, the details and variations discussed with respect to the first embodiment also apply to this second embodiment.

[0076] In Figure 3In an embodiment, a bypass path or connection 17 is formed near the IC gap 14a, the power feed regions 13a and 13b, and the RFID chip 3 to be connected to the power feed regions. The bypass path / connection 17 may be formed as a short extension outward from the intermediate member 12, or alternatively may be arranged as a separate intermediate member.

[0077] The bypass path / connection 17 is provided with a narrow gap 14b, sized to avoid voltage build-up at high frequencies in the same manner as described above. Thus, this narrow gap 14b in the bypass path / connection 17 has a gap length g of at least less than 80 μm, but preferably even smaller, as already discussed. Due to the above low impedance path obtained across the gap 14b at high frequencies, the bypass path / connection will conduct current at high frequencies (e.g., at conventional microwave oven frequencies, e.g., at about 2.45 GHz), but will block the conduction of current at lower frequencies (e.g., the operating frequency of the antenna in the UHF band).

[0078] The gap 14b may be provided at the center of the bypass path / connection 17, as in the illustrative example, but may alternatively be provided non-centrally, towards one side.

[0079] In such an embodiment, the IC gap 14a disposed between the power feed regions 13a and 13b may also be narrow, such that this gap also provides a low impedance path at high frequencies. Thus, the IC gap 14a may also have a gap length g of the same order of magnitude as the gap 14b in the bypass path / connection 17. However, the power build-up at high frequencies has been largely reduced by the bypass path / connection. The IC gap 14a may alternatively have a larger gap length G, e.g., having the same dimensions as a conventional RFID tag, e.g., in the range of 100 - 200 μm.

[0080] The antenna of the second embodiment may be arranged on the substrate 2 in the same manner as the first embodiment, as shown by the dashed lines, and the RFID chip 3 (also shown by the dashed lines) is attached to the power feed region to form the RFID tag 100'.

[0081] Whether the narrow gap length g is provided in the IC gap (i.e., between the power feed regions), or is provided as a separate gap in the bypass path / connection, the preferred gap width w is relatively long. Preferably, the width w of the narrow gap exceeds the gap length g (i.e., w > g), preferably is greater than 2 times the gap length (i.e., w > 2*g), more preferably is greater than 3 times the gap length (i.e., w > 3*g), and most preferably is greater than 5 times the gap length (i.e., w > 5*g). In one embodiment, the width of the first gap exceeds 150 μm, preferably exceeds 200 μm, and most preferably exceeds 250 μm.

[0082] Such a width w can be provided by providing a relatively wide conductive path in the intermediate member. At least as Figure 3 shown, the bypass path / connection 17 can be provided with a relatively large width.

[0083] However, alternatively, it is also feasible to provide a relatively large width only near the gap. Such an embodiment is schematically shown in Figure 4 and 5 In Figure 4 an example of forming a relatively wide rectangular region at the end where the gap is formed is shown. In the case where the gap is provided as an IC gap, the rectangular region here also serves as the power feeding regions 13a and 13b to connect to the RFID chip 3, as schematically shown by the dashed line in Figure 4 However, a similar rectangular region can also be used even when a narrow gap is provided in the bypass path / connection. However, the enlarged end region can also have other shapes, such as trapezoidal or semi-circular. One such alternative, as Figure 5 shown, the enlarged end region is triangular, with the bottoms facing each other to form a gap.

[0084] Another antenna design also provides a low-impedance bypass path for microwave frequencies, as Figure 15 shown. Here, similar to the embodiment discussed with respect to Figure 3 there is an IC gap 14a between the power feeding regions in the intermediate member 12, and an additional bypass gap 14b arranged in a bypass connection arranged at a certain distance from the power feeding regions.

[0085] The bypass gap is arranged to provide a low-impedance bypass path at microwave frequencies such as 2.45 GHz. However, different from the previously discussed embodiments, this is not obtained by using a very short gap length. Instead, this effect is obtained by a longer gap length and a very wide gap width (e.g., greatly exceeding the gap length).

[0086] In this embodiment, a very wide gap 14b is arranged in a loop formed between the intermediate members 12 and 16. However, it can of course also be arranged at other positions.

[0087] To make the bypass gap compact, the gap is arranged here in a meandering shape such that it extends in a meandering extension path. This meandering shape can be obtained by providing staggered arms 14b1 and 14b2, each arm 14b1 and 14b2 having a free end and each having an end connected to the antenna component, and the connecting ends being alternately connected to different sides of the antenna. Depending on the length of the arms and the number of arms, the meandering gap formed between them may have a very large width. In the illustrative example, the width can be about 60 mm here. Having a very large width allows for the use of a slightly longer gap length and still achieve the desired effect of a low impedance path formed across the gap at microwave frequencies. In the illustrative example, the gap length can be about 500 μm. Due to the meandering shape of the gap, the gap becomes very compact and can still be achieved with the same overall size of the antenna.

[0088] The production of the meandering gap may seem more complex than the embodiments discussed previously, but can actually be produced in an equally cost-effective or even more cost-effective manner because even though it includes more components, the acceptable tolerances are higher and thus the antenna pattern can be generated faster and with lower required precision. RFID is particularly suitable for the packaging of microwave-processable food. The RFID tag 100 or 100' can thus be attached, for example, by an adhesive to the housing forming the packaging 4, as Figure 6 schematically shown in. Alternatively, the RFID tag 100 or 100' can be formed as an integral part of the housing, in which case the dielectric substrate of the RFID tag can be formed, for example, from the material of the housing forming the packaging, as Figure 7 schematically shown in. Thus, for example, for the production of smart packaged products, the antenna of the RFID tag can be directly provided on the packaging material, for example in the form of a sheet or a mesh.

[0089] The housing of the packaging can be in the form of a box made of, for example, paper or plastic material. Furthermore, although the RFID tag is described here as being incorporated into the packaging of microwaveable food products, it should be understood that the RFID tag according to the present disclosure can be used in any of many possible applications, particularly when it is expected that they can be exposed to frequencies significantly higher than the expected operating frequency of the antenna of the RFID tag.

[0090] To evaluate the new concept, many experimental tests and simulations have been carried out.

[0091] In the first test line, having made of aluminum and belonging to regarding Figure 1The RFID tag of the general type of antenna discussed (but the antenna has an IC gap of 160 μm) is attached to one side of a conventional microwaveable food item made of paper. The food item is exposed to microwaves in a Samsung MS23K3523AK type microwave oven with a moving turntable. The microwave oven operates at full power of 800 W for 60 seconds.

[0092] After exposure to microwaves, it was noted that the paper became significantly darker and was charred in the area near the IC gap of the antenna.

[0093] The same test was also conducted on the RFID tag according to the present invention. For this test, the RFID tag and the antenna were the same as those of the first test, but with a reduced gap length at the IC gap. In the second test, the gap length was 20 μm. After the same type of microwave exposure, it was found that there was no darkening or discoloration on the paper of the packaging shell.

[0094] Conceptual tag antenna simulations were also carried out. For these simulations, antennas of the type Figure 1 disclosed were used, but with different gap lengths at the IC gap. For the simulations, a gap length of 200 μm was used as a comparative example, and then a gap length of 20 μm was used as an example of the present invention.

[0095] In the simulations, microwave exposure at 2.45 GHz was simulated, and the power and time period corresponding to the radiation in a microwave oven operating at 1000 W for 60 seconds were used.

[0096] The field maps of these simulations are as Figures 8 - 13 shown. Figures 8 - 10 The field map of the comparative example with a gap length of 200 μm is shown, and the temperature, surface current, and electric field strength are shown respectively. Figures 11 - 13 The field map of the example of the present invention with a gap length of 20 μm is illustrated, and the temperature, surface current, and electric field strength are also illustrated respectively.

[0097] Figure 8 The field map of the temperature of the antenna with a gap length of 200 μm (comparative example) is shown. It can be seen that there are very high temperature regions in a wide circle around the IC gap. The highest temperature that appears at the center of this circle (i.e., below the IC gap) exceeds 1600 °C, while the lowest temperature at a certain distance from the IC gap is the same as the surrounding room temperature, approximately 20 °C.

[0098] Figure 9 Shows the same as Figure 8Field pattern of surface current (A / m) on the conductive surface of the same antenna, with an IC gap of 200 μm in length (comparative example). It can be seen from the field pattern that there is a very high surface current (exceeding 5 A / m) throughout the intermediate component, with a maximum of 16.3 A / m near the IC gap.

[0099] Figure 10 Shows the Figure 8 and Figure 9 Field pattern of the electric field strength (V / m) of the conductive component of the same antenna, with an IC gap of 200 μm in length (comparative example). It can be seen from this field pattern that there is a very high electric field strength near the IC gap, with a maximum value of approximately 100 kV / m.

[0100] Figure 11 Shows the field pattern of the temperature of the antenna with a gap length of 20 μm (example of the present invention). It can be seen that the temperature is relatively low throughout the antenna, and there is only a very limited temperature increase near the IC gap. At a certain distance from the IC gap, the highest temperature that appears near the IC gap is less than 50 °C, which is only slightly higher than the lowest temperature (which is the same as the ambient room temperature (about 20 °C)).

[0101] Figure 12 Shows the Figure 11 Field pattern of the surface current (A / m) on the conductive surface of the same antenna, with an IC gap of 20 μm in length (example of the present invention). It can be seen from the field pattern that there is a very limited surface current (less than 2 A / m) in most of the intermediate component, with a maximum of 3.9 A / m near the IC gap.

[0102] Figure 13 Shows the Figure 11 and 12 Field pattern of the electric field strength (V / m) at the conductive component of the same antenna, with an IC gap of 20 μm in length (example of the present invention). It can be seen from this field pattern that there is a very limited electric field strength everywhere, including near the IC gap, with a maximum value of approximately 3 - 7 kV / m.

[0103] In further simulations, other gap lengths have been analyzed in a similar manner. It has been found that the highest temperature obtained at or near the IC gap highly depends on the gap length. The following table shows the highest temperatures obtained for many different gap lengths under the same conditions as the examples illustrated by the field patterns discussed above.

[0104] <![CDATA Gap length > <![CDATA Highest temperature > 20μm 48℃ 50μm 110℃ 100μm 1041℃ 200μm 1600℃ 500μm 191℃ 1mm 75℃ 3mm 30℃ 6mm 23℃

[0105] From this, the following observations can be made:

[0106] · It should be noted that, for example, within the conventional operating gap length range of the IC gap (i.e., between 100 and 200 μm), very high maximum temperatures can be obtained.

[0107] · For different materials, the ignition temperature (i.e., the temperature at which something catches fire and burns on its own) is naturally different. The ignition temperature of ordinary paper is about 233 °C. However, even though many materials used in packaging for microwaveable food items have higher ignition temperatures, the maximum temperatures seen in the 100 - 200 μm gap length range are far higher than the ignition temperatures of most conventional packaging materials. The above simulations show the relative temperature differences when assuming a simple microwave source is relatively close to the RFID tag. Naturally, the environment inside a real-world microwave oven is much more complex, and the absolute temperature levels may differ somewhat from the simulated situation. However, the simulations still clearly show the sharp reduction in temperature obtained by providing a narrow gap.

[0108] · At very long gap lengths (at least over 500 μm, preferably over 1 mm), the maximum temperature is low enough not to pose any safety risks, etc. However, using such long gap lengths is generally not actually feasible.

[0109] · When the gap length is below 100 μm, the maximum temperature drops sharply and can be as low as 110 °C when the gap length is 50 μm. This is far below the ignition temperatures of all or at least most available packaging materials.

[0110] · It can be concluded that the gap lengths that sufficiently reduce the temperature to sufficiently prevent voltage build-up and significantly mitigate any safety risks occur between 50 μm and 100 μm. Considering a suitable step size for the maximum temperature, it is fair to conclude that this occurs at a gap length of approximately 80 μm.

[0111] · The gap lengths discussed above are optimal for the antenna design used in the simulations. However, for other antenna designs, the optimal gap length may be different, for example, less than 50 μm, such as 20 μm.

[0112] To further improve safety, the dielectric substrate can be made of a non-flammable material. It is also feasible to manufacture the packaging / package at least in the part adjacent to the RFID tag with a non-combustible material.

[0113] Those skilled in the art recognize that the present invention is not limited to the above embodiments. For example, as is well known in the art, the general antenna design can be changed in many ways. The antenna can also be adapted to different operating frequencies. As mentioned above, the narrow gap can be arranged in the IC gap or in a separate bypass path / connection, and in such cases, the bypass path / connection can be provided in many different ways.

[0114] These and other obvious modifications must be considered to be within the scope of the present invention, as defined by the appended claims. It should be noted that the above embodiments are illustrative and not restrictive of the present invention, and those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs in parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in the claim. The word "a" before an element does not exclude the presence of a plurality of such elements.

Claims

1. An antenna (1) for an RFID tag (100), comprising: two antenna components (11a, 11b), which are arranged in opposite end regions of the antenna; at least one intermediate component (12), which forms a bridge between the antenna components, and one of the at least one intermediate component includes a power feed region (13a; 13b) to be connected to an integrated circuit (3), wherein the power feed regions are separated by a gap, and the gap is bridged by an RFID chip; and a bypass gap (14b), which is arranged in a bypass connection (17), the bypass connection (17) being arranged at a distance from the power feed region (13a; 13b), and the bypass gap being arranged to provide a low impedance bypass path at microwave frequencies, wherein the gap of the bypass gap has a meandering extension.

2. An RFID tag (100), comprising: the antenna (1) according to claim 1; an integrated circuit (3), which is connected to the power feed region (13a; 13b) of the antenna (1); and a dielectric substrate (2), on which the antenna is provided.

3. The RFID tag according to claim 2, wherein the dielectric substrate (2) is made of at least one of the following: paper, board, polymer film, fabric, and non-woven material.

4. A package (4) for microwaveable food products, comprising: a housing; and the RFID tag according to claim 2 or 3, which is fixed to the housing.

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