Patent Publication Number: US-2015076238-A1

Title: Integrated loop structure for radio frequency identification

Description:
TECHNICAL FIELD 
     The present invention relates to inlays for Radio Frequency (RF) communication. In particular the invention relates to RF Identification (RFID) inlays and RFID tags that are used in packages, articles, or products having only limited space available for the RFID inlay. 
     BACKGROUND OF THE INVENTION 
     RFID tags are small sized devices, typically in a label format, that can be applied to or incorporated into a product, device or even animal for the purpose of identification and tracking of the item in question using radio waves. Some RFID tags can be read from several meters away and beyond the line of sight of the reader. These capabilities make the use of RFID tags very interesting over optical bar codes in product logistics, even if the data contained in the RFID tags would be equal to the UPC (Universal Product Code), EAN (European Article Number) codes traditionally used in bar codes. EPC (Electronic Product Code) codes used globally in RFID tags make it possible to store more information in a standardized manner to the RFID tags than has been possible in case of basic optical bar codes. Thus, RFID tags are becoming increasingly popular in everyday product logistics in many commercial fields. 
     Typically RFID tags (or in some cases the RFID inlays) are attached to the articles or packages thereof. In case the article or package thereof is small in size, the RFID tag can take up a large portion of the article or package. Therefore, there is a need for smaller RFID tags. 
     SUMMARY OF THE INVENTION 
     Despite of a wide variety of different existing RFID tag solutions there still is a clear need for a solution that would facilitate improved capability to tag small sized items and to utilize the full potential of RFID tags including the possibilities to use RFID. In order to improve the capability to tag small sized items, an assembly for a radio frequency (RF) communication circuits disclosed. In addition, a radio frequency transponder, comprising the assembly for the RF communication circuit is disclosed. Still further, an item comprising the radio frequency transponder is disclosed. 
     The assembly for a radio frequency (RF) communication circuit comprises, 
     an electrically insulating substrate having a first side and a second side, 
     a first electrically conductive structure arranged on the first side of the substrate, wherein 
     the first electrically conductive structure has the structure of a split loop, wherein the split loop structure comprises a first end and a second end, wherein the RF communication circuit is arranged to be attached to a site for the RF communication circuit between the first end and the second end such that the RF communication circuit closes the split loop, and 
     a second electrically conductive structure arranged on the second side of the substrate, wherein 
     the second electrically conductive structure is arranged with respect to the first electrically conductive structure in such a manner that the site for the RF communication circuit overlaps the second electrically conductive structure. 
     The second electrically conductive structure increases the capacitance of the assembly for the RF communication circuit. 
     These and other technical features are disclosed in the specification and the claims  1  to  24 . The structure increases the capacitance of the joint between the RF communication circuit and the assembly for the RF communication circuit thereby decreasing operating frequency of the assembly, and, in effect, decreasing the size of the assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which 
         FIG. 1   a  shows an assembly for a radio frequency (RF) communication circuit, as seen from top, 
         FIG. 1   b  shows the assembly of  FIG. 1   a  for a radio frequency (RF) communication circuit, as seen from bottom, 
         FIG. 1   c  shows the assembly of  FIG. 1   a  for a radio frequency (RF) communication circuit, in a perspective view, 
         FIG. 1   d  shows a RF transponder comprising the assembly of  FIG. 1   a  and a circuit attached to the assembly, in a perspective view, 
         FIGS. 2   a - 2   c  shows examples of split loop structures, 
         FIG. 2   d  shows a circular split ring structure, the split ring structure being also a split loop structure, 
         FIGS. 2   e   1 - 2   e   3  show an example of a split ring structure on the first side of the substrate, a corresponding split ring structure on the second side of the substrate, and the two loop structures aligned, 
         FIGS. 2   f   1 - 2   f   3  show an example of a split loop structure on the first side of the substrate, a corresponding split loop structure on the second side of the substrate, and the two loop structures aligned, 
         FIGS. 2   g   1 - 2   g   3  show an example of a split loop structure on the first side of the substrate, a corresponding split loop structure on the second side of the substrate, and the two loop structures aligned, 
         FIG. 3   a  shows an assembly for a radio frequency (RF) communication circuit comprising two overlapping split ring structures as seen from top, 
         FIG. 3   b  shows an assembly for a radio frequency (RF) communication circuit comprising two overlapping split ring structures as seen from bottom, 
         FIG. 3   c  shows an assembly for a radio frequency (RF) communication circuit comprising two overlapping split ring structures, in a perspective view, 
         FIG. 4   a  shows two overlapping split ring structures of an assembly for a radio frequency (RF) communication circuit as seen from top, 
         FIG. 4   b  shows two overlapping split ring structures of an assembly for a radio frequency (RF) communication circuit as seen from top, 
         FIG. 5   a  shows an assembly for a radio frequency (RF) communication circuit, comprising two overlapping split ring structures as seen from top, the structure further comprising an antenna, 
         FIG. 5   b  shows an assembly for a radio frequency (RF) communication circuit, comprising two overlapping split ring structures as seen from top, the structure further comprising an antenna, 
         FIG. 5   c  shows an assembly for a radio frequency (RF) communication circuit, comprising two overlapping split ring structures as seen from top, the structure further comprising an antenna, 
         FIG. 6  shows an assembly for a radio frequency (RF) communication circuit, comprising multiple overlapping split loop structures as seen from top, the structure further comprising two mutually perpendicular dipole antennas, 
         FIG. 7   a  shows an assembly for a radio frequency (RF) communication circuit, comprising multiple co-centric overlapping split loop structures, in an exploded perspective view, 
         FIG. 7   b  shows an assembly for a radio frequency (RF) communication circuit, comprising multiple co-centric overlapping split loop structures, in an exploded perspective view, and 
         FIG. 7   c  shows an assembly for a radio frequency (RF) communication circuit, comprising multiple co-centric overlapping split loop structures, as seen from top, the figure also showing the ring structures of different layers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An RFID tag typically comprises an RFID inlay and an overlay structure forming the RFID tag. The RFID inlay is an electrically fully functional RFID transponder device, that is, a device that works as a transmitter and responder. The main components of the transponder are an RF communication circuit (i.e. an electronic integrated circuit) and an antenna. An inlay further comprises a substrate and other optional layers to support the transponder. The overlay structure of an RFID tag forms further mechanical support for the inlay and it can be used for printing trademarks, brand names etc. Overlays can be e.g. laminated or molded on the inlay. A typical RFID inlay is flexible, and, depending on the overlay, the RFID tag can be flexible or rigid. RFID inlays are typically sold in reels or rolls comprising hundreds to thousands of inlays. Generally the RFID tags can be either active or passive depending on whether they include an internal energy source, or they are operated with the electro-magnetic field generated by the RFID reader device. 
     RFID tags can operate on several frequencies. Four frequency ranges are generally defined as: (1) low frequency (LF); frequencies below 135 kHz, (2) high frequency (HF); frequencies around 13.56 MHz, (3) ultra high frequency (UHF); frequencies between 860 MHz and 960 MHz, and (4) microwave; frequencies around 2.54 GHz. RFID tags can be designed to operate near the reader device, or far from the reader device. 
     In case tags are designed to work near the reader device, the tags are known as near field tags, and the energy transfer from the reader device to the RFID tag is mostly through the magnetic field generated by the RFID reader. Data transfer from the tag to the reader device in near field case is enabled by inductive coupling, where the RFID tag changes its impedance, and the alternating load is detected by the reader device. Sometimes the communication in the near field is known as near field communication (NFC). 
     In case the tags are designed to work far away from the reader device, the tags are known as far field tags, and the energy transfer from the reader device to the tag is mainly through the electric field. Part of the RFID tag operates as an antenna, and the RFID device gets its energy from the electric field. In the far field case, data transfer from the tag to the reader device is enabled by field backscattering. In addition to the antenna, the RFID tag may comprise an impedance matching loop to fit the impedance of the RF communication circuit with the antenna. 
     The theoretical limit between the near field and the far field is proportional to λ/2π, where λ is the wavelength of the electromagnetic radiation generated by the reader device, equaling to c/f, where c is the speed of radiation (i.e. light) and f is the frequency. As a result, the limit between near and far fields for a HF RFID system would be 3.5 m and for an UHF RFID system the limit would be 5 cm. One can also define a transition zone between the near field and the far field. 
     In the near field tags, the strength of the inductive coupling between the RFID tag and the RFID reader is proportional to the area enclosed by the wiring of the RFID inlay. In the far field tags, the wiring of the RFID inlay performs as an antenna, and the length of the wiring must therefore be proportional to the wavelength λ. Even if these wirings can be made to meander in the inlay, these physical principles determine size limits for the RFID inlays, e.g. the minimum size. 
     RFID devices and the RF communication devices discussed above are generally energetically essentially passive. Such energetically essentially passive RFID tags are tags that operate while being in the reader field and being able to draw energy from the field. The field may be an electromagnetic field. The energetically essentially passive tags may comprise a capacitor to allow for short operation even when the field is turned off. 
     In addition to the identification, such RFID tags may be used for measurements. Therefore, in addition to identification, also other types of RF communication may be enabled with RF communication devices. The present invention is related particularly to energetically essentially passive RF transponders and their assemblies. Such energetically essentially passive RF transponders may be RFID tags, or may be able to perform other functions while engaged using an electromagnetic field. The energetically essentially passive RF communication devices may, in addition to drawing energy from the field, store the energy e.g. in to a capacitor. Thus they may operate for a while even without the presence of the field. 
     The size of such energetically passive RF communication devices is limited from below in principle in at least two ways:
         1) the frequency of the RF communication device need the match the specification for the device, and   2) the size of the device must be so large as to be able to draw energy from an electromagnetic field.       

     How the operating frequency is related to the size of an assembly will be apparent later. 
       FIGS. 1   a - 1   c  show an embodiment of the invention from different viewing angles.  FIG. 1   a  shows an assembly  100  for a radio frequency (RF) communication circuit as seen from top. The assembly  100  comprises an electrically insulating substrate  110  having a first side and a second side. In  FIG. 1   a , only the first side is shown. The assembly  100  further comprises a first electrically conductive structure  120  arranged on the first side of the substrate  110 . 
     The first electrically conductive structure has the structure of a split loop, wherein the split loop structure comprises a first end  122  and a second end  124 . A RF communication circuit is arranged to be attached to the first end  122  and to the second  124 . A split  125  is arranged in between the first end  122  and the second end  124 . Thus the loop is a split loop. The RF communication circuit is arranged to be attached to a site for the RF communication circuit. The site is at the split, i.e. between the first end  122  and the second end  124 . The RF communication circuit is arranged to be attached to its site such that the RF communication circuit closes the split loop. Thereby a closed loop is formed from the split loop and the communication circuit. 
     A loop, by definition is a structure that starts and ends at the same point. A loop further has a length, i.e. a loop is not a single point. Therefore a loop encircles a central part, and the angle of view of the loop, as viewed from the central part, is the full circle, i.e. 360 degrees. A split loop is splitted by the split  125 . Therefore, the angle of view of a split loop is less than the full circle. The ends  122  and  124  of the split loop are located relatively close to each other such that the RF communication circuit is can be attached to both the ends. The linear size of such circuits may be e.g. from 0.1 mm to 5 mm. Thus, the width of the split may be e.g. less than 5 mm. Typically the linear size of an RFID chip is about 0.5 mm. 
       FIG. 1   b  shows the assembly  100  for a radio frequency (RF) communication circuit of  FIG. 1   a  as seen from bottom. The assembly  100  comprises a second electrically conductive structure  140  arranged on the second side of the substrate. 
     The second electrically conductive structure arranged with respect to the first electrically conductive structure in such a manner that the site for the RF communication circuit overlaps the second electrically conductive structure. For example, at least one of the first end  122  and the second end  124  of the split loop  120  may overlap the second electrically conductive structure  140 . In  FIGS. 1   b  to  1   d  the site for the RF communication circuit overlaps the second electrically conductive structure. In  FIGS. 1   b  to  1   d  both the ends  122  and  124  overlap the second electrically conductive structure  140 . 
       FIG. 1   d  shows a radio frequency transponder  200  comprising the assembly of  FIG. 1   c  and further comprising the RF communication circuit  210 . RF communication circuit  210  is attached to the assembly  100  such that a part of the RF communication circuit  210  is attached to the first end  122  of the split loop and another part of the RF communication circuit  210  is attached to the second end  124  of the split loop, whereby the RF communication circuit  210  and the split loop structure form a closed loop. 
     The structures are such aligned for the following reason: the operating frequency of such a transponder depends, among other things, on the inductances and the capacitances of the device. In principle the operating frequency f is related to the inductance L and the capacitance C such that the frequency f is proportional to inverse of the square root of (LC), i.e. f is proportional to (LC) −1/2 . Therefore, increasing the inductance decreases the frequency. Furthermore, increasing the capacitance decreases the frequency. Inductance is related e.g. to the length of the wirings in the device. Decreasing the length decreases the inductance. When decreasing the size of the device, the wires tend to get shorter. This in effect decreases the inductance and increases the operating frequency. However, the operating frequency of the device is limited by the reader device and by standards. Therefore, in order to compensate for the decreasing inductance, capacitance should be increased. 
     The capacitance depends e.g. on the capacitance on the RF communication circuit  210  and on the capacitance experienced by the circuit  210  due to the assembly  100 . The latter capacitance depends on the capacitance of the joint, by which the RF communication circuit is attached to the assembly  100 , and on the internal capacitances of the assembly. In principle, the total capacitance may be written as (1/C)=(1/C chip )+(1/C chip-assembly ). Here C is the capacitance as defined above, C chip  is the internal capacitance of the chip  210  and C chip-assembly  is the capacitance experienced by the circuit  210  due to the assembly  100 , when the chip  210  is attached to the assembly  100 . 
     It has been noticed that the second electrically conductive structure  140  on the second side of the substrate  110  increases the capacitance C chip-assembly  significantly. The chip  210  not only experiences the capacitance of the joint by which the chip  210  is attached on to the first side of the substrate  110 , but in addition experiences an additional capacitance in relation to the second electrically conductive structure  140  on the second side of the substrate  110 . Therefore, the capacitance of the device increases, as compared to a structure without the second conductive structure  140 . 
     As discussed above, when targeting to a small structure, the decrement in inductance should be taken into account by an increment in the capacitance. Therefore, the assembly  100  comprises the second electrically conductive structure  140  in order to increase the capacitance of the assembly  100  for RF communication circuit. 
     To increase the capacitance the second electrically conductive structure  140  may overlap at least one of the ends  122  and  124 . In order to better characterize overlapping, it is noted that the substrate defines a direction, e.g. a direction perpendicular to the first surface. This direction is referred to as the direction of the substrate thickness. If the substrate is planar, the direction of substrate thickness is the direction from the first side to the second side. The first end  122  overlaps the second conductive structure  140 , when a first line, that comprises the first end  122  of the first electrically conductive structure  120 , and that is parallel to the direction of the substrate thickness, also comprises a point of the second electrically conductive structure  140 . Moreover, the second end  124  overlaps the second conductive structure  140 , when a second line, that comprises the second end  124  of the first electrically conductive structure  120 , and that is parallel to the direction of the substrate thickness, also comprises a point of the second electrically conductive structure  140 . Still further, the site for the RF communication circuit overlaps the second electrically conductive structure, when a third line, that comprises a point of the site for the RF communication (e.g. a point of the split  125 ), and that is parallel to the direction of the substrate thickness, also comprises a point of the second electrically conductive structure  140 . It is noted that a line is a set of points. 
     It has further been noticed that increasing the capacitance C chip-assembly  the operating frequency of the manufactured RF communication devices show less variation. As was discussed, the operating frequency depends e.g. on the capacitance. Furthermore, this capacitance depends on the capacitance of the joint, by which the RF communication circuit is attached to the assembly  100 , and on the internal capacitances of the assembly. However, the capacitance of the joint has some variations, since it depends on the joint, e.g. the shape of the joint that joins the chip to the assembly. The shape joint on the other hand depends on the translational and rotational positions of the chip with respect to the assembly. These have some variation due to the manufacturing process. Moreover, the joining pressure may affect these positions. Therefore, the capacitance of the joint has some variation. However, the capacitance C chip-assembly  is further affected by the internal capacitances of the assembly. Therefore, the proportional variation becomes much smaller, as the capacitance is increased by the second electrically conductive structure  140 . 
       FIGS. 2   a - 2   d  show split-loop structures. In  FIG. 2   a  the first conductive structure  120  has the shape of a split square. The first conductive structure  120  comprises an electrically conductive wire  126 , and two conductive pads  128 . The pads are arranged at the ends of the structure  120 . The pads may be used for connecting the chip  210  ( FIG. 1   d ) to the first conductive structure  120 . The split square forms the split loop. In addition, a focusing mark  220  is shown. The focusing mark  220  may be used to facilitate locating of the second electrically conductive structure  140  on the second side of the substrate  110 , with respect to the first electrically conductive structure  120  on the first side of the substrate  110 , to a location such that the second conductive structure increases the capacitance. The focusing mark  220  may be arranged in at least one of the first side of the substrate and the second side of the substrate. 
       FIG. 2   b  shows a split loop, wherein the structure is a split ellipse.  FIG. 2   c  shows a split loop, wherein the structure is a split arbitrary loop.  FIG. 2   d  shows a split loop, wherein the structure is a split ring. The ring refers to an essentially circular structure. Thus split ring refers to a structure, wherein the circular ring is broken by the split  125 . Even if not explicitly shown with a reference numeral, the split  125  is present in all the split loop structures of  FIGS. 2   a - 2   d.    
     In general, an electromagnetic field does not penetrate a metal sheet as well as it penetrates air. As an energetically passive device may draw its energy from the field, it may be preferable, that the field is not required to penetrate a conductive sheet. As shown in  FIGS. 1   a - 1   c , the second electrically conductive structure may therefore have such a shape, that it does not overlap the whole split loop. The second electrically conductive structure  140  does not overlap the whole split loop, when a line that penetrates a central part of the split loop structure, and that is parallel to the direction of the substrate thickness, does not comprise a point of the a point of the second electrically conductive structure. The line that penetrates a central part of the split loop structure is a line that is surrounded by the split loop structure. Moreover, the line that penetrates a central part of the split loop structure is a line that does not comprise a point of the first conductive structure  120 . In order to keep the area for field penetration relatively large, preferably at least half (50%) of the central area of the split loop on the first side of the substrate  110  is not overlapped by the second electrically conductive structure  140  on the second side of the substrate  110 . The term overlapping is understood in the sense described above for a single point. 
     The split ring structure ( FIG. 2   d ) is a preferred shape for near field tags, since in near field tags the area of the loop should be large. A large area means that more magnetic energy can be extracted from the field with the loop. A circular shape (i.e. a split ring) has a large area with respect to the linear size (i.e. diameter) of the split loop. 
     It has also been noticed that as the first conductive structure  120  and the second conductive structure  140  are separated by the insulating substrate  110 , a capacitance is formed between the first  120  and second  140  structures. Also this capacitance has the tendency of reducing the frequency, as discussed above. Therefore, preferably a large portion of the area of the first electrically conductive structure  120  overlaps the second electrically conductive structure  140 . Moreover, to ease the field penetration, the second structure  140  should have an open area corresponding to the central area of the first split loop structure  120 . An open area and relatively large overlap between the structures may be achieved, when the second structure  140  is either a loop or a split loop. The split loop structure is preferred, as it prevents the formation of an electric short circuit in the second electrically conductive structure. Therefore, preferably also the second electrically conductive structure is a split loop structure. 
     Thus, the second electrically conductive structure  140  may also have the shape of a split loop. As the second electrically conductive structure  140  and the first electrically conductive structure  120  are arranged on different surfaces of the substrate  110 , the first and the second structures are capacitively coupled to each other. Furthermore, when also the second electrically conductive structure  140  is a split loop structure, the second electrically conductive structure  140  may be used to guide a magnetic field penetrating the first and the second split loop structures. In particular also the second electrically conductive structure  140  may be used to extract energy from an electromagnetic field. 
     As a large portion of the area of the first electrically conductive structure  120  may overlap the second electrically conductive structure  140 , e.g. at least 50%, at least 66%, or at least 85% of the area of the first electrically conductive structure  120  may overlap the second electrically conductive structure  140 . The second electrically conductive structure may also be a split loop structure. 
     Even more preferably the first electrically conductive structure  120  essentially completely overlaps the second electrically conductive structure  140 , wherein the second electrically conductive structure  140  is also a split loop structure. The term “essentially completely overlaps” refers to the situation, where the structures overlap except for the splits. 
     More specifically, the substrate  110  defines a direction, e.g. a direction perpendicular to the first surface. This direction is referred to as the direction of the substrate thickness. The substrate may be planar. In the planar case, the direction of the substrate thickness is a direction from the first side to the second side. The second electrically conductive structure  140  may be arranged in relation to the first electrically conductive structure  120  such that each line that comprises a point of the first electrically conductive structure  120  and that is parallel to the direction of the thickness of the substrate either 
     (i) also comprises a point of the second electrically conductive structure  140 , or
 
(ii) penetrates the split  125  of the split loop structure of the second electrically conductive structure  140 .
 
     In the case where a large portion a large portion of the area of the first electrically conductive structure  120  overlaps the second electrically conductive structure  140 , overlapping is understood in the same sense as discussed above for the case of essentially complete overlapping. 
       FIG. 2   e   1  shows a first electrically conductive split ring structure  120 . The structure has the first end  122  end the second end  124 . The split  125  is arranged in between these ends. The split  125  is also a site for a RF communication circuit. In addition, a focusing mark  220  is shown.  FIG. 2   e   2  shows a corresponding second electrically conductive split ring structure  140  with the split  145 .  FIG. 2   e   3  shows the first structure of  FIG. 2   e   1  and the second structure of  FIG. 2   e   2  when aligned with respect to each other. It is understood, that a substrate  110  is located between these structures (cf.  FIG. 1   c ), even is the substrate is not shown in the figures. When the structures  120  and  140  are aligned, first of all, the site for the RF communication circuit on the first side of the substrate (i.e. the split  125 ) is being overlapped with the second electrically conductive structure  140  on the other side of the substrate. This is shown in the  FIG. 2   e   3  with the reference numerals  125  and  140 , particularly by the location for the numeral  140 . Furthermore, when the structures are aligned, the first electrically conductive structure  120  essentially completely overlaps the second electrically conductive structure  140 , and the second electrically conductive structure  140  is also a split loop structure. In some other embodiments, due to manufacturing tolerances, due to different pad configurations (pad  128 , cf.  FIG. 2   a ), or for other reasons, it is also possible that the overlap is not essentially complete. In this case a large portion of the area of the first electrically conductive structure  120  may overlap the second electrically conductive structure  140 .  FIGS. 2   e   3 ,  2   f   3 , and  2   g   3  show the overlap, however for the case of essentially complete overlap. 
       FIGS. 2   f   1 - 2   f   3  show conductive split loop structures. The reference numerals were explained in context of  FIG. 2   e   1 - 2   e   3 . The overlapping of different areas of the split loops were also discussed in context of  FIGS. 2   e   1 - 2   e   3 . In contrast to  FIGS. 2   e   1 - 2   e   3 ,  FIGS. 2   f   1 - 2   f   3  show conductive split loop structures, wherein the shape of the split loop is a rounded square. 
       FIGS. 2   g   1 - 2   g   3  show further conductive split loop structures. The reference numerals were explained in context of  FIG. 2   e   1 - 2   e   3 . The overlapping of different areas of the split loops were also discussed in context of  FIGS. 2   e   1 - 2   e   3 . In contrast to  FIGS. 2   e   1 - 2   e   3 ,  FIGS. 2   g   1 - 2   g   3  show conductive split loop structures, wherein the shape of the split loop is a rounded triangle. 
       FIGS. 3   a ,  3   b , and  3   c  show such an embodiment, wherein both the split loops  120 ,  140  are also split rings.  FIG. 3   a  shows the structure from a top view, wherein only the first electrically conductive structure  120  is shown.  FIG. 3   b  shows the structure from a bottom view, wherein only the second electrically conductive structure  140  is shown.  FIG. 3   c  shows the structure in a perspective view, wherein both the electrically conductive structures  120  and  140  are shown. The first structure  120  is shown in grey colour to distinct it from the second structure  140 . 
     As depicted in  FIGS. 3   a  to  3   c , the width of the second structure  140  may be greater than the width of the first structure  120 . Alternatively, the widths may be equal. The names of the structures  120  and  140  are interchangeable. The first structure  120  may be selected to describe the thinner (or otherwise smaller) of the structures  120 ,  140 . 
     Referring to  FIGS. 4   a  and  4   b , the split  125  of the first split loop structure and the split  145  of the second split loop structure are arranged, with respect to each other, in an angle. The situation is symmetric, and therefore, the angle may be measured in a clockwise or an anticlockwise direction. Thus the minimum value in principle could be zero degrees, and the maximum value 180 degrees. If the angle is very small, i.e. the splits are aligned, the increase in the capacitance, as discussed above, is lost. Therefore the angle may be e.g. at least 15 degrees. 
       FIG. 4   a  shows the structure in a top view, however showing both electrically conductive structures  120  and  140 , wherein the angle is small. In  FIG. 4   a , the angle is depicted with α, and the angle has the value of 25 degrees. In  FIG. 4   b , the angle is depicted with α, and the angle has the value of 180 degrees. Preferably the angle is large, e.g. more than 170 degrees, and even more preferably about 180 degrees. 
     In a preferred embodiment, both the first electrically conductive structure  120  and the second electrically conductive structure  140  have the shape of a split ring, and the inner and outer diameters of the split rings are equal, i.e. the shape of the second electrically conductive structure is similar to the shape of the first electrically conductive structure. The electromagnetic properties of the structure may be tuned with the angle α. 
     The substrate  110  may comprise polymer material. The polymer material may be e.g. polyethylene terephthalate (PET). PET has good electric properties for the purpose, and can be manufactured in relatively thin sheets. As known from the theory of plate capacitors, a thin substrate may increase the capacitance more than a thick substrate. The thickness of the substrate, Ts ( FIG. 1   c ), may be from 5 μm to 100 μm, or preferably in the range from 20 μm to 40 μm, to increase the capacitance. In addition or alternatively, the substrate  110  may comprise fibrous material such as paper. In addition or alternatively, the substrate may comprise ferromagnetic material to improve the magnetic coupling of the RF communication device and the reader device. In addition or alternatively, the substrate may comprise dielectric material, such a ceramics with a high permeability, to further increase the capacitance and thus decreasing the size or frequency. 
     Thickness, Ts, width, Ws, and length, Ls, of the substrate  110  are shown in  FIG. 1   c . The width, Ws, of the substrate depends on the use, and may be e.g. from 3 mm to 20 cm. The length, Ls, of the substrate depends on the use, and may be e.g. from 3 mm to 20 cm. In an embodiment, the outer diameter of the split ring is 7 mm, and the width and the length of the substrate are slightly more, about 8 mm. 
     At least one of the first electrically conductive structure  120  and the second electrically conductive structure may comprise metal. At least one of the structures  120 ,  140  may comprise at least one of the following metals: copper, aluminium, silver, and gold. The thickness of the conductive structure may be from 1 to 50 μm, preferably from 5 to 10 μm. 
     Copper and aluminium are relatively cheap conductor materials, and can be easily etched. In an embodiment, the electrically conductive structures are formed by etching. Therefore, in some embodiments one of copper and aluminium are preferred for the conductor materials. 
     In an embodiment one or several of the following features may be present:
         the first electrically conductive structure  120  comprises aluminium,   the second electrically conductive structure  140  comprises aluminium,   the thickness of at least one conductive structure is 9 μm,   the substrate  110  comprises polyethylene terephthalate (PET),   the thickness of the substrate is 38 μm,   the width of electrical wiring forming the split loop structure of the first electrical structure is less than 1.5 mm, preferably about 0.75 mm, and   the outer diameter of the split loop is less than 15 mm, preferably less than 10 mm, e.g. about 7 mm.       

     The diameter of a non-circular split loop may be regarded as the smallest of the dimensions from one boundary of the split loop to an opposite boundary of the split loop. 
     The assembly of two split loop structures as described above may also be used in connection with an antenna structure.  FIG. 5   a  shows an assembly comprising the first  120  and second  140  electrically conductive structures as discussed above. The embodiment of  FIG. 5   a  further comprises an antenna structure  520 . The split loop structures  120  and  140  are located a distance apart from the antenna structure  520 . Therefore, at least one of the split loop structures  120 ,  140  is capacitively or inductively coupled to the antenna structure  520 . In this way, a radio frequency antenna for boosting radio frequency transmission is formed. In  FIG. 5   a , the antenna structure  520  is arranged on the same substrate  110  as the split loop structures  120 ,  140 . 
     Referring to  FIG. 5   b , the antenna structure  520  may also be arranged onto another substrate  510 . The loop structures  120  and  140  and the substrate  110  in between the structures may be attached to the other substrate  510 . 
     Referring to  FIG. 5   c , one of the loop structures  120 ,  140  may be galvanically connected to the antenna structure  520 . In a galvanic contact there is no distance between the loop structure and the antenna structure. Thus the electromagnetic field in the loop  120  or  140  may propagate galvanically, i.e. through the conductive material, to the antenna structure  520 . 
     The dual-layer structure of the split loops, as discussed above, may diminish the size of the frequency matching loop of an antenna structure. Also, if more space is available for an antenna, the meandering antenna structure may be made somewhat straighter, which improves the properties of the antenna. The antenna structure  520  may be e.g. a dipole antenna. 
       FIG. 6  shows another structure, wherein two dipole antennas  520   a  and  520   b  are arranged perpendicularly to each other in a plane. The structure is capable of operating in various rotational positions with respect to a reader device. A first electrically conductive structure is shown in the figure with the reference numerals  120   a ,  120   b ,  120   c , and  120   d . Each of these parts of the first electrically conductive structure forms a split loop. Two ends ( 122 ,  124 ) of the split loop  120   a  structure are also shown. In addition, the ends comprise pads  128  for attaching a RF communication circuit to the assembly. As is clear from the figure, the first electrically conductive structure comprises also other ends; four ends in total. The structure is designed for a RF communication circuit comprising four terminals. Each end of the first electrically conductive structure corresponds to a terminal of the RF communication circuit. The second electrically conductive structure  140  is not shown in  FIG. 6 . It is understood, that the second electrically conductive structure  140  is arranged on the second side of the substrate at least to the central part, in order to increase the capacitance as discussed above. The second electrically conductive structure  140  may further comprise at least one area forming at least one split loop. The area or areas may be aligned with at least one of the split loop structures  120   a ,  120   b ,  120   c , and  120   d.    
     Assemblies with dipole antennas may be used e.g. in far field communication, wherein the energy is extracted from electromagnetic field, mostly from the electric part of the field. 
     In near field communication, wherein the energy is extracted from electromagnetic field, mainly from the magnetic part, the magnetic coupling between the reader device and the RF communication device may be further enhanced by increasing the number of overlapping split loops.  FIG. 7   a  shows, in an exploded perspective view, layers of an RF assembly. In addition to the part that have been described earlier, the assembly of  FIG. 7   a  comprises
         a second substrate  710  comprising a first side and a second side, and   a third electrically conductive structure  720  arranged on a first side of the second  710  substrate, wherein   the first electrically conductive structure  120  or the second electrically conductive structure  140  is arranged on the second side of the second substrate  710 ,   the third electrically conductive structure  720  at least partly overlaps the first or the second electrically conductive structure  120 ,  140 , and   the third electrically conductive structure  720  has the shape of a split loop.       

     This assembly further guides a magnetic field penetrating the split loop structures, and enhances to magnetic coupling between the RF communication assembly and the reader device. In  FIG. 7   a  the split loop structures have the shape of a (circular) split ring structures. 
     Referring to  FIG. 7   b , the structure with three split loops may be manufactured e.g. by manufacturing a first assembly comprising the first substrate  110  with the first and second electrically conductive layers  120  and  140 ; manufacturing a second assembly comprising the second substrate  710  and the third electrically conductive layer  720 ; and attaching the second assembly to the first assembly.  FIG. 7   c  shows an assembly with three the co-centric overlapping split ring structures. 
     Referring to  FIG. 1   d , an RF communication circuit  210  may be attached to the assembly of any of the  FIGS. 1   a - 1   c ,  2 - 6 , and  7   c . In addition, the RF communication circuit  210  may be attached to a partial assembly (e.g. the first or the second assembly discussed in the context of  FIG. 7   b ). The assembly with the RF communication circuit forms a radio frequency transponder  200 . In the transponder, the RF communication circuit  210  is attached to the assembly  100  such that a part of the RF communication circuit  210  is attached to the first end of the split loop and another part of the RF communication circuit is attached to the second end of the split loop, whereby the RF communication circuit and the split loop structure form a closed loop. 
     The RF communication circuit  210  may be attached to the assembly  100  by using known join techniques such as adhesive joining or solder joining. Adhesive joining may be done using electrically conductive or non-conductive adhesives. Conductive adhesives may be isotropically conductive or anisotropically conductive. Adhesives may be supplied in the form of film or paste. Anisotropic adhesives are particularly suitable for attaching small RF communication circuits  210  to the assembly  100 . Solder joining may also be applied. 
     The radio frequency transponder  200  may be arranged to extract its operating power from an electromagnetic field using the closed loop formed by the RF communication circuit  210  and a split loop, whereby the radio frequency transponder may be energetically essentially passive. 
     The transponder may be attached to an item. The item may be e.g. a commercial item. The commercial item may be available for sale in a store. The item may be stored in a warehouse and/or tracked for inventory purposes. The item may be a vessel arranged to contain samples, whereby the RF transponder may be used to identify the sample. 
     A particularly attractive application is one, where several small objects are to be identified from a close distance. The objects may need to be identified all at substantially the same time or in sequence. As the objects are small, a large coil structure is not a feasible solution. The objects may be arranged in a row or in a matrix. In near field communication, the area within a loop affects the magnetic coupling between a reader and the RF communication device. As some of the embodiments have multiple (two or three) split loops, the magnetic coupling is good even if the size of a single loop is relatively small. Moreover, because of the overlapping structures and increased capacitance, a reasonably operating frequency is obtained with small structures.