GB2467185A

GB2467185A - Antenna Arrangement of RFID Tag

Info

Publication number: GB2467185A
Application number: GB0901326A
Authority: GB
Inventors: David Rose; Richard Cresswell
Original assignee: NAVIGATOR SYSTEMS Ltd
Current assignee: NAVIGATOR SYSTEMS Ltd
Priority date: 2009-01-27
Filing date: 2009-01-27
Publication date: 2010-07-28
Also published as: WO2010086596A1; GB0901326D0

Abstract

A RFID tag arrangement comprises an RFID tag attached to a generally cylindrical object (100). The RFID tag comprises an antenna and an integrated circuit coupled to the antenna wherein the tag is attached to the cylindrical object in such as way so that at least part of the antenna extends around a circumference of the object. The antenna may extend around the object in a helical path, and may comprise two or three parts each following a different path around the object. The first and second parts may be produced by folding part of a longitudinal antenna pattern. The object may be a carrier (100) for arrangement around a second generally cylindrical object (50) such as a cable or pipe. The carrier may comprise a split cylinder.

Description

RFID Tag This invention relates to the arrangement of RFID tags on generally cylindrical objects.

It is well understood that RFID tags can be used to track and identify goods and other items. A RFID system typically comprises a reader and a RFID tag attached to the object to be tracked. The reader, which is often referred to as an interrogating device, is an electronic device used to communicate with the RFID tag. A reader has one or more antennas, which transmit and receive radio waves to and from the RFID tag.

RF]D tags contain a number of components, typically including an integrated circuit for storing and processing information, modulating and demodulating a radio frequency (RF) signal, and an antenna for receiving and transmitting a RF signal.

Although there are many known types of RF]D tag, they can be split into three broad categories: active tags, semi-active tags and passive tags.

Active RFID tags have a battery-powered receiver and transmitter. Semi-active tags have a receiver that is powered by the incoming RF signal and an active transmitter.

Passive tags do not have their own power source, and rely solely on power from the received RF signal from the reader to power the tags.

When RF waves from the reader reach the antenna of a passive tag, the energy from the RF waves is converted into electrical energy that can power the integrated circuit in the tag. The tag is then able to send back information stored on the integrated circuit, powered by the energy provided from RF waves from the reader. The RFID tag communicates with the reader by modulating the load on the RFID antenna, causing a portion of the incident electromagnetic energy to be back-scattered to the RFID reader.

The reader receives the back-scattered electromagnetic radiation and decodes the modulated signal.

Reading a passive RFJD tag therefore involves a two-way communication between the reader and the RFID tag. The distance at which a passive RFID tag can be read depends on a large number of factors. These factors include the output power of the reader, the environment around the RFID tag, and the efficiency with which the RFJD tag interacts

with the incident electromagnetic field.

Passive RFID tags are designed to work at a predetermined frequency, or predetermined frequency range. For example, UHF passive tags can be designed to work in the frequency range of 868-928 MHZ. Due to a lack of a global standard, different countries use different ranges for UHF tags. For example, the preferred frequency in North America is 915 MHz, 868 MHz for Europe, and 956 MHz for Japan. Ut-IF RFID tags can be made to be tuned to particular region's frequency or frequency band, or can be tuned to the entire UHF spectrum.

Figure 1 shows a schematic of a conventional passive RFID tag, that could be used as an UHF RFII) tag. Such a RFID tag can be manufactured in high volume, and can be inexpensive. The tag 10 comprises a substrate 20, on which there is an integrated circuit 40 and an antenna pattern 60. The tag 10 in Figure 1 is in the form of an elongate strip, with the antenna pattern 60 extending in a generally longitudinal direction.

As an example, the tag 10 could have a length of around 100mm and a width of around 14 mm. For example, the tag 10 could be an ALN-9540-WR tag. Of course, embodiments of the invention are applicable to a wide range of tags as will be discussed.

The substrate 20 can be rigid or flexible. For example, the substrate 20 can be a flexible plastic material provided with a suitable adhesive for adhering the tag to an object.

The antenna pattern 60 is typically formed of a metal wire pattern that is printed (or otherwise transferred) onto the substrate 20. Although not shown in Figure 1, the antenna pattern 60 may be covered by a protective layer, for example a layer of transparent plastic.

The antenna pattern 60 in tag 10 comprises a first portion 65 in the region of integrated circuit 40. Second and third portions 61 and 62 of the antenna pattern extend from the first portion 65 in opposite longitudinal directions of the tag 10. In the arrangement of Figure 1, the first portion 65 includes a loop that connects the individual patterns of the second and third portions 61 and 62 of the antenna pattern.

The second and third portions 61 and 62 of the antenna pattern each comprise tuning regions 63 in which the wire of the antenna pattern is arranged in a wave-like pattern.

The tuning regions 63, in conjunction with the rest of the antenna pattern 60, enable the tag 10 to respond to the desired predetermined frequency range.

The terms extension "direction" or extension "path" of the antenna pattern or portions of the antenna pattern will be used extensively in this description. It is to be understood that, for example, the while the actual antenna pattern may have a complicated internally folding design (e.g. as shown in the tuning regions 63 in Figure 1), the extension "direction" of a portion of an antenna pattern is the general direction in which that portion extends. As an example, it can be said that the extension direction of the antenna pattern 60 in Figure 1 is in a longitudinal direction along the strip shape of the tag 10, even though the metal (or other material) forming the antenna pattern 60 has numerous folds and curves.

The integrated circuit 40 receives the received RF signals from the antenna and modulates the back-scattered signal so as to send information stored on the integrated circuit 40 back to reader.

The antenna pattern 60 needs to be efficient to absorb incident electromagnetic radiation from the reader and to back-scatter electromagnetic radiation back to the reader.

Furthermore, the connection between the antenna pattern 60 and the integrated circuit needs to be efficient to supply sufficient energy to the integrated circuit.

The efficiency of the interaction of the electromagnetic field with the tag 10 depends on the antenna pattern and the efficiency of coupling the electromagnetic energy from the antenna pattern 60 into the integrated circuit 40. This efficiency is related to the impedance of the antenna 60 and the impedance of the integrated circuit 40.

Efficient coupling of electromagnetic energy between the antenna 60 and the integrated circuit 40 depends on substantially matching or exactly matching the impedance of the antenna to the impedance at the input connections of the integrated circuit.

Passive UI-IF tags 10 of the type schematically shown in Figure 1 can be considered as dipole antennas, and would typically have a length in the order of one-half wavelength of the preferred frequency. Circular polarisation is used to minimize the signal loss due to angular misalignment of the tags. It will be appreciated that the precise design of passive UHF RFID tags is well known, and therefore no further detail will be provided.

For example, it will be appreciated that passive UI-IF RFID tags can include various antenna patterns, and are not limited to the elongate strip forming a simple dipole antenna of the type shown in Figure 1.

A limitation of RFID tags (particularly passive UHF RFID tags) is that they do not operate well nearby any objects that changes the resonance of the antenna or any object that affects the loading of the antenna. This places limits on the suitability RFID tags (particularly passive UHF RFID tags) for tracking metal objects, objects that contain large parts of metal, water or any material with a hard carbon content (e.g. rubber).

Using a conductive material in sufficient close proximity to the antenna of an RFID tag, can de-tune' the antenna so that the RFID tag will effectively be inoperable. If a conductive material is in electrical contact with the antenna of the tag, or located so close to the antenna that it appears to be in electrical contact at RF frequencies, then the material therefore can effectively become a part of the antenna. This has the effect of changing the "length" of the antenna (for example, by changing the inductance/capacitance of the antenna), meaning that the antenna is no longer tuned as it was designed to be. Therefore, such materials in close proximity to the tag may alter the response of the antenna so that the tag no longer responds in the predetermined frequency range assigned to the tag.

Furthermore, various other environmental factors can detune the RFID tag's antenna, resulting in a shifting in the frequency to which the tag antenna is sensitive. For example, a RFTD tag attached to a liquid filled container can experience antenna detuning due to a parasitic capacitance provided by the container.

It will be appreciated that materials such as metal can also reflect or block RF energy in a way that reduces performance of the antenna. For obvious reasons, this can severely limit the usefulness of the tag, as if detuned it would not provide an appropriate back-scattered signal to the reader.

A conventional way of improving the sensitivity of a passive RFIID tag when attaching it to or near a metal (or otherwise conductive) object is to place an inlay of a spacing material between the tag and the metal. Such an arrangement is schematically shown in figure lb. In figure ib, a tag 10 (for example of the type schematically shown in figure la) is mounted on a spacing material 80. The spacing material 80 is then mounted on a metal object 90. The spacing material 80 could be, for example, a plastic material of thickness of around 6mm or more. The spacing material 80 separates the tag 10 from the metal object 90. A cover (not shown in figure ib) would typically be placed over the tag 10 so as to protect it.

Such a conventional arrangement is associated with a number of problems. The combination of the cover and the spacing material 80 will mean that the entire tag arrangement is bulky. Furthermore, as the metal object 90 will act as a radio reflector, and the off-axis sensitivity of the tag is reduced. When the receiver is arranged on an axis that is tag normal to metal object 90, the orientation of the tag around this axis is not important as RFID tags typically use circular polarisation. Therefore, when compared to the simple dipole antenna of the tag of Figure la when in free space, the orientation of the tag in Figure lb with respect to the reader becomes important.

As a result, when mounted as in Figure 2, it is very important that the tag needs to be orientated correctly with respect to the receiver for the tag to be effectively read. In other words, when placed near metal object 90, even with a spacer 80, the gain of the antenna (i.e. the directionality of the antenna) is increased with respect to the tag when in free space.

For many applications, it is desirable that the gain of the antenna of the RFJD tag is as low as possible. This is because if the antenna of the RFID tag has a high gain, it is necessary to orientate the tag in a more precise way with respect to the reader.

There are many instances where it would be desirable to use RFID tags to track the location of cables or other generally cylindrical objects. For example, the cables that are used to connect professional lighting or sound equipment may be specialist, high value items.

Using professional lighting or sound equipment as an example, it will be appreciated that such equipment is often hired out. The company hiring out the lighting or sound equipment may wish to place RFID tags on each component to enable them to keep track of each component. When the lighting or sound equipment is then returned, an RFID reader could be used to check whether all the hired components have been properly returned.

It will be appreciated that cables typically comprise metal cores that are sheathed in rubber. Both metal and the rubber (due to its high carbon content) have the undesirable property of detuning a typical RFID tag placed in proximity to the cable. This is because these materials can be considered as being conductive at RF frequencies, which can detune the antenna of the RF1D rag.

As a result, those skilled in the art would expect that simply attaching a typical RFTD to a cable would lead to unsatisfactory tag performance. It would be expected that the metal core of the cable and the rubber sheath of the cable would detune the antenna of the RFID tag and so lead to poor performance.

Therefore, the conventional way of tagging a cable is to provide a RF]D tag on a support (similar to that shown in Figure ib), and attaching the RFID tag and support combination to the cable using a cord.

Such an arrangement is unsatisfactory for a number of reasons. For example, such an arrangement will be relatively expensive, as a tag, support, cover and cord are needed.

Furthermore, by their very nature, cables are often laid on the ground and laid around objects, through holes etc. A bulky RFIID tag arrangement attached to the cable by a cord is liable to break off in normal use. For example, when a cable that has an RFID tag attached to it via a cord is pulled out from a bundle of other cables, the cord attached to the RFID tag may well get snagged or tangled with another cable/cord and then snap off.

As a result, it is desirable to produce better arrangements for tagging cylindrical objects such as cables. It is particularly desirable to produce arrangements for tagging cylindrical objects in a way that has reduced sensitivity to the orientation of the tag with respect to the reader, and that helps minimises the detuning effect of the cylindrical object on the antenna of the tag. -According to a first aspect of the invention there is provided an RFIID tag arrangement comprising an RFID tag attached to a generally cylindrical object, the RFID tag comprising: an antenna and an integrated circuit coupled to the antenna; wherein the RFID tag is attached to the cylindrical object in such as way so that at least part of the antenna extends around a circumference of the cylindrical object.

In some embodiments, the said at least part of the antenna extends around a circumference of the cylindrical object in a helical path.

In some embodiments, the antenna comprises an antenna pattern with a first portion and a second portion, with the integrated circuit being located in the region of the first portion (e.g. connected to the first portion), and the first portion being connected to the second portion, wherein the first portion extends on the circumference of the cylindrical object in a first path and the second portion extends on the circumference of the cylindrical object in a second path that is different to the first path.

In some embodiments, the second path is helical. The second path may have a helix angle of between 00 to 90°, for example 35° and 60°. This helix angle may be 45°. The helix angle may be measured in either rotation direction.

In some embodiments, the third portion extends on the circumference of the cylindrical object in a different rotation direction to the second path. In other words, some embodiments have second and third helical paths that rotate in different directions to each other. This particular arrangement of the antenna pattern has been found to help minimise coupling between the cable and the antenna.

It has been found that the arrangement with the first portion of the antenna pattern in a first helical path around the object and with the second and third portions each arranged in transverse helical paths to the first helical path is effective at minimising coupling between the object and the antenna. This arrangement of the antenna pattern has also been found to produce particularly effective at improving the angular response of the tag. This is important, because there will be many practical applications for tagging objects in which the object is to be tagged is, for example, lying in a box in a random orientation.

In some embodiments, the first and second portions of the antenna pattern are produced by folding part of a longitudinal antenna pattern. For example, the longitudinal antenna pattern could have a length of around 50 mm to 150 mm, e.g. 100 mm. Such tags could, for example, be used on carriers (or other cylindrical objects) of diameters 15 mm to 25 mm.

In other embodiments, the first and second portions of the antenna pattern are produced by providing a base antenna pattern with a suitable unfolded shape so that when the RFID tag attached to the cylindrical object, the first and second paths are produced around the cylindrical object without any folding of the tag. For example, a 2D base antenna pattern could be produced on a substrate of the RFID tag with suitable bends in the 2D base pattern so that the first and second portions of the antenna pattern produce the desired first and second portions of the antenna pattern. For example, such a 2D base pattern could be "S" shaped.

In some embodiments, the antenna pattern comprises a third portion, with the first portion being connected between the second portion and the third portion, wherein the third portion extends on the circumference of the cylindrical object in a third path that is different to the first path. The third path may be helical. The third path may have a helix angle of between 00 to 900, for example between 35° and 60°. This helix angle may be measured in either rotation direction. This helix angle may be 45°.

In some embodiments, the first path is helical. The first path may have a helix angle of between 0° to 90°, for example 25° and 60°. This helix angle may be measured in either rotation direction. This helix angle may be 45°.

In some embodiments, the cylindrical object is a carrier for arrangement around a second generally cylindrical object. The carrier may comprise a split cylinder to enable the carrier to be arranged around the second generally cylindrical object.

As a result of the split, the carrier can be flexed so as to open the split to enable the carrier to be placed around the second generally cylindrical object. As a result, the carrier can be arranged over the second generally cylindrical object without having to slide the carrier over an end of the second generally cylindrical object.

The carrier provides separation between the RFID tag and the second generally cylindrical object, which helps to minimise RF coupling between the second cylindrical object and the tag and minimise the blocking effect of the second cylindrical object on the tag.

In some embodiments, the RFID tag arrangement further comprises an outer member that is arranged to cover the antenna pattern. This outer member may have rounded corners.

According to a second aspect of the invention there is provided a method of producing a RFID tag arrangement that includes attaching an RFJD tag around a generally cylindrical object, the RFID tag comprising an antenna and an integrated circuit coupled to the antenna, the method comprising: attaching the RFID tag to the cylindrical object in such as way so that at least part of the antenna extends around a circumference of the cylindrical object.

Such methods could be used to producing a RFID tag arrangement according to the first aspect.

According to a third aspect of the invention there is provided a RFJD tag comprising an antenna and an integrated circuit coupled to the antenna, wherein: the antenna comprises an antenna pattern with a first portion and a second portion, with the integrated circuit located in the region of the first portion and the first portion being connected to the second portion; the first portion and second portion are arranged at an angle so that when the RFID tag is attached around a circumference of a cylindrical object, the first portion extends on the circumference of the cylindrical object in a first path and the second portion extends on the circumference of the cylindrical object in a second path that is different to the first path.

In some embodiments, the first and second portions of the antenna pattern are produced by folding part of a longitudinal shaped base antenna pattern. The antenna pattern may comprise a third portion, with the first portion being connected between the second portion and the third portion, so that when the RFID tag is attached around a circumference of the cylindrical object the third portion extends on the circumference of the cylindrical object in a third path that is different to the first path; and the third portion may be produced by folding part of the longitudinal shaped base antenna pattern.

In some embodiments, the antenna pattern comprises the first and second portions in an unfolded state of the RFID tag, so that when the REID tag attached to the cylindrical object, the first and second paths are produced around the cylindrical object. The antenna pattern may comprises a third portion in an unfolded state of the RFID tag, with the first portion being connected between the second portion and the third portion, so that when the RFID tag is attached around a circumference of the cylindrical object, the third portion extends on the circumference of the cylindrical object in a third path that is different to the first path. The tag may comprise a substrate on which the antenna pattern is disposed, wherein the first, second and third portions are deposited onto the substrate to form the antenna pattern. The antenna pattern may comprise a metal wire pattern that is printed (or otherwise transferred) onto the substrate, In some embodiments, the RFID tag may be "S" shaped in an unfolded state.

Embodiments of the invention include manufacturing a RFID tag as described above.

According to a third aspect of the invention there is provided a RF1D tag arrangement comprising an RFIID tag attached to a generally cylindrical carrier, the RFID tag comprising: an antenna and an integrated circuit coupled to the antenna; wherein the carrier is suitable for arrangement around a generally cylindrical object.

In such an aspect, the RFID tag could be attached to the carrier in any orientation. The carrier provides separation between the RFID tag and the generally cylindrical object, which helps to minimise RF coupling between the cylindrical object and the tag and minimise the blocking effect of the cylindrical object on the tag.

The carrier may comprise a split cylinder to enable the carrier to be arranged around the second generally cylindrical object. In some embodiments, the RFJD tag arrangement further comprises an outer member that is arranged to cover the antenna pattern. This outer member may have rounded corners.

Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which:-Figure 1 a is a schematic representation of a RFID tag; Figure lb is a schematic representation of a conventional way of mounting a passive RFJD tag onto a metal object; Figures 2a, 2b to 13a, 13b are plots of RSSI for different angles for various orientations of a RFID tag around a cylindrical object; Figure 14 is a schematic representation of a cross section of a RFID tag around a cylindrical object; Figure 15 is a schematic representation of an apparatus for testing a RFID tag around a cylindrical object; Figure 16 illustrates folding a RFID tag according to an embodiment of the invention; Figure 17 illustrates a RFID tag arrangement according to an embodiment of the invention; Figure 18 illustrates a carrier around an object according to an embodiment of the invention; Figure 19 illustrates an outer case around an object according to an embodiment of the invention; Figure 20 illustrates carrier and outer case arrangements according to embodiments of the invention; Figure 21 illustrates the helix angle made by some embodiments of the invention; Figure 22 shows an arrangement of two wires; and Figure 23 shows an arrangement analogous to a transformer.

Embodiments of the invention relate to the arrangement of RFID tags on generally cylindrical objects. Cables provide one example of such generally cylindrical objects.

However, embodiments of the invention can be used for any cylindrical objects, whether flexible or rigid. For example, embodiments of the invention could be used to tag rods, hoses, poles, bars etc. A first embodiment of the invention will now be discussed in relation to Figures 16 to 18. In this embodiment, a RF]D tag 10 is first folded into a folding pattern and then arranged around the circumference of a cable 50. In this embodiment, the tag 10 is of the type schematically shown in Figure 1. This produces a RFID tag arrangement.

As discussed above, the tag 10 is in the form of a strip and comprises an antenna in the form of an antenna pattern 60. Therefore, the tag 10 can be thought of as having a longitudinally shaped "base antenna pattern". The antenna pattern 60 comprises a first portion 65 that includes a loop around the IC 40 (not shown in Figure 16, but shown in Figure la), and second and third portions 61 and 62 that, in the unfolded state of the strip, extend from the first portion 65 in opposite longitudinal directions. The second and third portions 61 and 62 each comprise a tuning region 63 (not shown in Figure 16, but shown in Figure 1 a) that enable the tag 10 to respond in the desired predetermined frequency associated with the tag 10.

In this embodiment, the substrate 20 of the tag is a flexible material and the antenna pattern 60 is formed by printing metal onto the substrate 20. As a result, the whole tag is flexible and can be folded in various orientations.

Figure 16 shows a schematic view of a cable 50 with a folded RFIID tag 10 of the general type shown in Figure 1. The cable 50 in this embodiment is 12 mm cable with a metal core and rubber sheath. However, embodiments of the invention can be used with any type of known cable or cable diameter.

As shown in Figure 16, the tag 10 is folded at region 68 at a point roughly corresponding to the boundary between the first portion 65 of the antenna pattern (that includes a loop around the IC 40) and the second portion 61 of the antenna pattern 60.

The tag 10 is also folded at region 69 at a point roughly corresponding to the boundary between the first portion 65 of the antenna pattern and the third portion 62 of the antenna pattern. Therefore, when considered in 2 dimensions, the tag 10 is folded into an "S" shape, as shown in Figure 16.

In this embodiment, the tag 10 is not directly to the cable 50. Instead, as shown in Figure 17, a carrier 100 is first placed around the cable 50 so that the tag 10 and the carrier 100 together form a RF1D tag arrangement. As shown in Figure 18, the carrier is in the form of a cylinder with a split region 101. As a result of the split region 101, the carrier 100 can be flexed so as to open the split to enable the carrier 100 to be placed around the cable 50. As a result, the carrier 100 can be arranged over the cable without having to remove the ends of the cable 50. This is beneficial, as it enables the carrier 100 to be fitted non-destructively over existing cables.

The carrier 100 in this embodiment is 20mm in diameter and is made of a synthetic rubber and is shorter than the unfolded length of the tag 10. However, the carrier 100 could be made out of a variety of different materials, such as plastics, ceramics or any material that is non-conductive at RF frequencies, or be of different lengths.

In Figure 17, the tag 10 is attached to the carrier 100 in such as way that the first portion of the antenna pattern extends around a circumference of the carrier 100 and along the axis of the carrier 100. Thus, the first portion 65 extends in a first helical path around the carrier 100, and thus also around the cable 50.

As shown in Figure 17, as a result of the fold 68, the second portion 61 of the antenna pattern extends around the circumference of the carrier 100 and along the axis of the carrier 100 in a second helical path. The second helical path has a different handedness to the first helical path. In other words, the fold 68 causes the second portion 61 of the antenna pattern to extend in a second helical path that is transverse to the first helical path. More detail on these helical paths will be provided later.

In addition, the fold 69 causes the third portion 62 of the antenna pattern to extend around the circumference of the carrier 100 in a third helical path that is transverse to the first helical path. In this embodiment, the second and third helical paths are congruent. However, in other embodiments, the second and third portions may be arranged to make different angles with the axis of the carrier 100. In other words, the second and third portions may extend along helical paths with different helix angles.

The carrier 100 holds the tag 10 away from the cable, which helps to minimise RF coupling between the cable and the tag. This also improves off-axis response, as will be discussed in more detail later.

Figure 19 shows an outer case 150 that can be provided over the carrier 100. The outer case 150 in this embodiment is provided as two halves 151 and 152 that can be secured together. For example, the two halves 151 arid 152 could be clipped together. As a result, the outer case 150 can be arranged over the carrier 100 without having to remove the ends of the cable 50.

In other embodiments, the two components of the outer case 150 could be attached to each other in different ways. For example, they could be screwed together, glued together or secured with a clasp. Furthermore, in other embodiments, other arraignments of the outer case 150 are possible. For example, the outer case 150 may comprise two hingedly joined members that can be closed together or more than two components attached together.

The outer case 150 in this embodiment has a generally cylindrical shape, with rounded ends 155. The rounded ends 155 of the outer case 150 are provided to help prevent the outer case 150 from snagging on other cables or objects. By having rounded ends 155, the outer case 150 is less likely to get caught on other objects.

Therefore, the first embodiment of the invention provides a RFID tag 10 arranged in an "S" shape and laid over a carrier 100, and provided with an outer case 150. This arrangement has numerous benefits when compared to conventional RFTD tag arrangements for cables.

As discussed above, the conventional way to attach an RFID tag to a cable, is to mount the tag on a suitable substrate and cover and attach the whole arrangement to the cable via a cord. This is done because it is known that putting the RFID tag in close proximity to the cable 50 would detune the antenna of the cable, due to a variety of effects such as the RF coupling of the material of the cable (rubber, metal etc) with the antenna.

Attaching the RFID tag to a cable via a cord in this way is expensive and cumbersome.

Furthermore, such arrangements are often not practical in situations in which the cables are moved around a lot, because the cords are liable to get snagged and pulled off.

In the first embodiment of the invention, the particular arrangement of the antenna pattern 60 has been found to help minimise coupling between the cable and the antenna.

In particular, it has been found that arranging the first portion 65 of the antenna pattern in a first helical path around the cable 50, with the second and third portions 61 and 62 each arranged in transverse helical paths to the first helical path is particularly effective at minimising coupling between the cable 50 and the antenna.

This arrangement of the antenna pattern 60 has also been found to produce particularly effective at improving the angular response of the tag. In other words, as will be discussed in more detail later, this arrangement of the anterma pattern 60 can be read with the tag at a particularly large range of angles around the axis of the cable with respect to the reader. Therefore, this arrangement of the antenna pattern 60 does not have to be orientated in a particular way with respect to the reader. This is important, because there will be many practical applications for tagging cables in which the cable to be tagged is, for example, lying in a box in a random orientation.

Other benefits of this arrangement of the antenna pattern 60 will be discussed later.

The carrier 100 holds the tag 10 away from the cable, which helps to minimise RF coupling between the cable and the tag. This also improves off-axis response, as it helps minimise the "blocking" effect of the cable on the tag, as will be discussed in more detail later.

The outer case 150 is provided to protect the tag 10. The rounded ends 155 of the outer case 150 help prevent the outer case 150 from snagging on other cables or objects.

While a first embodiment has been discussed in relation to a tag folded into an "S" shape and then laid over a carrier 100, the invention is not limited in this way. For example, while the carrier 100 is beneficial to the first embedment, in other embodiments, the tag 10 could be attached directly to the cable 50.

In addition, while the outer case 150 protects the tag 10 in the first embodiment, other embodiments need not use such an outer case. For example, in some situations, it is not necessary to provide a case or other means for protecting the tag 10. In addition, if protecting the tag is desired, other embodiments may use a different method of protecting the tag 10. Furthermore, other embodiments may use types of tags that do not require such a protective case. For example, such tags may be manufactured with a suitable protective coating.

While the first embodiment has been discussed in relation to a RFID tag arrangement with a folded tag 10, it will be appreciated that folding the tag is not necessary to achieve the "S" shape (or other shape) that is laid over the carrier 50. The tag 10 in Figure 1 is in the form of an elongate strip, but other shapes of tag are possible. For example, a tag with all the functional features of the tag 10 in Figure 1 could be formed with an antenna pattern printed in an "S" shape in an unfolded state. As a result, there would be a change of direction in the tag between the first portion 65 of the antenna pattern (that includes a loop around the IC 40) and the second portion 61 of the antenna pattern. There would also be a change of direction in the tag between the first portion 65 of the antenna pattern and the third portion 62 of the antenna pattern. Therefore, when considered in 2 dimensions, the tag would appear in the "S" shape, and would form first, second and third helical paths when arranged over a cylindrical object.

Therefore, when considering the functions and characteristics of the antenna pattern on the tags, it does not matter whether the tag is folded into an "S" shape from an elongate strip or formed into the "S" shape.

As a result, references in this description to "folding" of an antenna pattern equally apply to arrangements in which the same arrangement of the antenna pattern is created by suitable printing (or otherwise) of the antenna pattern.

Although, the arrangement of the tag in "S" shape has been found to be beneficial, embodiments of the invention are not limited to this shape.

In addition, embodiments of the invention can use carriers and outer covers (both of which are themselves optional) of different shapes. Figure 20 shows a perspective view of one half portion 160 of an outer case design, with perspective views of two alternative carrier designs 110 and 120.

Carrier 110 is generally cylindrical, and comprises a split 111 to aid attachment over an object such as a cable. The carrier 110 comprises a central portion 112 and two end portions 113. The end portions 113 have a slightly larger outer diameter than the central portion 112. The outer diameter of the central portion 112 may be chosen so as to provide sufficient separation between the tag mounted on the carrier and the cylindrical object (e.g. the cable) to help minimise coupling between the tag and the object.

Furthermore, increasing the separation between the tag mounted on the carrier and the cylindrical object can help minimise the "blocking" effect of the object on the tag's antenna. In this embodiment of the invention, the carrier is 18 mm in diameter.

However, any other suitable carrier diameters (e.g. 20 mm or 22mm) may be chosen.

The inner diameter of the carrier 110 in this embodiment is matched to the outer diameter of the cylindrical object (e.g. the cable) that the carrier 110 is intended to go around.

The half portion 160 of the outer case comprises a central portion 162 and two end portions 163. The inner diameter of the central portion 162 of the half portion 160 in this embodiment is matched to the outer diameter of the end portions 113 of the carrier 110. As a result, the half portion 160 fits closely around the carrier 110.

The inner diameter of the two end portions 163 is smaller than the inner diameter of the central portion 162. The inner diameter of the two end portions 163 of the half portion in this embodiment is matched to the outer diameter of the cylindrical object (e.g. the cable) that the outer case is intended to fit around. Therefore, each end portion 163 of the half portion 160 fits closely around the cylindrical object.

Therefore, in this embodiment, when two half portions 160 are fitted together, the outer case fits around the carrier 110 and the cylindrical object so that there is substantially no gap between the cylindrical object and the outer case. As a result of this arrangement, debris cannot easily get from the outside into the outer case via a gap between the cylindrical object and the outer case.

The half portion 160 of the outer case comprises also comprises a ridge 165 and a corresponding slot 164. The ridge 165 of one half portion 160 can fit into the corresponding slot 164 of another half portion 160. Furthennore, the half portion 160 is provided with protrusions 167 and holes 168. The protrusions 167 are fitted into holes 168 of another half portion 160. Hence, the ridge 165, slot 164, protrusions 167 and holes 168 enable one half portion to be secured to another half portion.

Carrier 120 is an alternative carrier arrangement that (compared to carrier 110) is intended for use with smaller diameter cylindrical objects. Carrier 120 is generally cylindrical, and comprises a split 121 to aid attachment over an object such as a cable.

The carrier 120 comprises a central portion 122 and two end portions 123. The end portions 123 have a slightly larger outer diameter than the central portion 122.

Compared to carrier 110, the outer diameters of central portion 122 and two end portions 123 of carrier 120 are the same as for carrier 110.

The inner diameter of the carrier 120 in this embodiment is matched to the outer diameter of the cylindrical object (e.g. the cable) that the carrier 120 is intended to go around. The inner diameter of the carrier 120 is smaller than that of carrier 110.

Compared to carrier 110, carrier 120 comprises two neck portions 124 that are respectively connected to the end portions 123. The neck portions 124 have an outer diameter that is equivalent to the outer diameter of the cylindrical object that carrier 110 is designed for. Therefore, carrier 120 can be fitted into a case framed by two half portions 160 without any gap between the cylindrical object and the outer case. This is because the inner diameter of the central portion 162 of the half portion 160 in this embodiment is matched to the outer diameter of the end portions 123 of the carrier 120 (as they are the same diameter as the corresponding portions of carrier 110). As a result, the half portion 160 fits closely around the carrier 120.

As discussed, when used with carrier 110, each end portion 163 of the half portion 160 fits closely around the cylindrical object. However, when used with carrier 120, each end portion 163 of the half portion 160 fits closely around one of the neck portions 124 of the carrier 120. As a result of this arrangement, debris cannot easily get from the outside into the outer case via a gap between the cylindrical object and the outer case.

As a result of the neck portions 124, standard size half portions 160 can be produced that can be fit onto a carrier 110 for a certain diameter cylindrical object or onto a carrier 120 for a smaller diameter cylindrical object, with both carriers 110 and 120 providing the same separation between the tag and the object. In other words, the same half portion 160 can be used for carriers suitable for different cables.

In other embodiments, the neck portions 124 on carrier 120 could be provided as separate inserts into the end portions of a carrier such as carrier 110.

It will also be appreciated the carriers suitable for tags thus far described could be arranged over non-cylindrical objects. In other words, a carrier according to an embodiment of the invention could provide a cylindrical surface on which to mount a tag around a non-cylindrical object -e.g. one with a square cross section. In will be appreciated that the problems associated with mounting tags over cylindrical objects will apply to some non-cylindrical objects.

It will also be appreciated that at RF frequencies (particularly UHF), many solid objects appear as they are solid in terms of their RF characteristics. Therefore, it may be desirable to use embodiments of the invention around any object that appears as a solid object at RF frequencies.

When producing a RFID tag for a cable (or other cylindrical object), there are a number of factors to consider, with the relative importance if each factor depending on the practical implementation and the use of the cable. While the most important factor may be for many applications the sensitivity of the tag at random orientations, it is also desirable to produce an arrangement that has the smallest possible diameter and the smallest possible length.

These factors can work against each other. For example, to produce arrangement that has the smallest possible diameter, one could directly attach the tag to the cable (i.e. have no carrier). However, due to coupling effects, this may detune the tag in such arrangements. Furthermore, to produce arrangement that has the smallest possible length, one could attach the tag 10 around the cable in the form of a ring. However, this would inevitably place the two ends of the antenna pattern 60 close together, reducing the sensitivity of the antenna, as will be discussed later.

A discussion will now be provided of various tag shapes attached to cylindrical objects and their relative merits. Various orientations of tags with respect to cables are shown in Figures 2a to 13a, and comparative signal strength measurements (Received Signal Strength Indication -RSSI) for these arrangements are shown in Figures 2b to 13b.

In each of these arrangements, a carrier 100 of 20 mm (such as the one schematically shown in Figure 17) was placed over a cylindrical metal rod 500 of diameter 12 mm. A tag (such as the one schematically shown in Figure 1) was then arranged over the carrier 100. Figure 14 shows a schematic cross-section ofatag 10 folded over a 20mm diameter carrier 100, with the carrier 100 placed over a cylindrical metal rod 500 of 12 mm diameter. The cylindrical metal rod 500 was provided with large coils of attached wire (not shown) at either end.

In each example arrangements, the tag was of the same elongate strip type and was folded in the various orientations shown. Folding of the same tag type (that shown in Figure 1) was done for ease of comparison. Therefore, the reference numerals for the parts of the tag discussed in relation to Figure 1 will be used in the discussion of Figures 2a to I 3a for convenience. However, it will be appreciated that the same tag shapes could be achieved through printing tags with differently shaped antenna patterns. In other words, the embodiments of the invention are not limited to tags of the exact type shown in Figure 1, but this is merely an illustrative example.

Furthermore, it is noted that although a carrier of the same axial length was used for each of the arrangements of Figures 2ato 13a, this was done for the sake of convenience. It will be appreciated that some of these arrangements could use shorter carriers than others.

In Figures 3a to 1 3a the tags are shown prior to being arranged around the circumference of the carrier. In other words, the shape of the tag is shown in two dimensions for ease of representation. In will be appreciated that the actual measured arrangements have the ends of the tags arranged around the circumference of the carrier.

To help explain the arrangements in Figures 3a to 13a, it is noted that if a portion of the tag (for example the antenna pattern) extends around the circumference of the carrier (or more generally a cylindrical object), then that portion of the tag can be considered as extending in a helical path around the cylindrical object. This is, of course, unless the portion extends around the circumference of the cylindrical object at an angle of 900 with respect to the axis of the cylindrical object.

The helix angle is the angle that the helical path makes with the axis of the cylindrical object. Helices can be either right-handed or left-handed. To illustrate the difference, if an observer has as his line of sight the helical axis, the helix will be considered right-handed helix if a clockwise movement of the helix corresponds to axial movement away from the observe. The helix is a left-handed helix if anti-clockwise movement corresponds to axial movement away from the observer. Thus, left-handed helix and right-handed helices are mirror images of each other. Therefore, the handedness of the helix is a property of the helix that is independent of the observer.

An object extending along a helical path (whether it is a left-handed helix or a right-handed helix) can rotate around the axis of the cylindrical object in either a clockwise or anti-clockwise direction (which will correspond to movement along the axis in different directions). Thus, for a given helix, rotation around the cylindrical object in either a clockwise or anti-clockwise direction will result in movement either forwards or backwards along the axis of the cylindrical object.

A first helical path can be thought of as being a transverse helical path to a second helical path if the first helical path crosses the second helical path.

Furthermore, where the bend or fold is shown before the tuning section 63 of the second or third portions 61, 62 of the tag, the bend or fold could be after the tuning section 63 or as part of the tuning section 63. Only one position of bend is shown for clarity. In other words, the precise region of the bend or fold (i.e. the boundary between the first/second portion and the first/third portion) is not limited to what is shown in these figures. Furthermore, in some practical tag arrangements, the IC portion of the tag is more rigid than the rest of the tag (due to the IC 40). Therefore, it may be beneficial to locate the folds (i.e. the boundaries between the first/second portion and the first/third portion) at the adjacent the more rigid IC portion of the tag.

In order to measure the Received Signal Strength Indication (RSSI) of each arrangement of tag shape, each arrangement was tested using the equipment schematically shown in Figure 15.

As shown in figure 15, a RFID reader 210 was provided with au RFID reader antenna 200. The reader antenna 200 was placed 1 m from the tag arrangement to be tested.

The RF1D reader antenna 200 sends an RF signal to the tag and receives a backscattered signal. The output of the received signal is provided to the RFID reader 210, where the results are processed and send to a logger 220.

To take measurements of RSSI, a tag would be arranged in certain orientation with respect to the RFID reader antenna 200. A measure of the received signal strength backscattered from the tag would then be obtained and the results logged by the logger 220. The tag would then be orientated at a different angle around the axis of the cylindrical metal rod 500 with respect to the RFID reader antenna 200.

The RSSI plots in Figures 2b to 13b are referenced to the angle of incident radiation around the axis of the cylindrical metal rod 500, where 0° is the point where the IC 40 portion of the tag is directly facing the RFID reader antenna 200.

The RSSI plots are shown as a relative figure of signal strength (in dB) with respect to the maximum signal strength measured (which was at 00 for the Figure 8a arrangement). Therefore, a reading of-9 dB at an angle of 90° (e.g. for the Figure 9a arrangement) means that for the IC 40 of the tag to be facing 90° from the RFJD reader antenna 200, the received signal strength is 9 dB lower than for the maximum signal strength measured.

As a result, the RSSJ plots in Figures 2b to 13b show comparative received signal strength values for different tag shapes at the full range of orientations around the axis of the cylindrical metal rod 500.

The received signal strength values for different tag shapes will depend on a number of factors, including how the cylindrical metal rod 500 blocks the signal, and both the near field and far field properties of electromagnetic waves transmitted and received by the antenna of the tag.

In will be appreciated that in the near field' characteristics of an antenna system, where the tagged object close to the test antenna, the electric field is predominant and is used to generate the electricity to power the receiver chip. At short ranges this means that the cylindrical metal rod 500 can be thought of as a transformer core and the antenna pattern on the tag as windings on the core of the transformer. It is desirable that the arrangement of the antenna pattern on the cylindrical metal rod 500 is optimised to reduce the coupling between the cylindrical metal rod 500 and the antenna pattern as this can cancel signals received by the opposite ends of the dipole of the antenna pattern 500.

In the far field model, the arrangement of the antenna pattern is designed to keep the tips of the antenna pattern as far apart as possible to approach an effective resonant length when matched by the tuning regions of the antenna pattern. The tuning regions of the antenna pattern are used to match the impedance of the aerial to the IC 40, and are affected by the proximity of metals and other conductive materials such as carbon. As a result, it is desirable that the arrangement of the antenna pattern on the cylindrical metal rod 500 is optimised to reduce the coupling of the object to the tuning regions by physical separation or by the angle at which they pass over the object. This prevents the detuning of the tuning regions 63 of the antenna pattern and therefore keeps the impedance match to the IC 40 as close as possible and results in the lowest possible loss.

In the arrangement of Figure 2a, the tag 10 is arranged on the carrier 100 along the axis of the cylindrical metal rod 500. As a result, the antenna pattern 60 of the tag does not extend around the circumference of the cylindrical metal rod 500. In other words, the anterma pattern extends in a longitudinal path along the axis of the cylindrical metal rod 500.

The RSSI plot in Figure 2b shows that for when the IC 40 of the tag is 00 with respect to the RFID reader antenna 200, the received signal strength is 6 dB lower than for the maximum signal strength measured. The received signal strength decreases as the rotation angle changes from 0° to around 160°. The received signal strength then rapidly decreases to almost zero at 180°. The received signal strength at angles from 180° back to 0° corresponds to those angles from 0° to 180°, and therefore will not be discussed again.

As a result, the RSSI plot for Figure 2b shows a large lobe at 0° and two smaller lobes at around 160° and 200°, with a received signal strength of almost zero at 180°.

It is considered that for angles between 160° and 200°, the cylindrical object is located substantially between the antenna pattern and the RFID reader antenna 200. As a result, the signal from the RF]D reader antenna 200 is blocked by the cylindrical object for angles between 1600 and 200. This blocking effect is greatest for 180°, as the antenna pattern will be completely blocked by the cylindrical object.

For small angles, the cylindrical object can act as a RF reflector which further increases the sensitivity to angular rotation of the reader around the axis of the cylindrical object.

As a result, the tag arrangement of Figure 2a is not ideal for attaching to a cable or other cylindrical object, because there is no signal at some orientations of the tag with respect to the RFID reader antenna 200. This high degree of directionality is undesirable for many applications, as discussed above.

The tag arrangement of Figure 2a also leads to the longest carrier size. Therefore, the tag arrangement of Figure 2a is also not desirable for this reason.

Although not shown in Figure 2b, when the antenna pattern extends in a longitudinal path along the axis of the cylindrical metal rod 500, and the tag is directly attached to the cylindrical metal rod 500, there is significant detuning of the antenna pattern due to coupling between the cylindrical metal rod 500 and the antenna pattern. Therefore, compared to other orientations, the Figure 2 arrangement requires a relatively large separation between the cylindrical metal rod 500 and the antenna pattern. Therefore, a relatively large carrier diameter is required for the tag arrangement of Figure 2a.

This requirement of a larger carrier diameter to prevent coupling of the antenna to the object and also does not work at angles where the object gets between the antenna and the test antenna and so blocks signal from the test antenna. In a presentation of a small number of degrees the object can act as a reflector which further increases the arrangement of Figure 2a's sensitivity to angular rotation around the axis of the object.

It is considered that directly attaching a tag to a cable in the Figure 2a arrangement would produce unsatisfactory results as there would be too much parallel coupling between the cable and the tag and the blocking effect would be too great.

In the arrangement of Figure 3a, the tag 10 is arranged on the carrier 100 such that the first portion 65 of the antenna pattern 60 is arranged along the axis of the cylindrical metal rod. The second portion 61 of the antenna pattern 60 extends in a helical path H3. 1 around the cylindrical metal rod. The third portion 61 of the antenna pattern 60 extends in a helical path H3.2 around the cylindrical metal rod. As can be seen helical paths H3.1 and H3.2 have different handedness.

From the point of view of an observer located along the axis of the cylindrical metal rod, looking in direction A.A (see Figure 3b), the ends of the second portion 61 and third portion 62 rotate around the axis of the cylindrical metal rod in an anti-clockwise direction. Therefore, the second portion 61 and third portion 62 can be considered as rotating around the axis of the cylindrical metal rod in the same direction.

The RSSI plot in Figure 3b shows that for when the IC 40 of the tag is 00 with respect to the RFID reader antenna 200, the received signal strength is about the same as that of Figure 2b (i.e. around 6 dB lower than for the maximum signal strength measured).

However, the arrangement of Figure 3a shows a high degree of sensitivity to angular orientation around the axis of the cylindrical metal rod.

As the second portion 61 and third portion 62 extend in helical paths H3.1 and H3.2 that have different handedness, the second portion 61 and third portion 62 begin rotating around the same side of the cylindrical metal rod. As a result, the tag in the arrangement of arrangement of Figure 3a is largely located on one side of the cylindrical metal object. As a result of this, the cylindrical metal object has a large blocking effect on the antenna pattern.

Furthermore in the arrangement of Figure 3a, the second portion 61 and third portion 62 couple to the cylindrical metal rod and any signal induced in the rod appears in phase to both portions 61 and 63 and therefore are cancelled out at the receiver. In other words, in the arrangement of Figure 3a, the design has the two antenna elements passing across the object in the same direction and so the signal is in phase at both ends of the antenna, reducing the sensitivity.

However, in contrast to the Figure 2a arrangement, the Figure 3a arrangement requires a smaller diameter carrier (it is less sensitive to deturiing due to proximity to the cylindrical metal rod) and produces an axially smaller RFID arrangement -therefore using a carrier of smaller length.

In the arrangement of Figure 4a, the tag 10 is arranged on the carrier 100 such that the first portion 65 of the antenna pattern 60 is arranged along the axis of the cylindrical metal rod. The second portion 61 and the third portion 62 of the antenna pattern 60 extend in annular paths around the cylindrical metal rod, with the annular paths having the same rotation direction.

The RSSJ plot in Figure 4b shows that for all angles, the received signal strength is poor. Since both the second portion 61 and the third portion 62 of the antenna pattern 60 rotate around the cylindrical metal rod in the same direction, this arrangement does not work at angles where the object gets between the antenna pattern 60 and the RF[D reader antenna 200, blocking the signal from the RFTD reader antenna 200.

Furthermore in the arrangement of Figure 4a, the second portion 61 and third portion 62 couple to the cylindrical metal rod and any signal induced in the rod appears in phase to both portions 61 and 63 and therefore are cancelled out at the receiver. In other words, in the arrangement of Figure 4a, the design has the two antenna elements passing across the object in the same direction and so the signal is in phase at both ends of the antenna reducing the sensitivity.

In addition, the arrangement of Figure 4a requires a large carrier diameter so that the ends of the second portion 61 and the third portion 62 of the antenna pattern 60 do not wrap around to come close to the tuning regions 63, because this detunes the tag by coupling to the object. As will be explained in more detail later, if the second portion 61 and the third portion 62 are too close, the antenna pattern can be though of as being analogous to a winding on a transformer core, which changes the resonant frequency of the antenna pattern. However, the Figure 4a produces an axially smaller RFID arrangement (therefore using a carrier of smaller length) than the Figure 2a arrangement.

In the arrangement of Figure 5a, the tag 10 is arranged on the carrier 100 such that the first portion 65 of the antenna pattern 60 is arranged along the axis of the cylindrical metal rod. The second portion 61 and the third portion 62 of the antenna pattern 60 extend in annular paths around the cylindrical metal rod, with the annular paths having a different rotation direction.

The RSSI plot in Figure 5b shows that for all angles, the received signal strength is poor.

The arrangement of Figure 5a requires a large carrier diameter so that the ends of the second portion 61 and the third portion 62 of the antenna pattern 60 do not wrap around to come close to the tuning regions 63, because this detunes the tag by coupling to the object. As for Figure 4a, if the second portion 61 and the third portion 62 are too close, the antenna pattern can be though of as being analogous to a winding on a transformer core, which changes the resonant frequency of the antenna pattern. However, the Figure 4a produces an axially smaller RFID arrangement (therefore using a carrier of smaller length) than the Figure 2a arrangement.

In the arrangement of Figure 6a, the tag 10 is arranged on the carrier 100 in a simple helix. In other words, the first portion 65, second portion 61 and the third portion 62 are arranged along the same helical path H6.

The RSSI plot in Figure 6b shows that the angular sensitivity of the tag arrangement is quite good. When the IC 40 of the tag is 0° with respect to the REID reader antenna 200, the received signal strength is 9 dB lower than for the maximum signal strength measured. The received signal strength does not change significantly for rotation angle changes from 0° to around 90°. The received signal strength then decreases slowly to a minimum of around -10 dB at 180°. The received signal strength at angles from 180° back to 0° corresponds to those from 0° to 180°.

Jn order to help understand the results of Figure 6b (and the rest of Figures 2b to 1 3b), it is helpful to consider the various coupling effects that occur between the tag 10 and the cylindrical metal rod 500.

It is desirable to stop any energy in the antenra of the tag being induced into the cylindrical metal rod 500 via RF coupling. This coupling: a) transfers energy from where it is wanted to send a signal to the reader; and b) changes the resonant frequency of the antenna by changing its inductance and therefore its resonant frequency It can be considered that there are two RF coupling effects that occur. The first coupling effect can be explained by considering a first wire 91 and a second wire 92 that are arranged to cross each other at different angles. In will be appreciated that, for the purpose of considering the RSSI plots, the antenna pattern and the cylindrical metal rod 500 can both be considered as being analogous to wires that "cross" each other at various angles when considered from the point of view of the RFID reader.

If a current is passed through the first wire 91, a voltage may be induced in the second wire 92 which can be seen on a voltmeter. If the wires were at right angles (see Figure 22) there would be no induced voltage in the second wire 92. Therefore, considering this first coupling effect (which could be called the "parallel" coupling effect), one might have expected the best result would be a tag arranged onto the cylindrical metal rod 500 at right angles, with the worst results being for parallel arrangements. However, Figures 2b to 13b show this not to be the case for the measured arrangements, which is due to the second coupling effect.

The second coupling effect (which could be called the "turn" coupling effect) can be explained by considering the antenna pattern arranged onto the cylindrical metal rod 500 in terms of it being analogous to a winding on a transformer coil.

It will be appreciated that a coil wound round a rod changes the inductance of the coil.

For example, some radios used the method of sliding a metal or graphite rod in and out of a coil to change the inductance of the coil. This changes the resonant frequency of the circuit and thereby changes the radio channel.

The change in inductance of the coil is increased if the rod is connected to itself at either end it forms a ioop. In this case, any energy transferred into the rod is then effectively fed into a short circuit which draws even more energy out of the circuit.

When considering RF coupling, at high frequencies two objects do not have to be electrically connected to act as though they are connected. A wound up loop of cable around an object can appear to be a solid ring at RF. Therefore, it can be thought of as a core of a toroidal transformer.

If the antenna pattern of the tag is wound all the way around the circumference of the cable it becomes like a turn' of wire round a transformer core (a single winding on a transformer coil can be referred to as a turn'). This provides the most efficient way of coupling to a transformer core. The more times the antenna pattern turns or coils round the cylindrical metal rod 500 the stronger the coupling effect becomes. Therefore, it has been found that the less of a turn that the antenna pattern makes round the cylindrical metal rod 500, the better -as there is less RF coupling.

Figures 23a and 23b provide a further explanation how a tag arranged around an object can act like a winding on a transformer core. Figure 23a is a side view of a loop of cable 55, with a carrier 100 arranged around a portion of the loop 55. The carrier 100 is provided with a tag 10 arranged on it in a spiral pattern.

Figure 23b shows two cross sections taken at points P23.1 and P23.2. The cross section at point P23.1 shows the circular cross section of the loop 55. The cross section at point P23.2 shows the circular cross section of the loop 55, with the carrier 100 arranged around it. As shown, as a result of the tag 10 being arranged on it in a spiral pattern, the cross section at point P23.2 shows that the tag forms an almost complete ioop around the ioop 55.

In Figures 4a and 5a, antenna portions 61 and 62 individually form turns round the object, and therefore these antenna portions 61 and 62 couple with the object. In figure 6a, antenna portions 61, 65 and 62 can be considered as one long section which form a long turn' or winding. This can reduce the near field performance of the tag.

Therefore, because a ioop round any metal object inductively couples the ioop to the object just like a transformer, it is desirable in some embodiments of the invention to arrive at an arrangement of the antenna pattern that forms the least possible loop around the object.

The RSSI of a spiral arrangement such as that in Figure 6a will improve as the carrier diameter increases. This is because, as the carrier diameter increases, the tag 10 forms less of a "turn" around the object. Therefore, spiral arrangement such as that in Figure 6a may require larger carrier diameters than some other embodiments of the invention.

In the arrangement of Figure 7a, the tag 10 is arranged on the carrier 100 with the first portion 65 arranged at 90° to the axis of the cylindrical metal rod. The second portion 61 and the third portion 62 are arranged along the respective helical paths H7.1 and H7.2. Helical paths H7.1 and H7.2 are the same handiness with different rotation directions.

Such an arrangement differs from the pure helix of Figure 6a in a number of ways. The produce the arrangement of Figure 7a, the antenna pattern is folded at region 68 to produce the second portion 61, and the antenna pattern is folded at region 69 to produce the third portion 62 (though as discussed, the arrangement could be produced without folding).

As a result, compared to the arrangement of Figure 6a, the arrangement of Figure 7a is located more asymmetrically with respect to the axis of the cylindrical metal rod. In other words, compared to the arrangement of Figure 6a, the tag in Figure 7a is less evenly spread around the axis of the cylindrical metal rod.

As a result of this, one might have expected the RSSI plot in Figure 7b to show worse angular sensitivity that the RSSI plot in Figure 6b. This is because one might expect that the tag in Figure 7b is blocked to a greater extend by the cylindrical metal rod at angles around 1800. However, the RSSI plot in Figure 7b shows that the angular sensitivity of the tag arrangement is better than the RSSI plot in Figure 6b.

When the IC 40 of the tag is 0° with respect to the RFID reader antenna 200, the received signal strength is around 7 dB lower than for the maximum signal strength measured. The received signal strength then decreases slowly to a minimum ofjust under -6 dB at 180°, with little change between 90° and 180°. The received signal strength at angles from 1800 back to 00 corresponds to those from 00 to 180°.

In the arrangement of Figure 7a, there is reduced coupling to the object caused by the antenna pattern largely not running parallel to the object. Therefore, for the reasons discussed above, this reduces the first coupling effect mentioned above.

The arrangement of Figure 7a can also use a smaller diameter carrier compared to some embodiments. This is because in the arrangement of Figure 7a the ends of the first and second antenna portions are vertically spaced from the portions of their respective tuning regions 63 that are closest to the IC 40.

In the arrangement of Figure 8a, the tag 10 is arranged on the carrier 100 with the first portion 65 arranged along a helical path H8. 1 around the circumference of the cylindrical metal rod. The second portion 61 and the third portion 62 are arranged along the respective helical paths H8.2 and H8.3. Helical paths H8.2 and H8.3 are the same handedness (but different rotation directions around the rod), while helical path H8.1 has a different handedness to helical paths H8.2 and H8. In this arrangement, helical paths H8.2 and H8.3 are congruent, i.e. they have the same helix angle, which in this arrangement is 45° (+1-5°). However, in other embodiments, helical paths H8.2 and H8.3 may have different helix angles.

The RSSI plot in Figure 8b produces a very good angular sensitivity. When the IC 40 of the tag is 0° with respect to the RFID reader antenna 200, the received signal strength is at the maximum signal strength measured for all the arrangements measured. The received signal strength then decreases slowly to a minimum of around -9 dB at 90°, and then increases again to just over -6 dB at 180°. The received signal strength at angles from 1800 back to 00 corresponds to those from 00 to 180°.

Even at its minimum (90°), the RSSI plot in Figure 8b is better than the maximum for the pure helical arrangement of Figure 6a.

It is considered that the arrangement of Figure 8a produces a better response than the arrangement of Figure 7a because the distance between the second 61 and third 62 antenna portions is increased, which has a number of beneficial effects. For example, this can enable the antenna pattern of the tag to better match the wavelength of the incoming signal.

In the arrangement of Figure 9a, the tag 10 is arranged on the carrier 100 such that the first portion 65 of the antenna pattern 60 is arranged at 90° to the axis of the cylindrical metal rod so as to extend in an annular path around the cylindrical metal rod. The second portion 61 and the third portion 62 of the antenna pattern 60 extend along the axis of the axis of the cylindrical metal rod, while pointing in different directions.

The RSSI plot in Figure 9b produces poor angular sensitivity with three distinct lobes -with three maxima at roughly 45°, 180° and 3 15°, and three minima at roughly 0°, 135° and 225°. This is because the second portion 61 and the third portion 62 of the antenna pattern 60 run parallel to the object and therefore closely couple to the object causing the tag to detune.

From Figures 2b to 9b, it is apparent that the arrangement of Figure 8a produces the best results. Figures lOa to 13b help explain why this is the case.

In the arrangement of Figure 1 Oa, the tag 10 is arranged on the carrier 100 such that the first portion 65 of the antenna pattern 60 extends along a helical path H10. The second portion 61 and the third portion 62 of the antenna pattern 60 extend along the axis of the axis of the cylindrical metal rod, while pointing in different directions.

The RSSI plot in Figure lOb shows poor angular sensitivity, but with a good maximum.

When the IC 40 of the tag is 00 with respect to the RFID reader antenna 200, the received signal strength is around 3 dB lower than for the maximum signal strength measured. The received signal strength then decreases sharply to a minimum of almost zero at 180°. The received signal strength at angles from 1800 back to 00 corresponds to those from 00 to 180°.

This result can be explained by the majority of the antenna pattern 60 being located on one side of the cylindrical metal rod, so that there is a large blocking effect. Therefore, the Figure 1 Oa arrangement is highly sensitive to the angle with respect to the reader.

Furthermore, the second portion 61 and third portion 62 have a large coupling effect with the cylindrical metal rod, as they are largely parallel with the cylindrical metal rod.

In the arrangement of Figure ha, the tag 10 is arranged on the carrier 100 with the first portion 65 arranged along a helical path Hi 1.1 around the circumference of the cylindrical metal rod. The second portion 61 and the third portion 62 are arranged along the respective helical paths Hi 1.2 and Hi 1.3. Helical paths Hi 1.2 and Hi 1.3 are the same handedness (with different rotation directions), while helical path H11.1 has a different handedness to helical paths Hi 1.2 and Hi 1. In this arrangement, helical paths Hii.2 and H 11.3 are congruent, i.e. they have the same helix angle.

To help explain the helix angle, reference will now be made to Figure 2 i. Figure 21 shows an axis on which there is a folded tag of the type schematically shown in Figure 1. The first portion 65 of the antenna pattern 60 makes an angle of a with the axis.

Therefore, a is the helix angle for the helical path of the first portion 65 of the antenna pattern 60. The second portion 61 of the antenna pattern 60 makes an angle of with the axis. Therefore, f3 is the helix angle for the helical path of the first portion 65 of the antenna pattern 60. It is noted that a and [3 have been shown in Figure 21 as being angles between 0° and 90° (even though they correspond in this example to helices of different handedness). However, is convenient to measure a and [3 in this way.

In Figure 8a, the helix angle a is 45° and the helix angle 3 is 45°. In the arrangement of Figure 1 Ia, the helix angle a is 45° and the helix angle 13 is 200.

The RSSI plot in Figure 1 lb shows good angular sensitivity. When the IC 40 of the tag is 0° with respect to the RFID reader antenna 200, the received signal strength is around 3 dB lower than for the maximum signal strength measured. The received signal strength then decreases slowly to a minimum of just under -6dB at 180°. The received signal strength at angles from 180° back to 0° corresponds to those from 0° to 180°.

The RSSI plot in Figure 1 lb therefore compares very favourably with the RSSI plot in Figure 6b.

In the arrangement of Figure 12a, the tag 10 is arranged on the carrier 100 in a corresponding way to Figures 8a and ha, with helical paths H12.1, H12.2 and H12.3.

In the arrangement of Figure 12a, the helix angle a is 45° and the helix angle f3 is 60°.

The RSSI plot in Figure 12b corresponds largely to that in Figure 8b, with the difference between a slightly reduced maximum at 0°.

In the arrangement of Figure 13a, the tag 10 is arranged on the carrier 100 with the first portion 65 arranged in a helical path H13. The second portion 61 and the third portion 62 of the antenna pattern 60 extend in annular paths around the cylindrical metal rod, with the annular paths having a different rotation direction.

The RSSI plot in Figure 1 3b shows that for all angles, the received signal strength is poor. It is considered that, like for Figure 5a, the second and third portions 61 and 62 wrap round the cylindrical rod, but end up too close together (vertically), so that there is not enough separation of between the second and third portions 61 and 62. This lack of separation, combined with the fact that the antenna pattern forms a "turn" round the object, causes the poor performance.

Figures lOb, lib, 8b, 12b and 13b (when considered in that order) show how the received signal strength varies as the helix angle 13 of the second and third portions of the antenna pattern varies. Although the second and third portions do not have helical paths in Figure lOb or 13b, these can be considered as paths with a helix angle 13 of 0° and 90° respectively. Therefore, Figures lOb, 1 ib, 8b, 12b and 13b illustrate how the response varies with helix angle 13 varying from 0° to 90°.

It is apparent from Figures lOb, 1 Ib, 8b, 12b and 13b that as the helix angle 3 increases from 0° (figures lOb) the angular sensitivity of the received signal strength improves dramatically, peaking at around 45° of Figures 8b. The total received signal strength then decreases as the helix angle 13 approaches 90°.

Considering the helix angle 13, from 0-30° the antenna of the tag is highly directional.

This because the second and third portions do not wrap around far enough around the cylindrical rod. Thus, there is a large blocking effect of the rod on the antenna.

For helix angles 13 of 30-65°, the directionality is improved, peaking at 45°.

For helix angles 13 of 65-90°, the signal becomes sharply attenuated due to the proximity of the ends of the second and third portions to each other. This is because, for the reasons given above, as the ends of the second and third portions approach each, the antenna becomes more and more analogous to a turn in a transformer. As a result, embodiments of the invention can use helix angles 13 for the second and/or third portions of between 0° to 90°, preferably 30-65°, and more preferably 45°.

In addition to the effect of the helix angle 13, the helix angle ct also has an effect on the results. This can be seen by comparing Figures 7b and 8b. In figure 7a, the first portion is at right angles to the axis of the object. This is equivalent to a helix angle cx of 90°.

As discussed above, the helix angle a in Figure 8a is 45°. In both figure 7a and figure 8a, the helix angle 13 is 45°.

The dramatic increase in signal strength in Figure 8b at practically all angles when compared to Figure 7b can be attributed to the change in helix angle a. Comparing Figure 7a to Figure 8a, it is apparent that the tag in Figure 7a will extend further around the rod. Therefore, the "turn" coupling effect is greater in Figure 7a than in Figure 8a, and it is better if the helix angle a for the first portion is less than 900, Furthermore, although not shown, a helix angle a of 00 will be associated with worse results than angles greater than 0°. If the angle a is 0°, then the first portion 65 will be parallel to the rod, which will maximise the "parallel" coupling effect mentioned above.

While it has been found that changing the helix angle a has less an effect on the angular sensitivity of the received signal strength than changing the helix angle 13, it can still be important to optimise the helix angle a. For a constant helix angle 13, the angular sensitivity of the received signal strength is best when the helix angle a is 45°. As a result, embodiments of the invention can use helix angles a of between 00 to 90°, preferably 25-65°, and more preferably 45°.

It will be appreciated that the optimum angles for the helix angle a and 13 may vary according to the carrier diameter. The optimum angles for the helix angle a and 13 will also vary according to the rod diameter. As the rod diameter increases, the "blocking" effect of the rod will increase.

Furthermore, as discussed, the arrangements whose results are primarily determined by the "turn" coupling effect benefit from having larger carrier diameters. This is because larger carrier diameters result in the ends of the second and third portions of the antenna pattern being located further apart. Increasing the carrier diameter also decreases the "blocking" effect of the object on the tag. However, increasing the carrier diameter increases the size of the whole tag arrangement.

When designing an optimum tag arrangement for mounting around cylindrical object such as a rod or cable, it is desirable to produce an arrangement that has the smallest possible diameter and the smallest possible length, and that can be read successfully at random presentations (as can be found in applications with loosely packed objects).

In some embodiments of the invention in which the tag extends around the circumference of the obj ect, there can be provided a better angular response that the "parallel" arrangement of Figure 2a. This is because in the Figure 2a arrangement, the object has a large blocking effect on the antenna of the tag. Providing at least part of the antenna pattern in a helical path around the object has been found to be beneficial.

Furthermore, if the antenna is considered as comprising three portions (first, second and third) mentioned above, it has been found to be beneficial to have the first andlor second and/or third portions not being parallel to the axis of the object nor at 900 to the object.

It has been found that an arrangement of a tag around the object in the general "S" shape shown in Figures 8b provides the optimum result. This shape has the best RSSI response at the shortest carrier length and requires the smallest carrier diameter.

However, as discussed, embodiments of the invention may use different arrangements (those these may require larger carrier diameters and have longer carriers).

It will be appreciated that the arrangements in, for example, Figure 8b have been shown in one orientation, but that the mirror image of this arrangement (i.e. where the respective helical paths have different handedness) is equivalent. This applies to all the discussed embodiments of the invention.

It should be understood that embodiments of the present invention are not limited to particular arrangements of the tag, optional carrier or optional outer case. Furthermore, while the embodiments of the invention have been described in relation to generally cylindrical objects (e.g. cables or rods), embodiments of the invention are not limited in this way. For example, embodiments of the invention could use a cylindrical carrier over a non-cylindrical object.

In addition, it will be appreciated that the "cylindrical" object for which the tag is to be attached (e.g. carrier or rodlcable) does not have to be exactly cylindrical. The object could have other similar shapes.

In addition, some embodiments of the invention have been discussed in relation to a tag that is formed in a dipole (or a tag printed in an "S" shape that approximates to a dipole by having an antenna pattern with two "arms"). However, embodiments of the invention could use tags that have antenna patterns comprising any number of "arms".

For example, a tag could be produced with an anteima pattern with an "X" shape when considered in 2 dimensions.

Such a tag could comprise a central region comprising the IC for the tag. Two arms could then extend from two respective different sides of the IC portion. Considering two arms extending from the same side, these two arms could extend on respective helical paths around the object. These helical paths could have different handedness and different rotation directions around the object. A pattern produced by such a tag arrangement would be analogous to overlaying the mirror image of the pattern of Figure 8a over the pattern of Figure 8a (although only one IC portion may be required).

Of course, the above is merely one example of a tag arrangement with more than two "arms". Other arrangements in which at least part of the antenna pattern extend around the circumference of the object are possible.

Although the embodiments of the invention have been described in relation to a passive RFID tag (e.g. an UHF RFID tag), embodiments of the invention could use tags of any type. For example, embodiments of the invention could be used with active tags or semi-passive tags, or tags tuned to different frequencies.

Many further modifications and variations will suggest themselves to those versed in the art upon reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the invention, that being determined by the appended claims.

Claims

CLAIMS: 1 A RFJD tag arrangement comprising an RFID tag attached to a generally cylindrical object, the RFiI tag comprising: an antenna and an integrated circuit coupled to the antenna; wherein the RF1D tag is attached to the cylindrical object in such as way so that at least part of the antenna extends around a circumference of the cylindrical object.
2. A RFID tag arrangement according to claim 1, wherein said at least part of the antenna extends around a circumference of the cylindrical object in a helical path.
3. A RF1D tag arrangement according to claim 1 or 2, wherein the antenna comprises an antenna pattern with a first portion and a second portion, with the integrated circuit located in the region of the first portion, and the first portion being connected to the second portion, wherein the first portion extends on the circumference of the cylindrical object in a first path and the second portion extends on the circumference of the cylindrical object in a second path that is different to the first path.
4. A RFID tag arrangement according to claim 3, wherein the second path is helical.
5. A RFID tag arrangement according to claim 4, wherein the second path has a helix angle of between 35° and 60°.
6. An RFID tag arrangement according to any one of claims 3 to 5, wherein the first and second portions of the antenna pattern are produced by folding part of a longitudinal antenna pattern.
7. An RFTD tag arrangement according to any one of claims 3 to 5, wherein the first and second portions of the antenna pattern are produced by providing a base antenna pattern with a suitable unfolded shape so that when the RFID tag attached to the cylindrical object, the first and second paths are produced around the cylindrical object.
8. An RFID tag arrangement according to any one of claims 3 to 7, wherein the antenna pattern comprises a third portion, with the first portion being connected between the second portion and the third portion, wherein the third portion extends on the circumference of the cylindrical object in a third path that is different to the first path.
9. A RF1D tag arrangement according to claim 8 wherein the third path is helical.
10. A RFID tag arrangement according to claim 9, wherein the third path has a helix angle of between 35° and 60°.
11. A RFID tag arrangement according to any one of claims 8 to 10, wherein the third portion extends on the circumference of the cylindrical object in a different rotation direction to the second path.
12. A RFIID tag arrangement according to any one of claims 3 to 11, wherein the first path is helical.
13. A RFID tag arrangement according to any one of claims 1 to 12, wherein the cylindrical object is a carrier for arrangement around a second generally cylindrical object.
14. A RFID tag arrangement according to claim 13, wherein the carrier comprises a split cylinder to enable the carrier to be arranged around the second generally cylindrical object.
15. A RFID tag arrangement according to any one of claims 1 to 14, further comprising an outer member that is arranged to cover the antenna pattern.
16. A method of producing a RFID tag arrangement including attaching an RFID tag around a generally cylindrical object, the RFTD tag comprising an antenna and an integrated circuit coupled to the antenna, the method comprising: attaching the RFID tag to the cylindrical object in such as way so that at least part of the antenna extends around a circumference of the cylindrical object.
17. A RFID tag comprising an antenna and an integrated circuit coupled to the antenna, wherein: the antenna comprises an antenna pattern with a first portion and a second portion, with the integrated circuit located in the region of the first portion and the first portion being connected to the second portion; the first portion and second portion are arranged at an angle so that when the RFID tag is attached around a circumference of a cylindrical object, the first portion extends on the circumference of the cylindrical object in a first path and the second portion extends on the circumference of the cylindrical object in a second path that is different to the first path.
18. A RFID tag according to claim 17, wherein the first and second portions of the antenna pattern are produced by folding part of a longitudinal shaped base antenna pattern.
19. A RF]D tag according to claim 18, wherein: the antenna pattern comprises a third portion, with the first portion being connected between the second portion and the third portion, so that when the RFI[) tag is attached around a circumference of the cylindrical object, the third portion extends on the circumference of the cylindrical object in a third path that is different to the first path; and the third portion is produced by folding part of the longitudinal shaped base antenna pattern.
20. A RF]D tag according to claim 17, wherein the antenna pattern comprises the first and second portions in an unfolded state of the RFID tag, so that when the RFID tag attached to the cylindrical object, the first and second paths are produced around the cylindrical object.
21. A RFJD tag according to claim 20, wherein the antenna pattern comprises a third portion in an unfolded state of the RFID tag, with the first portion being connected between the second portion and the third portion, so that when the RFJD tag is attached around a circumference of the cylindrical object, the third portion extends on the circumference of the cylindrical object in a third path that is different to the first path.
22. A RFID tag according to claim 20 or 21, further comprising a substrate on which the antenna pattern is disposed, wherein the first, second and third portions are deposited onto the substrate to form the antenna pattern.
23. A RFID tag according to any one of claims 19 to 22, wherein the RFJD tag is "S" shaped in an unfolded state.
24. A RFID tag arrangement comprising an RFID tag attached to a generally cylindrical carrier, the RFII) tag comprising: an antenna and an integrated circuit coupled to the antenna; wherein the carrier is suitable for arrangement around a generally cylindrical object.