Abstract:
Apparatus capable of enhancing an incident electric field to drive an electromagnetic tag ( 124 ) into operation, comprising a resonant dielectric cavity which extends out of a single plane defined between two conducting surfaces ( 102, 104, 106 ). The cavity may extend over two or more layers, and can adopt C or S shaped or spiral profiles.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to the local manipulation of electromagnetic fields, and more particularly, but not exclusively, to the use of radiation manipulating devices to allow RF (radio frequency) tags to be mounted on materials which would otherwise impede their use. 
     (2) Description of the Art 
     RF tags are widely used for the identification and tracking of items, particularly for articles in a shop or warehouse environment. One commonly experienced disadvantage with such tags is that if directly placed on a metal surface their read range is decreased to unacceptable levels and more typically the tag cannot be read or interrogated. This is because a propagating-wave RF tag uses an integral antenna to receive the incident radiation: the antenna&#39;s dimensions and geometry dictate the frequency at which it resonates, and hence the frequency of operation of the tag (typically 866 MHz, or 915 MHz, with 860-960 MHz being the approved range for a UHF (ultra-high frequency) range tag and 2.4-2.5 GHz or 5.8 GHz for a microwave-range tag). When the tag is placed near or in direct contact with a metallic surface, the tag&#39;s conductive antenna interacts with that surface, and hence its resonant properties are degraded or—more typically—negated. Therefore the tracking of metal articles such as cages or containers is very difficult to achieve with UHF RF tags and so other more expensive location systems have to be employed, such as GPS. 
     UHF RFID tags also experience similar problems when applied to any surfaces which interact with RF waves such as, certain types of glass and surfaces which possess significant water content, such as, for example, certain types of wood with a high water or sap content. Problems will also be encountered when tagging materials which contain/house water such as, for example, water bottles, drinks cans or human bodies etc. 
     This problem is particularly true of passive tags; that is tags which have no integrated power source and which rely on incident energy for operation. However, semi passive and active tags, which employ a power source such as an onboard battery also suffer detrimental effects on account of this problem. 
     One way around this problem is to place a foam spacer, or mounting between the RF tag and the surface, preventing interaction of the antenna and the surface. With currently-available systems the foam spacer needs to be at least 10-15 mm thick in order to physically distance the RF tag from the surface by a sufficient amount. Clearly, a spacer of this thickness is impractical for many applications and is prone to being accidentally knocked and damaged. 
     Other methods have involved providing unique patterned antennas which have been designed to impedance match a particular RF tag with a particular environment. 
     SUMMARY OF THE INVENTION 
     Accordingly, a first aspect of the invention provides apparatus comprising a resonant dielectric cavity defined between conducting surfaces, adapted to enhance an electromagnetic field at the edge of one of said conducting surfaces, wherein said dielectric cavity is non-planar. 
     Such apparatus provides a mounting or enabling component for an EM tag or device which is responsive to the enhanced field at a mounting site adjacent to the first conducting layer, at an open edge of the cavity. 
     The resonant cavity advantageously decouples or isolates the electronic device from surfaces or materials which would otherwise degrade the performance of the electronic device, such as metallic surfaces in the case of certain identification tags. This property is well documented in applicant&#39;s co-pending applications PCT/GB2006/002327 and GB0611983.8, to which reference is hereby directed. These applications describe radiation decoupling of a wide range of identification tags, particularly those that rely upon propagating wave interactions (as opposed to the inductive coupling exhibited by magnetic tags). Hence our preferred embodiment involves application to long-range system tags (e.g. UHF-range and microwave-range tags, also referred to as far-field devices) 
     The above referenced applications describe decouplers in which a planar dielectric layer is defined between two substantially parallel conducting layers. In certain described decouplers, the first layer does not overlie the second layer in at least one area of absence. This results in a structure which can be thought of as a sub-wavelength resonant cavity for standing waves being open at both ends of the cavity. Where the cavity length is substantially half the wavelength of incident radiation, a standing wave situation is produced, ie the mounting acts as a ½ wave decoupler as defined in the aforementioned PCT/GB2006/002327. 
     This structure results in the strength of the electromagnetic fields in the core being resonantly enhanced: constructive interference resulting in field strengths of 50 or 100 times greater than that of the incident radiation. Advantageously, enhancement factors of 200 or even 300 or more can be produced. In more specific applications typically involving very small devices, lower enhancement factors of 20, 30 or 40 times may still result in a readable system which would not be possible without such enhancement. The field pattern is such that the electric field is strongest (has an anti-node) at the open ends of the cavity. Due to the cavity having a small thickness the field strength falls off very quickly with increasing distance away from the open end outside the cavity. This results in a region of near-zero electric field a short distance—typically 5 mm—beyond the open end in juxtaposition to the highly enhanced field region. An electronic device or EM tag placed in this area therefore will be exposed to a high field gradient and high electrical potential gradient, irrespective of the surface on which the tag and decoupler are mounted. 
     An EM tag placed in the region of high potential gradient will undergo differential capacitive coupling: the part of the tag exposed to a high potential from the cavity will itself be charged to a high potential as is the nature of capacitive coupling. The part of the tag exposed to a low potential will similarly be charged to a low potential. If the sections of the EM tag to either side of the chip are in regions of different electrical potential this creates a potential difference across the chip which in embodiments of the present invention is sufficient to drive it into operation. The magnitude of the potential difference will depend on the dimensions and materials of the decoupler and on the position and orientation of the EM tag. 
     Typical EPC Gen 2 RFID chips have a threshold voltage of 0.5V, below which they cannot be read. If the entirety of the voltage across the open end of the cavity were to appear across the chip then based on a 1 mm thick core and simple integration of the electric field across the open end, the electric field would need to have a magnitude of approximately 250V/m. If a typical incident wave amplitude at the device is 2.5V/m—consistent with a standard RFID reader system operating at a distance of approximately 5 m—then an enhancement factor of approximately 100 would be required. Embodiments in which the field enhancement is greater will afford greater read-range before the enhancement of the incident amplitude becomes insufficient to power the chip 
     In such a decoupler, conveniently the length of the second conductor layer is at least the same length as the first conductor layer. More preferably the second conductor layer is longer than the first conductor layer. 
     Preferably a tag is mounted or can be mounted on a mounting site substantially over the area of absence. The electromagnetic field may also be enhanced at certain edges of the dielectric core layer, therefore conveniently the mounting site may also be located on at least one of the edges of the dielectric core layer which exhibits increased electric field. 
     RF tags may be designed to operate at any frequencies, such as for example in the range of from 100 MHz up to 600 GHz. In a preferred embodiment the RF tag is a UHF (Ultra-High Frequency) tag, such as, for example, tags which have a chip and antenna and operate at 866 MHz, 915 MHz or 954 MHz, or a microwave-range tag that operates at 2.4-2.5 GHz or 5.8 GHz. 
     The area(s) of absence are described as being small, discrete crosses, or L-shapes but more conveniently are slits wherein the width of the slit is less than the intended wavelength of operation. A slit may be any rectilinear or curvilinear channel, groove, or void in the conductor layer material. The slit may optionally be filled with a non conducting material or further dielectric core layer material. 
     The described structure can therefore act as a radiation decoupling device. First and second conductor layers sandwich a dielectric core. Where the first conductor layer contains at least two islands i.e. conducting regions separated by an area of absence or a slit, preferably the one or more areas of absence is a sub-wavelength area of absence (i.e. less than λ in at least one dimension) or more preferably a sub wavelength width slit, which exposes the dielectric core to the atmosphere. Conveniently, where the area of absence occurs at the perimeter of the decoupler to form a single island or where at least one edge of the dielectric core forms the area of absence then said area of absence does not need to be sub wavelength in its width. 
     It is noted that the sum thickness of the dielectric core and first conductor layer of the decoupler structure may be less than a quarter-wavelength in its total thickness, and is therefore thinner and lighter compared to prior art systems. Selection of the dielectric layer can allow the decoupler to be flexible, enabling it to be applied to curved surfaces. 
     The length G of the first conductor layer of certain described decouplers is determined by λ≈2 nG, where n is the refractive index of the dielectric, and λ is the intended wavelength of operation of the decoupler. Clearly this is for the first harmonic (i.e. fundamental) frequency, but other resonant frequencies may be employed. 
     Conveniently it may be desirable to provide a decoupler with length G spacings that correspond to harmonic frequencies other than the fundamental resonant frequency. Therefore the length G may be represented by λ≈(2 nG)/N where N is an integer (N=1 indicating the fundamental). In most instances it will be desirable to use the fundamental frequency as it will typically provide the strongest response, however harmonic operation may offer advantages in terms of smaller footprint, lower profile and enhanced battery life even though it&#39;s not idealised in performance terms. 
     Considering the dielectric cavity of other described decouplers, the first layer and the second layer are electrically connected at one edge, locally forming a substantially “C” shaped section. This results in a structure which can be thought of as a sub-wavelength resonant cavity for standing waves being closed at one end of the cavity. Where the cavity length is substantially a quarter the wavelength of incident radiation, a standing wave situation is produced, ie the mounting acts as a ¼ wave decoupler as defined in the aforementioned GB0611983.8 
     In such a decoupler, the two conductor layers can be considered to form a cavity structure which comprises a conducting base portion connected to a first conducting side wall, to form a tuned conductor layer, and a second conducting side wall, the first conducting side wall and second conducting side wall being spaced apart and substantially parallel. 
     The conducting base portion forces the electric field to be a minimum (or a node) at the base portion and therefore at the opposite end of the cavity structure to the conducting base portion the electric field is at a maximum (antinode). An electronic device or EM tag placed in this area therefore will be located in an area of strong field, irrespective of the surface on which the tag and decoupler are mounted. 
     Conveniently, the first conducting side wall has a continuous length of approximately λ d /4 measured from the conducting base portion, where λ d  is the wavelength, in the dielectric material, of EM radiation at the frequency of operation v. 
     Both the ½ and ¼ wave decouplers described above comprise a tuning conductor layer and a further conductor layer; preferably this further conductor layer is at least the same length as the tuning conductor layer, more preferably longer than the tuning conductor layer. 
     The two conductor layers are separated by a dielectric layer. They may be electrically connected at one end to create a closed cavity ¼ wave decoupler as hereinbefore defined, or contain conducting vias between the two conductor layers in regions of low electric field strength. However, there should be substantially no electrical connections between the two conductor layers in regions of high electric field strength or at the perimeter of the decoupler for open ended ½ wave versions, or at more than one end or perimeter for ¼ wave (closed end) versions. 
     It is noted that for a metallic body which is to be tracked by RFID, that at least one of the conductor layers of the decoupler can be part of said metallic body. RF tags generally consist of a chip electrically connected to an integral antenna of a length that is generally comparable with (e.g. ⅓ rd  of) their operational wavelength. The present inventors have found that tags having much smaller and untuned antennas (i.e. which would not normally be expected to operate efficiently at UHF wavelengths) can be used in conjunction with decoupling components as described herein. Usually tags with such ‘stunted’ antennas (sometimes referred to as low-Q antennas, as will be appreciated by one skilled in the art) possess only a few centimeters or even millimeters read range in open space. However, it has surprisingly been found that using such a tag with a low-Q antenna mounted on a decoupler of the present invention may be operable and exhibit useful read ranges approaching (or even exceeding) that of an optimised commercially-available EM tag operating in free space without a decoupler. Low-antennas may be cheaper to manufacture, and may occupy less surface area (i.e. the antenna length of such a tag may be shorter than is usually possible) than a conventional tuned antenna. Therefore the EM tag may be a low Q-tag, i.e. an EM tag having a small, untuned antenna. Conveniently the device will incorporate a low Q antenna, such that upon deactivation of the decoupler the read range of the low Q tag is caused to be that of a few centimeters or even millimeters. 
     In order to allow progressively smaller items to be tagged or monitored, it is desirable for the size of a decoupler to be reduced. Although the decouplers described in the above referenced applications can be made ‘stunted’ or low-Q tags, with the largest dimension only a half and a quarter of a wavelength respectively (at the intended frequency of operation) there is a demand to reduce this dimension further still. 
     In embodiments of the present invention, a standing wave is set up in the cavity as described above, but the cavity is not constrained to be monoplanar, that is, to extend only in a single plane or layer (which may be straight or curved), defined between substantially parallel upper and lower surfaces. Instead the cavity can extend beyond such surfaces, and in this way the cavity can be bent or folded at an angle. This arrangement allows a cavity having a given length or dimension, corresponding to an intended frequency of operation to occupy a smaller footprint, at the expense of increased thickness. Since the overall thickness remains small, and significantly less than arrangements employing ‘spacers’, such a device may have advantageous dimensions when absolute thickness is not critical. 
     Preferably the cavity comprises two or more layers, with each layer preferably being defined at least partially between a pair conducting walls, conveniently, each layer being offset. Preferably the layers are substantially parallel, and this arrangement advantageously allows the component to be built up in a laminated structure, with adjacent layers of dielectric being separated by a single conducting wall or surface. 
     Alternatively, the layers are not parallel, but are arranged at angles to one another. This allows for a corrugated or rippled effect. 
     In certain embodiments, the cavity defines a unique path length. In this way the cavity can be considered to be formed of a single plane, but bent or folded to change its physical configuration but not its topology. The cavity of such an embodiment therefore does not include any branches or junctions, and a single unique length for the cavity can be defined, which length is associated with the frequency of radiation at which enhancement occurs. 
     Alternatively, the cavity may be branched, and define a number of lengths, each corresponding to a frequency of enhancement. 
     In this specification, when referring to path lengths, the structure of a decoupler is assumed to have uniform width, unless otherwise stated. The path length is most easily understood by considering the cross section of a device, and is explained in greater detail below, with reference to the accompanying drawings. 
     A further aspect of the invention provides a mounting component for an electronic device comprising a first dielectric layer arranged between first and second conductor layers, and a second dielectric layer arranged between said second conductor layer and a third conductor layer, said first and third conductor layers being electrically connected at one end, thereby defining a first dielectric connecting region, joining said first and second dielectric layers, wherein said mounting component is adapted to enhance an electromagnetic field at a mounting site at an open edge of said third conductor layer. 
     The invention extends to methods apparatus and/or use substantially as herein described with reference to the accompanying drawings. 
     Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: 
         FIGS. 1   a  &amp;  1   b  illustrate two layer components 
         FIG. 2  shows a detailed embodiment of a two layer component 
         FIGS. 3 &amp; 4  illustrate physical properties of the embodiment of  FIG. 2   
         FIGS. 5   a  &amp;  5   b  illustrate three layer components 
         FIG. 6  is a detailed embodiment of a three layer component 
         FIGS. 7 &amp; 8  illustrate physical properties of the embodiment of  FIG. 6   
         FIG. 9  shows a two layer component having multiple path lengths 
         FIG. 10  shows a three layer component having multiple path lengths. 
         FIG. 11  shows an ‘L’ shaped component 
         FIGS. 12 ,  13  and  14  illustrate the configuration, field enhancement properties and chip voltage of a three layered spiral device. 
         FIGS. 15 to 20  similarly illustrate two possible four layer devices. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1   a  illustrates a cross section of a quarter wave component with the dielectric cavity formed on two layers. The layers are defined between conducting sheets  102 ,  104 ,  106 , with the bottom dielectric layer  110  between sheets  102  and  104 , and the upper dielectric layer  112  between sheets  104  and  106 . At the left hand end of the decoupler as viewed, conducting sheets  102  and  106  extend beyond sheet  104 , and are electrically connected by an end wall  116 . This arrangement results in the two dielectric layers being joined at this end. 
     The structure is uniform in the width direction into the plane of the paper as viewed, with the dielectric and conducting sheets exposed at the sides of the structure. 
     The path length  120 , is an approximation of the effective length of the cavity for the purposes of the wavelength of radiation which forms a standing wave in the cavity. In  FIG. 1   a  it is shown formed from three straight sections joined at right angles in a ‘C’ shape, however it will be understood that a standing wave formed in this cavity will not be governed by such a rigid geometry. It can nevertheless be seen that the structure of  FIG. 1   a  can be considered as a single layer decoupler, having approximately twice the length ‘A’ folded over upon itself singly. 
     The component of  FIG. 1   a  is a quarter wave decoupler, as end portion  118  causes a standing wave in the cavity to be at a minimum value of electric field adjacent to it, with a maximum value of electric field enhanced relative to the free-space-wave value, indicated at  122 . Region  122  can be considered, and is described in the earlier referenced applications as an area of absence of conductor  106 , which does not extend as far as conductors  104  and  102 . This region acts as a mounting site for an electronic device such as an RFID tag  124  which will experience electric field enhancement. 
     An equivalent half wave version is shown in  FIG. 1   b , with an open end  130 . 
       FIG. 2  is a more detailed illustration of a component having the general arrangement of  FIG. 1   a , with a PETG dielectric core, and with 75 micron thick aluminium conducting sheets. If we consider the path length as indicated in  FIG. 1   a , then the path length of  FIG. 2  can be seen to be approximately 51.8 mm, which corresponds to a quarter of a wavelength (with a refractive index of approx. 1.8 for PETG) of a resonant wave at approximately 805 MHz. 
       FIG. 3  is a plot of the absorption produced by the component of  FIG. 2 . Greater absorption results from stronger electromagnetic fields which peak at resonance by definition, thus  FIG. 3  reveals the resonant frequency of the component. It can be seen that the resonance is centred on approximately 850 MHz. Although this is greater that the theoretical approximation of 805 MHz derived above, it confirms that the effective length of the resonant cavity has been extended well beyond the external length of the decoupler by virtue of the two layer ‘folded’ structure. 
       FIG. 4  is a plot of the electric field strength in the core of the component of  FIG. 2  at 851 MHz. It can be seen that the field strength gradually increases along the path length, from the closed end  402  of the lower layer to a maximum at the edge  404  of the upper layer. Here the electric filed is enhanced by a factor of greater than 25 relative to the free space incident wave value of 1V/m. 
       FIG. 5   a  shows an extension of the arrangement of  FIG. 1   a , having three dielectric layers and four conducting sheets. Here the dielectric layers are joined at alternate ends, resulting in a reverse ‘S’ shaped path length  520 , extending from closed end  522  to the open end and enhancement region  524 , where a tag  530  may be mounted. Hence the component of  FIG. 5   a  can be thought of as a decoupler of approximately three times length B, folded twice upon itself.  FIG. 5   b  shows an equivalent arrangement for a half wave decoupler, having an open end at  526 . 
     Thus for a given frequency of operation, the arrangements of  FIGS. 5   a  and  5   b  result in a component having approximately a third of the overall length of the equivalent single layer device, but having increased overall thickness. Nevertheless, such three layer devices can still exhibit thickness of the order of 1 mm or less. 
     A specific implementation of the general arrangement of  FIG. 5   a  is shown in  FIG. 6 , and characteristics of this implementation are illustrated in the plots of  FIGS. 7 and 8 . As with  FIG. 2 , this implementation is formed of a PETG dielectric core, and with 75 micron thick aluminium conducting sheets 
     Considering an approximate path length arrangement as indicated in  FIG. 5   a , then the path length of  FIG. 6  can be seen to be approximately 50 mm, which corresponds to a quarter of a wavelength (with a refractive index of approx. 1.8 for PETG) of a resonant wave at approximately 833 MHz. 
     From the plot of  FIG. 7 , which is analogous to that of  FIG. 3 , it can be seen that the resonance is centred on approximately 905 MHz. Again this is greater that the theoretical value of 805 MHz, and implies that the effective length of the three layer structure is in fact less than the simple straight line approximation above, but it is confirmed that the multilayered structure allows resonance of a wavelength significantly greater than the overall dimensions of the device. 
       FIG. 8  is a plot of the electric filed strength in the core of the decoupler of  FIG. 6  at 905 MHz. Again it can be seen that the field strength gradually increases along the path length, from a minimum at the closed end of the lower layer  802 , through the middle layer  804  to a maximum at the open edge  806  of the upper layer. Here, electric field enhancement by a factor of approximately 75 occurs. 
     In the above described embodiments, the cavity, although folded back on itself, has a unique path length.  FIGS. 9 and 10  illustrate embodiments having multiple path lengths. 
       FIG. 9  illustrates a two dielectric layer arrangement in which the dielectric layers are joined at one edge of the structure. The uppermost conducting sheet  906  has an aperture or area of absence  908  in the form of a slot extending across the width of the structure (into the plane of the page as viewed), causing the upper dielectric layer to have an open end at a point midway along the structure, as opposed to the arrangement of  FIG. 1   a  where the upper layer is open at the edge of the structure. The arrangement of  FIG. 9  can therefore be thought of as a two layer decoupler in which the top layer of the dielectric cavity extends only part way along the structure, having a path length shown as  910 , together with a single layer decoupler extending along the remainder of the upper layer, and having a path length shown as  912 . If we consider the structure as having two sub-cavities, both sub-cavities will act to enhance an incident electric field at a mounting site in the vicinity of aperture  908  but at different frequencies/wavelengths. 
     This structure therefore acts as a dual frequency, or broadband decoupler with the frequencies of enhancement being determined by the various effective lengths defined by the dielectric cavity. 
     A more complex arrangement is shown in  FIG. 10 . Here, three dielectric layers  1002 ,  1004  and  1006  are separated by four conducting sheets  1012 ,  1014 ,  1016  and  1018 . Conducting end portions  1020  and  1022  enclose the full thickness of the structure at either end. Conducting sheet  1014  separating the lower and middle dielectric layers does not extend fully to either end portion  1020 ,  1022 , thereby joining the lower and middle dielectric layers at both ends. An upright conducting portion  1030  however is located part way along the lower dielectric layer, forming a closed end on either side. This closed end forces a standing wave in the cavity to have a minimum value of electric field in the known fashion for a quarter wave device, and therefore defines the end of a path length. 
     Sheet  1016  extends to contact end portion  1022 , but not portion  1020 , thereby joining the middle and upper dielectric layers only at one end. Sheet  1018  has an aperture  1032  part way along its length, thereby defining an open end, and thus a path length end. 
     It can be seen that three path lengths exist in this structure. Path  1040  defines a ‘C’ shape and extends part way along the upper and lower dielectric layers. Path  1042  extends at least partly along all three layers and defines an ‘S’ shape, and path  1044  extends along the upper dielectric layer only. 
     A tag  1050  placed over aperture  1032  will therefore experience enhancement of incident electric fields at multiple frequencies determined by the geometry of the structure described above. 
     In  FIG. 11 , a dielectric cavity extends into a solid conducting surface  1102 . The cavity is formed of a portion  1104  extending perpendicular to the surface, and a portion  1106  substantially parallel to the surface. In this way, the arrangement is analogous to a quarter wave decoupler ‘bent’ at right angles, with a device  1110  placed at the surface opening of the cavity experiencing electric field enhancement of incident radiation at a frequency dependent upon the effective length of the cavity. 
     A 3-layer dielectric cavity structure in which the cavity is folded one way then back on itself the other way, as shown in  FIGS. 5 ,  6  and  8 , creates a working design. It is also possible however to create a 3-layer device which appears as a spiral in cross-section—the cavity is folded over one way then folded over again the same way such a design is shown in  FIGS. 12   a  and  12   b . This has the same footprint as the former 3-layer structure but may offer manufacturing advantages. The chip and loop arrangement, or low Q tag, is shown at  1202  extending partially over the upper conducting plane, and partially over the exposed dielectric, or area of absence of the conducting plane. In  FIG. 12   b  the chip and loop is shown significantly spaced apart from the upper plane, for clarity. In reality the chip and loop may be separated and electrically isolated from the upper plane only by a thin polyester spacer of the order 0.05 mm in thickness. The loop in this example is approximately 12 mm by 18 mm in plan. 
     A cross-section through the 3-layer spiral structure of  FIG. 12  is shown in  FIG. 13 , illustrating the magnitude of the electric field on a sectional plane. In previous  FIGS. 4 and 8 , plots of the electric field were used to demonstrate the field-enhancing effect of the cavity, with  FIGS. 3 and 7  then demonstrating that the cavity is resonating at a tailored frequency by plotting the power absorbed by the structure as a function of frequency: the power absorbed is proportional to the square of the field strength hence greater absorption equates to greater field strength. 
     An alternative approach is employed in  FIG. 13  with the coupling element included in the model, lying substantially over the upper conducting plane as explained above. This allows the voltage across the chip as a function of frequency to be calculated which is arguably a more straightforward measure of performance of the device. 
     Turning to  FIG. 13  then, the region of strongest electric field occurs at the open end of the cavity  1302 . The scale runs from 0 V/m (black) to 170 V/m (white)—it can be seen therefore that the field has been enhanced by a factor of approximately 170 as the incident wave amplitude was set to 1 V/m. The field goes to zero at the closed end of the cavity  1304 . There are further regions of high electric field along the long edges of the loop ( 1306 ,  1308 ) which demonstrate the coupling between the cavity structure and the loop. The structure is mounted on a solid metal plate which appears white as the field has not been plotted on its surface ( 1310 ). The magnitude of the voltage across the chip as a function of frequency is shown in  FIG. 14 : the curve demonstrates resonant behaviour and is centred around 862 MHz. 
     It can also be seen in  FIG. 13  that a localised area of high field strength exists at the first ‘corner’ encountered by the cavity starting from the closed end, ie. at the edge of the conducting layer separating the first and second layers of the cavity, and around which the cavity is folded. It is therefore possible that an EM device or tag could exploit differential capacitive coupling, and be driven into operation, at this region in addition to region  1302 . 
     To illustrate that further number of dielectric layers are possible,  FIGS. 15   a  and  15   b  show a four dielectric layer device, with the layers in an M shape. Such a device resonates with incident radiation having a wavelength four times the total length of the cavity (ie roughly 16 times the overall length of the device), resulting in a region of strongly enhanced electric field at the open end of the cavity ( 1602  in  FIG. 16 ) It is noted that the chip and loop extends a proportionally greater distance across the length of the device, which has been reduced compared to  FIG. 13  by an additional ‘fold’ of the dielectric cavity. The field is close to zero at the closed end  1604 , and regions of high electric field again exist along the long edges of the loop ( 1606 ,  1608 ) 
     The resonance clearly visible from the plot of the electric field magnitude results in the voltage across the chip showing a resonant response as expected, as shown in  FIG. 17 . 
     Equally the spiral structure of  FIGS. 12 and 13  can be extended to four layers, as shown in analogous  FIGS. 18 and 19 . The same desired field characteristics (closed end  1904  close to zero; open end  1902  and loop ends  1906 ,  1908  having high field) are exhibited. The voltage across the chip is again plotted in  FIG. 20 . 
     Both  FIGS. 16 and 19  again show localised areas of high electric field strength within the folded structure, at the edges of the conducting planes forming the internal corners of the dielectric cavity, which could act as tag mounting sites as explained above. 
     It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. Although the embodiment of  FIG. 11  includes two dielectric layers at right angles to one another, it will be understood that the layers can equally be arranged at other angles such as 45 or 30 degrees, or combinations thereof. Examples of the positioning of electronic devices on mounting components have been provided, but it will be understood that alternative positions and orientations exist which advantageously experience electric field enhancement. 
     Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.