Field

[0001] The invention relates to highly durable Radio-Frequency Identification (RFID) tags for the identification of tires, which are embedded or otherwise incorporated into the tire, at the time of production of the tire.

Background

[0002] The identification and tracking of a tire with radio frequency identification (RFID) enables automated and fully digitized traceability over its entire lifecycle. This allows automatic data recognition for the purpose of optimizing intra- and extra-logistical process handling. Moreover, RFID technology provides supportive information about different manufacturing steps at an early stage of the fabrication process, which may be helpful for the correlation of processing data as well as for customer claims. A fully embedded transponder and its associated information remains inseparably connected to the tire over the entire product lifetime. The embedment of the transponder inside the tire, however, is required to have absolutely no negative influence on either the product lifetime or its functionality. In particular, defects and air enclosures, which in the worst case cause damage to, or the destruction of, the tire, have to be avoided. As an embedded transponder per se constitutes a material defect, it is required to integrate the transponder in a minimal disruptive way, i.e., the transponder is required to assume a minimal amount of space inside the tire.

[0003] An embedded transponder should have a minimal shape; its resonance frequency can be tuned only by shortening the length of the antenna. Specifically, the tunability of helix-antennas, whose length can be reduced only by reducing the number of windings, is limited by this practice. This can lead to rather large frequency steps of the antenna tuning, especially if the embedding medium is a high dielectric.

[0004] There exists a need for more precise resonance tuning of RFID antenna and thereby improve the overall performance of RFID transponders to be incorporated into tires during tire manufacture.

[0005] Therefore, more precise resonance tuning of RFID antenna, and constructions containing the same, that improve the overall performance of RFID transponders to be incorporated into tires during tire manufacture are described herein. Summary

[0006] To solve the problem of an insufficient tunability of Linear-Antenna-Transponders, one embodiment of the invention proposes to construct the antenna of a Radio Frequency Identification (RFID) device in ways as described herein. In one embodiment, the transponder is linear and contains or includes two radiators formed of a conductive material. In some embodiments, the two or more radiators are identical. In some embodiments, the radiators are divided into a multitude or a plurality of transmission line segments, wherein each segment is in the form of a linear segment or helical spring segment or cylindrical segment. In some embodiments, consecutive or adjacent segments are different in either shape and/or pitch of the helical spring.

[0007] The distinct transmission line segments provide additional degrees of freedom, which enable finer/improved tunability of the antenna resonance. Small reflections of the electro-magnetic signal arise at the transitions between the segments and influence the resonance frequency. Due to the small magnitude of the occurring reflections, the resonance frequency is less susceptible to changes of a single segment than to changes of the overall antenna length. This enables fine adjustments of the antenna. The final adjustment of the antenna arises as a combination of an adjustment of individual segments as well as the adjustment of the total antenna length.

[0008] In some embodiments, the antenna properties of a transponder having a proper matching network and a wire-type antenna can also be tuned. The shape of a RFID antenna determines the specific RF-properties of an RFID label. Likewise, the properties of a transponder having a regular matching network and a wire-type antenna can be varied by introducing different antenna segments.

[0009] In some embodiments, one or more (or all) of the antenna segments are designed to meet the mechanical and geometrical requirements of a desired application. For example, the properties of a single line segment can differ by its geometry, e.g., outer radius or winding number as well as by the material used, such as copper, bronze, steel and alloys thereof, wires with an isolation layer, or other materials. Examples of such line segments include, but are not limited to, cylindrical wire segments (referred to as dipole segments); conical wire segments; hollow lines; helices having an arbitrary pitch; helices or wires having envelope curves, that form a body of revolution; helices with contacted, i.e., shorted windings; and helices with contacted windings made from insulated wire.

[0010] In some embodiments, helical forms or segments are advantageous as they reduce the possibility of antenna breakage and thereby minimize the risk of damaging the tire. At the same time, the form or segment is open enough for the ingress of the rubber mass during the manufacturing process of the tire, e.g., vulcanization. [0011] In some embodiments, each of the two identical conductive radiators are divided into at least three segments. The more segments a dipole antenna has, the more degrees of freedom are available for fine tuning of the antenna, allowing for an adaptation that is not possible with solutions according to the prior art.

[0012] In still other embodiments, the segments on each of the two identical radiators directly adjacent to the IC chip are linear: the first segment serves as a linear dipole segment. In a dipole antenna only those parts that are in the direction of a dipole contribute to the radiation. In a helical antenna, only those vectors in the direction of the dipole contribute, while the vectors perpendicular to the dipole direction do not. A linear segment can increase the effective antenna length.

[0013] In an alternative embodiment, the segments on each of the radiators directly adjacent to the IC chip are of a helical spring form or shape with a larger pitch than the consecutive segment. A larger pitch increases the fraction of the antenna parallel to the dipole. This allows for an increase in effective antenna length without giving up the stable helical spring form/shape.

[0014] In another embodiment, at least one of the segments on each of the radiators is a helical spring segment with a pitch small enough that the windings are in electrical contact with each other. The windings can be formed using uninsulated wire and shortcut the windings. Alternatively, a hollow cylinder can be used. Such segments can form sharp transitions between antenna segments, creating signal reflections, which then contribute to the impedance, as described further below. Alternatively, the windings can be insulated against each other.

[0015] In other embodiments, all segments of the radiators in the RFID transponder device are the same length. In alternative embodiments, the segments of the radiators in the RFID transponder device are of at least two different lengths alongside the radiators.

[0016] In some embodiments, the RFID transponder device contains an IC chip attached to a primary antenna and a dipole antenna, wherein the dipole antenna is a one-piece helical spring antenna and the dipole antenna is capacitively or inductively connected to the IC chip by means of the primary (coupling) antenna, and the chip with the primary (coupling) antenna is located at the mid-way point of the length of the primary (coupling) antenna, so that the parts of the antenna extending on either side of the chip are of the same length and therefore electrically forming two identical radiators. This embodiment can also contain the dipole antenna divided into a multitude of segments. In a preferred version of this embodiment two consecutive segments are different in pitch of the helical spring form.

[0017] In another embodiment, the RFID transponder device contains a one-piece dipole antenna divided into at least six segments. The six segments may be symmetrically distributed over the length of the antenna. In some embodiments, the IC chip is coupled via the primary antenna in the middle of the one-piece antenna, creating two electrically identical halves of the dipole. In some embodiments, segments having similar form are in the same relative position on each half. This arrangement may be referred to as mirror-symmetry.

[0018] In other embodiments, the device is as described above and the segments directly adjacent to the IC chip have a larger pitch than the adjacent segment. A larger pitch increases the fraction of the antenna alongside the dipole. This allows for an increase in the effective antenna length without foregoing the stable helical spring form.

[0019] In still other embodiments, the device is characterized by at least two of the segments having a pitch small enough that the windings are in electrical contact with each other and wherein said segments are distributed symmetrically over the length of the antenna, such that they are in the same relative position In some embodiments, the windings can be from an uninsulated wire such that the windings are electrically shorted together. Alternatively, a hollow cylinder can be used. Such segments can form sharp transitions between antenna segments, creating signal reflections, which then contribute to the impedance, as described further below. Alternatively, the windings can be insulated against each other.

[0020] Methods for producing the RFID transponder devices described above for integration into a tire are also described herein.

[0021] In some embodiments, the method includes the following method steps:

(1) producing two identical radiators from a conductive material;

(2) providing an IC chip suitable for Ultra-High-Frequency RFID, having two contacts for contacting antenna radiators;

(3) attaching one of the two identical radiators to each of the contacts of the IC chip, so that the radiators are in electrical contact with the contacts of the IC chip.

[0022] In one embodiment, the conductive material may be a rod or wire. In some embodiments, the radiators are formed such that they contain a multitude of segments, wherein each segment is a linear form, a helical spring form, or a cylindrical form. In other embodiments, the radiator segments are formed such that two consecutive segments are different in either form, or if in a helical spring form, the pitch of the helical spring form.

[0023] In another embodiment, the radiators are divided into at least three segments each.

The more segments a dipole antenna has, the more degrees of freedom are available for fine tuning of the antenna. [0024] In some embodiments, the method is as described above and the method includes forming the segments on each of the radiators directly adjacent to the IC chip into a linear form.

[0025] In other embodiments, the method includes forming the segments on each of the radiators directly adjacent to the IC chip into a helical spring form with a larger pitch than the adjacent segment.

[0026] In other embodiments, the method includes forming at least one of the segments on each of the radiators into a helical spring form segment with a pitch small enough that the windings are in electrical contact with each other or into a hollow cylinder.

[0027] As discussed above, applications for the RFID tags/labels described herein include molden, molded, or vulcanized rubber articles, such as tires. In some embodiments, in order to increase the adhesion between the transponder and rubber, the transponder may be coated with a primer or coating material. In some embodiments, an aqueous overcoat adhesive suited for bonding elastomers may be used e.g. a mixture of dispersed mineral fillers, organic compounds, resins and polymer lattices in an aqueous medium. The primer or coating may be applied by technique known in the art, including spray or dip methods.

[0028] In another embodiment, the method of making the devices described herein includes

(1) producing a helical spring from conductive material; In a variant of the method, the conductive material may be a rod or wire;

(2) providing an IC chip suitable for Ultra-High-Frequency RFID, having two contacts for contacting antenna radiators; and

(3) attaching a primary or coupling antenna to the IC chip, such that the primary antenna couples the IC chip to the helical spring antenna capacitively or inductively. The primary antenna may be just a short loop antenna or may be a coil antenna that is suited for said coupling.

[0029] In a variant of the method above, the helical spring is formed such that it includes or contains a multitude or a plurality of segments, wherein the segments are distributed or partitioned symmetrically over the length of the helical spring. In a further variant of this method, the segments are formed such that two consecutive segments are different in the pitch of the helical spring form.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a drawing of a prior art RFID device for tire tagging formed as a homogeneous spring antenna (a) and an embodiment of the RFID transponder device with a segmented spring antenna according to the present disclosure (b). [0031] Figure 2 are graphs of the resonance behavior of a helical antenna by adaption of its winding number (a) in air and (b) embedded into tire rubber.

[0032] Figure 3 are drawings of various examples of a RFID transponder device. Figure 3(a) depicts a state-of-the-art transponder. Figures 3(b) - 3(f) showing various examples of the RFID transponder device according to the invention.

[0033] Figure 4 is a schematic drawing of the approximation of a dipole antenna as twin-lead wire.

[0034] Figure 5 are graphs of the complex impedances of (a) a loss-free transmission line, (b) a lossy transmission line and (c) a linear dipole antenna.

[0035] Figure 6 is a schematic illustration of reflections arising during the transmission of a wave through several antenna segments.

[0036] Figure 7 are graphs of the impedance modifications of an embedded helix antenna containing five segments as shown in Figures 3 (e) and (f).

[0037] Figure 8 is a schematic drawing of an antenna formed by a regular chain of two alternating elements.

[0038] Figure 9 is a graph showing the results of the parametric studies for the two-element chain with n = 3, a ₁ = 0.4 mm, a ₂ = 1 mm, l ₁ = 40 mm, and ε _r = 6. The plots show the frequency curves of the propagation factors for different parameter configurations.

[0039] Figure 10 is a graph showing the results of the parametric studies for the two-element chain with n = 3, a ₁ = 0.4 mm, a ₂ = 1 mm, = 40 mm, and ε _r = 6. The plots show the calculated resonance frequency as a function of the varied parameter.

[0040] Figure 11 is a schematic representation of an embodiment of the inventive RFID transponder device with a non-conductive coupling of the IC to the antenna.

DETAILED DESCRIPTION

[0041] For conventional receiver antennas, the antenna's impedance is matched to the load resistance of the antenna's transmission line or the receiver, which in both cases is typically 50 Ohms. The equality of antenna and load impedance ensures that a maximal amount of power is transferred from the antenna to the load. If an antenna, however, has to be matched to a complex-valued load impedance as for instance in the case of a (passive) RFID transponder IC, the resistance as well as the reactance have to be considered. In order to ensure maximal power transfer, the complex-valued antenna impedance has to be matched in such a way that the resistances of antenna and IC coincide as before and the reactances of both components cancel each other out. This procedure is known as complex-conjugated matching, or maximum power transfer matching, of the antenna. Due to the architecture, (passive) transponder ICs always exhibit capacitive-type reactances. To compensate for this, the transponder antenna needs to present the conjugated reactance, namely an inductance. At those frequencies at which compensation is happening, the transponder is resonant and can be operated optimally. Usually, a complex conjugated matching with respect to the load impedance is established by an electrical matching circuit, which is added to the antenna but also requires additional space.

[0042] If the available space for the embedment of the transponder is strongly constrained by a specific application, it can be difficult to find a properly designed matching circuit. Among other applications, this particularly applies to transponders which are to be embedded into tires. Because of the enormous safety risk due to material defects and air enclosures, extremely narrow shaped transponders are typically used for tire applications. For those narrow shaped transponders it is a fundamental problem to find sufficient space for a proper matching network. Such transponders are referred to as Linear Transponders herein. All transponders whose cross-section is insufficient to place a well-suited matching network will be referred to as Linear Transponders herein. It is irrelevant if the transponder antenna is a simple wire, a helix or something else.

[0043] A solution to the problem of finding a proper matching element is the usage of discrete circuit elements such as surface mount (SMD) inductors. Due to their inherently high losses, SMD inductors are most often inconvenient or cannot be used because of the lack of space. Yet, to achieve at least a partial matching between the antenna and IC of a Linear Transponder it is common practice to cut the antenna length down depending on the IC used, the surrounding dielectric, and/or the individual application. Even though an ideal, i.e., complex conjugate matching between antenna and IC, can practically never be achieved by changing the antenna length alone, it is sufficient to adjust the transponder's resonance point in an appropriate manner. This concept is based on the close relationship between antenna length, resonance frequency, and impedance; in a simplified picture a linear wire-type dipole antenna is assumed to be a common signal transmission line. Its complex antenna impedance Z _a is then approximately determined by the effective transmission line properties, namely the characteristic line impedance Z ₀ , the geometric length of the line I, the phase velocity v _ph of the conducted waves.

[0044] On its ends the antenna represents an open transmission line, whose properties can be explained by transmission line theory. In the case of a loss-free transmission line the input impedance is obtained by the relation

[0045] This expression is an approximation of the antenna impedance, which is capable of adequately describing the resonant properties of the antenna. "Loss-free transmission" is a rather simple assumption, because a proper antenna should always be approximated as a lossy line due to its radiation losses. That being said, a lossy transmission line consists of an appreciable value of series resistance and shunt conductance where different frequencies travel at different speeds. This is opposite to a lossless or loss-free transmission line, where the speed of wave propagation is the same for all frequencies.

[0046] The periodic interdependency of the impedance, circular frequency a), and antenna length I reflects the multitude of harmonic modes of the current distribution along the line. The poles and zeros of the imaginary part of the impedance indicate series- and parallel resonances of the antenna. A series resonance is characterized by a root of the imaginary part of the antenna's impedance function. On the other hand, a parallel resonance is characterized by a root of the imaginary part of the antenna's admittance function. At these points the antenna effectively behaves as a serial or parallel RLC (resistor, capacitor, and inductor) circuit.

[0047] In the range between a series and parallel resonance, the antenna has an inductor- like behavior and admits the compensation of the IC's capacitive impedance by proper selection of the antenna length.

[0048] By adjusting the antenna length, it is in principle possible to tune the resonance of a Linear Transponder at will, but the window of the length parameter might be severely limited. In particular, LinearTransponders, which are enclosed by a dielectric material, experience a strong detuning of the in air resonance frequency, because the dielectric densifies the electric field lines in its interior thereby modifying the antenna's effective transmission line parameters. This causes a contraction of the resonant antenna length by a factor of where ε _r is the dielectric constant of the material. Thus, parameter adjustments have a larger impact on the more delicate structures of embedded transponders.

[0049] In general, the higher the dielectric constant of the embedding material the more precise an antenna structure is required to be adaptable.

[0050] Figure 1 (a) depicts a RFID transponder device 100 for implementation into a tire according to the prior art. An IC chip 102 is protectively encapsulated and has a conductive connection 103 to two identical radiators 105, which combined form the helical spring dipole antenna 104. Because of its small shape, the transponder has no matching network attached to it and therefore belongs to the class of Linear Transponders. In contrast, Figure 1 (b) depicts a RFID transponder device 150 according to an embodiment of the present disclosure. The two identical radiators 155 contain segments 156, 157, 158, 159, 160 of different pitches of the helical spring.

[0051] As Figure 2 illustrates, in the case of helical spring antennas which are to be embedded into a tire but can be adjusted winding by winding only, the concept of tuning the antenna length reaches its limits; by cutting a single winding such a large frequency shift is induced, which cannot be compensated by any other degree of freedom.

[0052] Another aspect of this conventional device affects the handling of products of different lengths. Every adaptation of the design, e.g. to a customer-specific material or a specific IC, results in an individual antenna length. For every specific length a set of tools has to be made and prepared for production, which complicates the handling of numerous individual products and therefore increases the cost. In both examples the antenna is cut in the range of one to seven windings. The influence of the dielectric, assumed to be E _r = 6 for tire rubber in Figure 2b, causes a significant spread of the frequency shift per winding.

[0053] Figure 3 schematically depicts different RFID transponder devices. Figure 3(a) is a RFID transponder device 200 according to the prior art. An IC chip 202 is protectively encapsulated and has a conductive connection 203 to two identical radiators 205, which combined form the helical spring dipole antenna 204. Figure 3(b) is an RFID transponder device 250 with segmented dipole antenna 254 according to an embodiment of the present disclosure and containing a linear dipole segment 256 and a helical spring segment 257 on both of the radiators 255. While the linear dipole segment 256 is functioning like any standard linear antenna, reflections will occur at the transition to the helical spring segment 257, increasing the impedance. Thus two consecutive segments 256, 257 of the antenna are different in form.

[0054] Figure 3(c) illustrates an embodiment of the present disclosure and is a schematic drawing of an antenna with segmented radiators 255 containing two helical spring segments with different pitches: one very large pitch 258 and one with a narrow pitch 259. That is, a first helical spring segment having a first (large) pitch 258 and a second helical spring segment with a second (narrow) pitch 259, wherein the first pitch is larger than the second pitch. Thus two consecutive segments 258, 259 of the antenna are different in pitch.

[0055] In a helical spring form antenna, only those parts of an antenna that are in parallel with the length of the dipole radiator contribute to the effective antenna length. An antenna winding can be imagined as a vector resulting from a vertical and a horizontal part, with the horizontal part pointing in the direction of the dipole radiator's length and the vertical part pointing in the direction of the winding radius. The vertical parts will always nullify each other, because for one winding for each vertical vector there is another one pointing in the opposite direction and the opposing electromagnetic fields nullify each other. Only the part of the vector in parallel with dipole is not nullified and contributes to the effective length of the dipole. Accordingly, a larger pitch increases the vector part in parallel with the dipole radiator. The large pitch 258 offers stability, while the component of the vector in parallel with the extension of the antenna is still large. At the transition to section 259 with narrow pitch, reflections occur. Varying the pitch of both sections allows for more freedom in tuning the antenna, as is shown in Figure 3(d). Here, segment 260 has a narrower pitch than segment 258 of Figure 3(c) and segment 261 is slightly shorter than segment 259 of Figure 3(c), resulting in a different behavior of the antenna 254.

[0056] Figure 3(e) illustrates an embodiment of the present disclosure and is a schematic drawing of an antenna with segmented radiators 255, containing segments 262 with a narrow pitch, alternating with segments 263 and 264 with a larger pitch.

[0057] Figure 3(f) illustrates an embodiment of the present disclosure and is a schematic drawing of an antenna containing five segments 262, 265, 266, which differ in their individual lengths with an overall constant antenna length. At least some of the segments 262 may have a pitch small enough that the windings are in electrical contact with each other. Three of the five segments 262 have electrically shortened windings, which can be treated as hollow-cylindrical line sections. Alternatively, segments 262 can be replaced with hollow cylinders.

[0058] In the following the influence of partial reflections on a segmented linear antenna is examined. The explanations given here primarily serve the qualitative understanding of the present disclosure. Every line section of a segmented dipole antenna i = 1, . .. , n is associated with a characteristic impedance, an effective phase velocity v _ph,i , as well as the length of the section l _i A linear or segmented dipole antenna can be described in a simplified way as twin lead-wire with different diameters in order to simulate the different impedances of the segments. This is schematically depicted in Figure. 4.

[0059] In Figure 5, complex impedances of a loss-free transmission line (upper graph), a lossy transmission line (middle graph) and a linear dipole antenna (lower graph) are depicted. The curves indicate that in good approximation a dipole antenna of length L — measured from end to end — can be treated as an open homogeneous transmission line of length L/2 — measured from its feeding point to the end. Up to small deviations of minor order, the resonance points of all cases shown here essentially coincide. In the limit 0 the effective transmission line model is inapplicable.

[0060] Due to the fact that inductance and capacitance per length of an antenna section is interdependent on its distance to the antenna's food point, the characteristic impedance is continuously changing. Reflections, which originate from this variability, remain small and shall be neglected here to give a qualitative explanation.

[0061] At the transition between two sections, signal reflections, whose amplitude depends on the difference of the line impedances at the transition, occur and can be expressed by where p ₀ is the reflection coefficient of the transition between the first antenna section and a measurement line; p _n is the reflection coefficient, which occurs at the open end of the antenna. As the antenna can be seen as a open transmission line, p _n ~ 1. Due to multiple signal reflections arising from the transitions between the antenna sections, the actual reflection coefficient at the foot point of the antenna is not equal to p _Q but rather the coherent sum of all reflections along the line, see Figure 6. The upper picture of Figure 6 shows the general case, in which multiple reflections happen at the transition points. The two lower pictures show some examples of first order and third order reflections arising during the transmission of a wave through four sections.

[0062] According to relation [B] the reflection coefficient r = Z _A — Z ₀ )/ Z _A + Z ₀ ) is related to the actual and measurable antenna impedance Z _A .

[0063] In general, the plurality of signal paths cannot be summed in a closed expression. However, if the reflections at the transitions are small, i.e., p ₁₍ . . . , p _n-1 « 1, a simple approximation formula can be given. The approximation relies on the assumption that because of the small geometry the line segments differ little from each other. Considering the fact that reflections at the beginning and the end of the antenna are not small, in general, p _n ≈ 1 and p ₀ ≃ 1, the foot point reflection factor of an antenna with n segments is obtained by where the propagation factor RΠ _n represents the coherent sum of all first order reflections (by neglecting all higher order reflections the given result corresponds to the first order series expansion in terms of the reflection coefficients p ₁ ,...,p _n .) of the signal at the transitions between the n segments [0064] This nomenclature refers to the nature of the propagation constant as a coherent sum of n (free line) propagation factors and reflection coefficients. Here θ _i = ωl _i /v _ph,i denotes the electrical phase of the i-th segment with a geometrical length l _i, the phase velocity of the segment v _ph,i , and the angular frequency co.

[0065] Since all segments of the antenna contribute, the coherent sum [D] represents a unique combination of the individual properties of the segments. Consequently, all characteristic transmission line properties of a segment, i.e., its line impedance Z _i, its geometrical length l _i, as well as its phase velocity v _ph,i constitute an additional degree of freedom for tuning the resonance frequency of the antenna. The characteristic impedance of a single segment contributes to the reflection coefficients at its ends. Furthermore, there is a direct relationship between the antenna impedance and the propagation factor [D],

[0066] The resonances of the antenna are obtained from the zeros and poles of its impedance function. With that the resonance conditions of the propagation factor are obtained by

[0067] As the propagation factor is a complex number, conditions [Fa] and [Fb] constitute requirements to the complex angle arg (RΠ _n ) as well as the modulus IR _n I of the propagation factor.

[0068] Under ideal conditions a single antenna parameter of a Linear Transponder is needed to tune the resonance frequency properly. Typically, this parameter is the antenna length, which has a direct impact on the antenna's resonance points. In the following two examples it shall be demonstrated how the claimed effect differs from the trivial case of simply tuning the antenna length and how it allows for fine-tuning the antenna. Therefore the trivial case of a linear dipole antenna constituting of a single segment and the case of a periodic chain consisting of 2n + 1 segments will be considered.

[0069] A linear dipole antenna comprising a single cylindrical wire segment only is characterized by its electrical phase θ _dip = ωl _dip /c, where c is the speed of light. Eq. [D] and [E] yield the expression Rn _n = exp —2iθ _dip ) from which the well-known impedance [A] of an open transmission line follows. Then the resonance points of the dipole antenna are obtained as described before. Due to the fact, that the phase velocity v _{ph, dip} = c is constant and independent from the wire's geometry, the antenna length l _dip remains as the only degree of freedom usable as a tuning parameter. [0070] In Fig. 7, the different curves show the impedance as a function of the position of the central section for air Fig. 7a and tire rubber Fig. 7b. The originating shifts of the resonance frequency are much smaller as in the case of cutting single windings, see Fig. 2. The results shown are obtained by simulations of the embedded antenna and do not contain simplifying assumptions about effective transmission line parameters.

[0071] When looking at the non-trivial case, there are more degrees of freedom which can be used for tuning. To show this, we choose as an example a two-element chain comprising 2n + 1 segments arranged in a regular pattern. The characteristic line properties of the two elementary segments shall be and Z ₁ , β ₂ , l ₂ , see Fig. 8. As in the case of the homogeneous dipole antenna a closed expression of the propagation factor can be given. By the symmetry of the segment pattern it is p ₂ = —p ₁ and expression [D] reveals the result

[0072] The result consists of two parts; the dominating contribution is of the order 0(1), i.e., independent of p ₁ and is--j ust as in the trivial case-caused by reflections at the open end of the antenna. The electrical phase, which is associated with this contribution, is the sum of the electrical phases of all 2n + 1 segments. As a result of the interdependence of the leading order term and the parameters n, θ ₁ , and θ ₂ , a matching of the antenna resonance, which is beyond a simple adaption of the antenna length, can be done. Moreover, a small term of the order 0(p ₁ ), which considers all first order reflections, is added to the result. As this contribution is much smaller than the dominating one, it is less susceptible to changes of the antenna parameters. To be able to fine tune the antenna it is mandatory to find an appropriate parameter dependency, which allows to vary the first order contribution but keeps the leading order term constant.

[0073] As a concrete example for such a fine tuning consider a two-element chain consisting of four dipole segments and six helical segments, see Fig. 8. Assuming the windings of the helix to be in contact, i. e., electrically shorted, the helix segments can be treated as hollow cylinders. Because those hollow cylinders are excited below their cut-off frequencies, only the fields outside contribute to the effective transmission line parameters. Hence, the helical segments can be seen as dipole segments where the effective wire diameter equals the outer diameter of the helix. Let a ₁ = 0.3 mm and a ₂ = 1 mm the radii of the respective line segments, l ₁ = 30 mm, l ₂ = 40 mm the lengths of the segments, and ∈ _r = 6 the dielectric constant of the surrounding medium. Fig. 9 und Fig. 10 show results of parametric studies of the quantities a ₁ and l ₁ . In both cases the total length of the antenna has been kept constant. Both parameter studies show a weak sensitivity of the resonance frequency as a function of the respective parameter. However, only the parameter a ₁ represents a uniquely invertible mapping within the evaluated frequency range and is thus better suited for adjustments of the resonance frequency.

[0074] In Fig. 9, for all parameter configurations the total antenna length is preserved. The diamond-shaped markers indicate the points of fixed frequency, whereas the O-shaped marker indicates the ideal resonance point.

[0075] In Fig. 10 the shapes of the curves imply that the parameter ai is well suited as a tuning parameter, whereas li is not in the considered frequency range.

[0076] Fig. 11 shows a schematic drawing of an example of the RFID transponder device 350 that comprises a one-piece helical spring antenna 354. Instead of being conductively coupled to two identical radiators, the IC chip 352 is coupled to a primary (or coupling) antenna (not shown). The primary antenna couples capacitively or inductively to the helical spring antenna 354. The helical spring antenna 354 comprises a plurality of segments 356, 357, 358, wherein two consecutive segments 356, 357, 358 have different pitches of the helical spring form, as previously discussed. The primary antenna can be directly connected to the IC chip 352 or the IC chip 352 can be attached to a substrate and the primary antenna attached to the IC chip 352 by conductive connections, so that the primary antenna and IC chip 352 are not located together. However, because space is limited, a compact solution is generally preferred. The primary antenna may be located inside the helical spring antenna 354 or on the outside. The primary antenna may therefore be coaxial with the helical spring antenna 354. Ideally, the primary antenna is coupled in a way that the dipole created by the coupling electro-magnetically has two identical radiators 355, thus the dipole antenna may have segments 356, 357, 358 which are mirror-symmetrically distributed over the length of the one-piece dipole antenna 354. This is the case if the coupling is located in the middle (half of the length) of the one-piece helical antenna 354. Other, asymmetrical, variants may be chosen for special applications. The dipole antenna 354 may be divided into at least six segments 356, 357, 358 as shown. In some embodiments, segments directly adjacent to the IC chip 352 may have a smaller pitch than the consecutive or adjacent segment. In an embodiment with six segments, as illustrated in Figure 11, segments 356 directly adjacent to the IC chip 352 may have a smaller pitch than the segments 357 consecutive thereto, and a larger pitch than the next segments 358. In this way, the middle segments 357 have larger pitches than any of their adjacent segments. [0077] Tuneable linear-antenna transponders and methods of making and using thereof are described herein. In some embodiments, the transponder includes two radiators formed of a conductive material, wherein the radiators may be identical or different. In some embodiments, the radiators are divided into a multitude of segments, wherein each segment is in the form of a linear segment or helical spring segment or cylindrical segment. In some embodiments, two of the consecutive segments are different in either shape and/or pitch of the helical spring. The distinct transmission line segments provide additional degrees of freedom, which enables finer/improved tunability of the antenna resonance.

[0078] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0079] Features, integers, characteristics, compounds, or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0080] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.