Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q7/00—Loop antennas with a substantially uniform current distribution around the loop and having a directional radiation pattern in a plane perpendicular to the plane of the loop
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/12—Supports; Mounting means
  • H01Q1/22—Supports; Mounting means by structural association with other equipment or articles
  • H01Q1/2208—Supports; Mounting means by structural association with other equipment or articles associated with components used in interrogation type services, i.e. in systems for information exchange between an interrogator/reader and a tag/transponder, e.g. in Radio Frequency Identification [RFID] systems
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/12—Supports; Mounting means
  • H01Q1/22—Supports; Mounting means by structural association with other equipment or articles
  • H01Q1/2208—Supports; Mounting means by structural association with other equipment or articles associated with components used in interrogation type services, i.e. in systems for information exchange between an interrogator/reader and a tag/transponder, e.g. in Radio Frequency Identification [RFID] systems
  • H01Q1/2225—Supports; Mounting means by structural association with other equipment or articles associated with components used in interrogation type services, i.e. in systems for information exchange between an interrogator/reader and a tag/transponder, e.g. in Radio Frequency Identification [RFID] systems used in active tags, i.e. provided with its own power source or in passive tags, i.e. deriving power from RF signal
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q13/00—Waveguide horns or mouths; Slot antennas; Leaky-waveguide antennas; Equivalent structures causing radiation along the transmission path of a guided wave
  • H01Q13/10—Resonant slot antennas
  • H01Q13/16—Folded slot antennas
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
  • H01Q15/0006—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices
  • H01Q15/0086—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices having materials with a synthesized negative refractive index, e.g. metamaterials or left-handed materials

Definitions

• the present invention is applicable to the antenna miniaturization design, for example, in the technical field of Radiofrequency Identification
• RFID radio-frequency identification
• the invention that is disclosed herein relates to an antenna that achieves self-resonance without needing any external matching network between the antenna and the source (for example an RFID chip) and can be reduced in size arbitrarily, just adjusting different parameters of the resonant structure (at the expense of a reduced read range).
• This tiny antenna is especially suitable for RFID applications because it can be fabricated in a single layer substrate, with small dimensions as the antennas used in RFID tags require.
• Antennas should be integrated in different electronic products as mobile phones, laptops, personal digital assistant (PDA), etc., and they require a small antenna capable of being integrated with different products.
• PDA personal digital assistant
• RIFD Frequency Identification
• This technology allows the identification of any object with the aid of an electronic tag attached to it.
• This electronic tag is composed by a small antenna and a micro-chip.
• Radiofrequency Identification the tiny antennae of the RFID electronic tag can operate in a low-frequency band (LF), around 125 kHz, others in the high-frequency band (HF) at 13.56 MHz and some last ones are developed to work in the 900 MHz range, in the ultra-high-frequency (UHF) band.
• LF low-frequency band
• HF high-frequency band
• UHF ultra-high-frequency
• Different implementations of the RFID tags carrying in the interior thereof the microchip connected to the printed circuit antenna are known, for example, implemented in self-adhesive labels, capsules, coins, cards, badges, etc.
• the size of a given antenna is in the order of the wavelength. This restriction means that antennas for low frequencies will be larger than antennas for high frequencies.
• small antennas herein are commonly defined as antennas that fit in a sphere of radius ⁇ /(2 ⁇ ), being ⁇ the wavelength.
• a resonant dipole is a balanced antenna formed by a wire with length slightly shorter than half a wavelength fed at the centre.
• a self-resonant antenna as the resonant dipole, is an antenna whose input impedance is purely real.
• the maximum power transfer theorem states that, for a linear network with fixed source impedance, the maximum power is delivered from the source (antenna) to the load (chip) when the load impedance is the complex conjugate of the source impedance.
• the maximum power will be delivered when the antenna and source impedance are equal.
• theorem Based on the maximum power transfer theorem, if the antenna is not self-resonant, usually a matching network is needed in order to achieve the maximum power transfer between antenna and load.
• the self-resonant antennas known so far have a size in the order of the wavelength, which for some applications is very large. If the size of the antenna is required to be reduced, the input impedance becomes reactive (inductive or capacitive, depending of the structure of the antenna).
• the Split Ring Resonator (SRR), introduced by Pendry (see “Magnetism from conductors and enhanced non linear phenomena" by J. B. Pendry et al., IEEE Transactions on Microwave Theory and Techniques, vol. 47, pp. 2075-2084, November 1988) is a great contribution to the field of metamaterials since it is the first particle able to achieve negative values of effective magnetic permeability.
• the structure of such resonator consists of two concentric metallic rings. Both rings have a certain thickness (c) and small gaps etched on opposite sides, as shown in Figure 1 A.
• the SRR has a mean radius (r 0 ) measured just in between the two concentric rings.
• the induction (L 5 ) can be approximated by the induction of a single ring with a radius equal to the mean radius (r 0 ) of the SRR and width (c) of each concentric ring.
• B 2 is the axial magnetic component of the electromagnetic field
• Qo is a geometric factor
• ⁇ 0 is the resonant frequency of the SRR.
• a periodic array of these resonators can be used as a filter for millimetre waves and microwaves.
• An example of this use is EP 1675212 A1 , wherein a planar transmission element, such as a microstrip line or a central metallic plane with dielectric substrate on both sides and a conducting strip formed on it, is mounted in magnetic coupling with an in-series insertion of several SRRs.
• EP 1675212 A1 provides an antenna or a battery of antennae which incorporates the described filter comprising said array of SRRs for emission and reception of electromagnetic waves, because the behaviour of the array of SRRs as an effective medium allows the propagation of fast waves for a given frequency, and then it behaves as a leaky wave antenna.
• NBSRR Non-bianisotropic SRR
• Double-Slit SRR (D-SRR) or Distorted/Dual Split Ring Resonator, shown in Figure 5: it also presents the aforementioned symmetry, thus avoiding cross polarization; however, the D-SSR equivalent circuit differs from that of the SRR, being the frequency of resonance twice than that of a SRR of identical size.
• SR Spiral resonator
• DSR Double spiral resonator
• Figure 7 The SR presents a structure composed by a spiral element with two radii.
• the DSR has two coupled spiral elements In both cases, the resonant frequency does not only depend on the overall size. As can be seen from their equivalent circuits, the SR as well as the DSR allow for a reduction of the resonant frequency with respect to the SRR.
• the SRR has a dual counterpart which is so-called Complementary SRR (CSRR).
• Metal parts of the SRR are changed by slots in a conducting plane in the CSRR.
• electric currents in the rings are changed by magnetic currents in the slots and electric and magnetic fields surrounding the SRR are swapped by each other in the CSRR.
• Magnetic currents in the slots do no physically exist; actually they are a mathematical model for modelling the electric currents on the conducting plane. The currents are not confined to the edges of the slot but rather spread out over the conducting plane. In the SRR, the currents are more confined, and a higher current density flows through the rings. Because of this, power loss in SRR due to metal losses can be higher (lower efficiency) than in CSRR.
• a solution to overcome the reduction of the radiation resistance due to the miniaturization of the antenna is using a folded structure, which allows a x4 increment of the radiation resistance.
• a folded structure which allows to increase the real component of the input impedance (radiation resistance and loss resistance) without varying the resonant frequency.
• a dipole antenna and a single folded- dipole antenna are shown in Figure 8A and 8B respectively.
• the present invention is intended to resolve the problem outlined above on miniaturized antenna design without needing to introduce a matching network in the antenna and satisfying both of two antenna design requirements: small size and matching to the source.
• one aspect of this invention deals with an antenna which comprises a self-resonant radiating structure that is perfectly adaptable to manufacture of micro- antennas for Radiofrequency Identification (RFID).
• RFID Radiofrequency Identification
• another aspect of the invention refers to an RFID tag which comprises an antenna configured with this self-resonant radiating structure as described as follows.
• the antenna proposed in this invention comprises at least a radiant element consisting of a resonant structure, built in a planar substrate and excited at a feed point, which produces an electric current through the feed point when said resonant structure is excited by a magnetic field (or an electric field in case the complementary resonant structure, applying the Babinet principle, is used) pointed in a direction transversal to the planar substrate.
• the resonant structure can be modelled by an equivalent electric circuit with inductance and capacitance that determine its resonant frequency.
• such self- resonant structure can be used as a near field UHF tag antenna, because it can be excited by the magnetic near (or evanescent) field from a reader antenna.
• an antenna comprising a radiant element that is a self- resonant structure as defined above is one based on the possible split ring resonator configurations (SRR, CSRR, DSRR, etc.).
• the split ring resonator structure can resonate at a frequency not only dependent on its overall size, this means that the size of the resonator can be reduced arbitrarily for a given frequency and so, when the structure is fed to produce electromagnetic radiation, the SRR is a self- resonating and radiating element which becomes an antenna as small as required.
• a small slit or gap can be done in the middle of the SRR, without modifying significantly the resonance frequency, because the equivalent circuit of this SRR behaves as a RLC series circuit at the resonant frequency and said resonant frequency is not affected by introducing a series resistor in the feeding point or feeding port etched in the external or internal ring.
• An advantage of the antenna based on the SRR configuration is that the resonant antenna overall size can be reduced as much as needed just by increasing the overall inductance and capacitance between rings of the SRR.
• a main difference of the present invention from the antennae described in EP 1675212 A1 lies in the electromagnetic radiation originated by the rings of the SRR.
• the radiation pattern of the SRR antenna described here is almost omni-directional with maximum gain in the plane containing the rings. If these rings have a radius much smaller than the wavelength, the SRR can be modelled, at the resonant frequency, as a loop antenna with an equivalent radius equal to the mean radius (r 0 ) of the SRR and an equivalent width equal to the width (c) of the rings.
• the radiation pattern of the SRR is similar to the one generated by a loop antenna.
• the SRR radiating structure is self-resonant, whilst the loop is purely inductive and requires a matching network to maximize transferring of power to the load of the antenna.
• the load must be the complex conjugated of the antenna impedance and the inductive component must be cancelled.
• the SRR does not need any matching network to the load and, at the same time, the resonant frequency can be kept independent from the SRR size, being an optimal configuration to be applied in miniature antennae.
• a magnetic field through the SRR-based antenna induces a current around the rings.
• another way to feed the antenna proposed here is exciting the SRR by means of a metallic loop which can be etched inside or outside the SRR. This feeding method is especially suitable when the inductance of the SRR is not high enough to cancel out the capacitive component of the antenna load, for example, the capacitance of an RFID chip.
• One way to increase the capacitance between rings is by decreasing the separation between them, and to do it so without increasing the resolution needed in the fabrication of the SRR structure, an option for implementation of the invention is to put each ring on a different layer of a common substrate as if it where a capacitor.
• the physical dimension of the SRR is reduced keeping the resonant frequency constant, i.e., not dependent on the ring size (because of the reduction in size is compensated by changing other parameters of the structure), one of the consequences is decreasing of the radiation resistance, and in turn, the reduction of the radiation efficiency of the antenna.
• a folded SRR antenna can be used to increase more than four times the radiation resistance for a given (constant) resonant frequency with respect to the SRR antenna for matching purposes.
• Another way to increase the radiation resistance is to shift the feed port along the ring. Because of the current density in each ring decreases as it gets close to the gap of the ring, the feed point displacement along the external or internal ring achieves higher radiation resistance without modifying the resonant frequency. Moving the feed port results in an unbalanced antenna, so it is not suitable for applications which require a balanced transmission line, but it is perfectly valid for RFID applications.
• SRR-based antenna can be placed inside a cap on a bottle so that the RFID reader can interrogate the SRR-based antenna when the reader antenna reaches the cap, being the optimum read direction the natural one defined by the major surface of the cap.
• D-SRR the spiral resonator
• DSR Double-spiral resonator
• the CSRR Complementary SRR
• the resonant (CSRR) structure is excited by an electric field perpendicular to the plane containing the CSRR structure, according to the Babinet principle. Because of the swapping between magnetic and electric fields in the complementary SRR, the antenna can be fed through capacitive coupling.
• the CSRR antenna can have an RFID chip just soldered across the gap (i.e., in the radial direction).
• an interdigitated split ring resonator increases the capacitance of the equivalent circuit due to a longer gap and wider surface of the interdigitated rings, resulting in a reduction of the resonant frequency.
• the thickness of the rings in the interdigitated split ring resonator leads to decreasing the inductance of the equivalent circuit.
• a meandered split ring resonator may be used.
• the capacitance of the meandered split ring resonator equivalent circuit is increased with respect to the SRR, though the equivalent capacitance of the meandered split ring resonator is lower than the equivalent capacitance of the interdigitated split ring resonator, but the resonant frequency is reduced with respect to both previous cases (SRR and interdigitated split ring resonator) because of the increase of the equivalent inductance.
• any of the different options presented in this invention has the slot counterpart, i.e. can be realized just swapping metal parts by slots on a metal plane.
• a main benefit of the present invention in any of the diverse implementation ways disclosed here, is that the antenna can be fabricated very easily using a planar, rigid or flexible, substrate. This means that the fabrication process involves lower costs and also the fabricated antenna can be easily integrated for numerous applications that demand strictly reduced dimensions.
• Figure 1. It shows the structure (A) with relevant dimensions and the equivalent circuit model (B) of a split ring resonator, according to prior art.
• Figure 2. It shows the uniform and high current density distribution in a SRR at the resonant frequency, according to the behaviour at magnetic polarization of the SRR studied in prior art.
• Figure 3. It shows an effective medium composed by a plurality of split ring resonators, according to the normal behaviour studied in prior art of the effective magnetic permeability with negative values near the resonant frequency.
• Figure 4. It shows the structure (A) and the equivalent circuit model (B) of a non-bianisotropic split ring resonator, according to prior art.
• Figure 6. It shows the structure (A) and the equivalent circuit model (B) of a spiral resonator, according to prior art.
• Figure 7. It shows the structure (A) and the equivalent circuit model (B) of a double spiral resonator, according to prior art.
• Figure 8. It shows a dipole (A) and a folded-dipole (B) antennae, according to prior art.
• Figure 9. It shows a structure (A) and an equivalent circuit model (B) of an antenna based on the normal SRR configuration with a feed point, according to a possible embodiment of this invention.
• Figure 10. It shows a three-dimensional representation of the radiation pattern from the antenna shown in Figure 9(A), being the SRR rings placed in the XY plane with their centre at the origin.
• Figure 11. It shows an antenna based on a non-bianisotropic SRR structure, according to another possible embodiment of this invention.
• Figure 12. It shows the optimum directions pointed by arrows for reading a conventional RFID tag, according to prior art, and a SRR-based RFID tag according to a possible embodiment of the invention.
• Figure 13 It shows the SRR-based RFID tag depicted in Figure 12 put inside a cap of a bottle to be read by an RFID reader antenna.
• Figure 14 It shows an antenna based on a loop feeding SRR structure, with a smaller metallic loop inside the SRR to feed the antenna, according to another alternate possible embodiment of this invention, and a detail (C) of an RFID chip connected to the antenna in the loop.
• Figure 15. It shows an antenna based on a loop feeding SRR structure built in slots, with a smaller loop etched inside the SRR to feed the antenna, according to yet another alternate possible embodiment of this invention, and a detail (C) of an RFID chip connected to the antenna in the loop.
• Figure 16. It shows an equivalent circuit model of the antenna based on the loop feeding SRR structure depicted in Figure 14.
• Figure 17. It shows an antenna based on a broad side coupled SRR structure with each one of the two rings is built in a different layer of the planar dielectric substrate, according to another alternate embodiment of this invention.
• Figure 18 It shows an antenna based on a folded SRR structure, configured with two arms, according to yet another alternate embodiment of this invention.
• Figure 19 It shows an antenna based on a feed port shifted SRR structure, according to another example for embodiment of this invention.
• Figure 20. - It shows an antenna based on an interdigitated SRR structure, according to another possible example for embodiment of this invention.
• Figure 21. - It shows an antenna based on a meandered SRR structure, according to yet another possible example for embodiment of this invention.
• Figure 22 It shows an antenna based on a complementary SRR structure fed by two electrodes creating an electric field, according to a last example for embodiment of this invention.
• Figure 23 It shows an antenna based on the complementary SRR structure with an RFID chip soldered directly across the gap of the slot-ring
• Figure 24 It shows an antenna based on a single loop resonant structure.
• Figure 25 It shows an antenna based on a resonant structure consisting of a single split ring.
• Figure 26 It shows an antenna based on a folded SRR structure configured with three arms, according to another option for embodiment of this invention.
• Figure 27 It shows an antenna based on a meandered SRR structure configured by defining spirals with inclined lines, according to yet another possible example for embodiment of this invention.
• one of the preferred embodiments of the invention as an antenna comprising a radiant element which is a split ring resonator (2) composed by an internal ring (R1 ) and an external ring (R2) with respective gaps etched in diametrical opposition.
• the two rings (R1 , R2) are concentric, and can be made of metal, built in a common planar substrate.
• the SRR antenna structure, drawn in Figure 9A can be modelled by an equivalent electric circuit, depicted in Figure 9B, including the feeding source for the antenna.
• the resonant frequency is achieved by properly selecting the dimensions of the SRR: width and separations between the rings (R1 , R2) and overall size.
• the antenna input impedance is generally composed by real and imaginary components. If the source impedance is not purely real, as usual in an RFID chip, the SRR antenna still resonates but at another frequency.
• the inductive and capacitive values of the equivalent circuit model determine the value of the resonant frequency.
• the radiation and loss resistances (R rad , Ri oss ) of the antenna are included in determining the real component of the antenna input impedance in the equivalent circuit, in which there is a resistance from the source (R SO ur ce )- The inductance (Lsrr) and capacitance
• the SRR structure can be applied in fabricating a slot antenna wherein the two concentric rings (R1 , R2) are configured as slots in the common planar substrate, and carry magnetic current instead of electric current.
• This configuration can be useful, for example, to build an RFID tag out of a metal sheet, such as those covering the pills in a blister packaged pharmaceutical product.
• the SRR antenna is excited at a feed point (1 ) producing an electric current through said feed point (1 ) when the resonant structure is excited by a magnetic field pointed in a direction transversal to the planar substrate containing the SRR structure
• feeding said antenna either just exciting one of the rings of the SRR structure at the feed point (1 ) so that the antenna is fed directly through said ring as a dipole, either through a loop built in said substrate that generates the magnetic field pointed in a direction transversal to the said common substrate.
• the radiation pattern of the SRR antenna is almost omnidirectional with maximum gain in the plane defined by the common substrate, as shown in Figure 10, where the SRR antenna is placed with the ring axis parallel to the Z direction.
• the polarization is almost linear with the electric field contained in the XY plane. However, there can be a small cross-polarization due to the bianisotropy of the SRR. Different modifications can be done in order to improve the axial ratio, as for example the utilization of a NB SRR or Non- bianisotropic SRR (3), whose structure is depicted in Figure 11 and showing the feed point (1 ) for the NB SRR antenna. This alternative is very suitable for building RFID tags because it allows an optimum reading direction just in the same plane of the label or the RFID tag.
• Figure 12 shows the reading direction of a conventional RFID tag (4), wherein the maximum direction of radiation is perpendicular to axis of the dipole-like antenna, i.e. there is a null radiation direction in the axis, versus the reading direction of a SRR-based RFID tag (5).
• the optimum reading direction is in the XZ plane, whilst the SRR-based RFID tag (5) has its optimum reading direction in the plane of the antenna itself, i.e. XY plane in Figure 12.
• This property derived by the SRR and that can be improved by an NB SRR allows to insert either the NB SRR-based RFID tag or SRR-based RFID tag (5) inside, for example, a cap of a bottle (7) so that a conventional RFID reader antenna (6) can read such label when going up to the neck of the bottle (7), as shown in Figure 13.
• the RFID antenna of this label is based on a loop feeding SRR structure, as the example given in Figure 14.
• the SRR is excited with a small loop (8) configured inside the internal ring (R1 ) of the SRR.
• Another alternative is to feed the SRR with a larger loop outside the external ring (R2) of the SRR.
• a complementary design can be realized just swapping metallic rings and feeding loop by slot rings and slot loop in a metal plane, and magnetic and electric fields by electric and magnetic fields, respectively.
• Figure 15 shows the loop feeding SRR in slot, complementary to that of Figure 14, wherein the RFID chip (9) can be soldered, within the internal loop, in the radial direction.
• the loop feeding SRR structure shown in Figure 14 can be modelled by the equivalent electric circuit drawn in Figure 16, wherein the antenna impedance (Za) at the SRR resonant frequency is given by:
• R rad + R loos being Z srr and Z
• OOP have corresponding SRR and loop inductances (L srr , L
• This equivalent circuit model helps to match the antenna input impedance to the input impedance of an RFID chip.
• RFID chips usually have capacitive input impedance that can be well matched by the loop feeding SRR structure.
• the loop feeding SRR structure can work also at frequencies below or above resonance in order to achieve a better matching. For example, for frequencies just below resonance, the inductive behaviour of the antenna input impedance is increased. This can be used to compensate the capacitive behaviour of an RFID chip with a smaller overall size.
• Another possible embodiment of the invention refers to print one of the rings of the SRR in a top layer of the substrate and the other ring is printed in the opposite layer of said substrate, as depicted in Figure 17, defining a broad side coupled SRR.
• the capacitance between the rings (R1 , R2) of the broad side coupled SRR can be controlled with the thickness of the substrate and the width of the metal strips.
• RFID label substrates usually have a thickness of about 40 microns, so the capacitance between the rings (R1 , R2) printed in different layers can be very high.
• Yet another possible embodiment of this invention deals with folding one or both rings of the SRR in order to achieve a larger radiation resistance.
• An example of a folded split ring resonator (11 ) structure is drawn in Figure 18.
• Table 1 shows the self-resonant frequency and real part of the input impedance of different resonant structures. All the structures have been simulated with a full wave MoM [Method of Moments] simulator assuming they are in free space, made of copper and have the same size with a maximum value about 20 millimetres.
• the single split ring resonates at 2.36 GHz. At this frequency the length of the ring is approximately half a wavelength. This means that it is equivalent to the resonant frequency of a dipole of the same length (however, a dipole would have a larger overall size because it is straight).
• the higher input resistance is for a single loop antenna; however the resonant frequency is 5.5 GHz.
• the input impedance of the single loop at 1.72 GHz is 15.2 + J979 ⁇ , which is highly inductive. If we cancel out the reactive part by means of a series capacitor, the input resistance would be 15.2 ⁇ , which is very close to the folded split ring resonator, but the folded SRR does not need any matching network or external lumped components.
• the folded structure used in the folded split ring resonator has two arms. In order to increase more the input resistance, more arms can be used. As instance, a three-arms folded SRR, drawn in Figure 26, with identical overall size, would have a resonant frequency of 1.72 GHz and input impedance of 27 ⁇ .. The input resistance is about N 2 times the input resistance of the non-folded structure, where N is the number of arms.
• Another option to increase the radiation resistance is to shift the feed point (1 ) along the ring, as shown in Figure 19, because the current in the ring decrease as you get closer to the gap.
• the antenna becomes unbalanced, but for RFID applications, since the small RFID chip (9) is placed directly on the antenna, this is no disadvantage. For instance, for a 20 mm diameter SRR, shifting the feed port 30 degrees results in an increase of the radiation resistance of about 17%.
• an antenna configured with an interdigitated split ring resonator (12) shown in Figure 20, or an antenna based on a meandered split ring resonator (13) that is shown in Figure 21 and additionally allows to increase the inductance of the normal SRR structure.
• interdigitated split ring resonator (12) and meandered split ring resonator (13) either by configuring the interdigitated/meandered rings by straight lines, as drawn in Figures 20/21 , or defining spirals with inclined lines, as depicted in Figure 27, etc.
• the equivalent capacitance is higher than the capacitance of the simple SRR structure since a wider surface and longer gap is defined between the interdigitated rings.
• the equivalent inductance of the rings can be increased by either increasing the ring lenth or reducing the ring thickness, in meandered split ring resonator (13) the equivalent inductance is also higher than in the simple SRR structure.
• two metallic layers (15a, 15b) of a substrate (15) are shown, depicted in black and grey respectively, in which the antenna based on the CSRR or complementary split ring resonator (14) is built, being fed through capacitive coupling.
• the antenna based on the CSRR or complementary split ring resonator (14) is built, being fed through capacitive coupling.
• the two concentric rings defined as slots conforming the CSRR that is, the part without metal in the layer (15a) is the CSRR.
• the other layer (15b) of the metal substrate (15) there are two electrodes connected to the feed point (1 ), so that an electric field is generated between the two electrodes and crosses through the slots/rings in the former layer (15a).
• the antenna While in a conventional SRR, the antenna is excited by a magnetic field crossing the rings, in the complementary SSR, i.e., the CSRR, since the magnetic and electric fields are swapped, the excitation of the antenna is given by the electric field produced between the electrodes.
• the RFID chip (10) can be soldered at the same metal layer (15b) with the electrodes of the metal substrate (15).
• Figure 23 shows another way to feed the CSRR antenna by soldering the RFID chip
• any of the different options of implementation presented for this invention included double-slit SRR, spiral resonator and double spiral resonator structures, has the slot counterpart, i.e. can be realized just swapping metal rings by slots.

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