WO2005048476A1

WO2005048476A1 - Radio frequency transponder with electrically short uhf antenna

Info

Publication number: WO2005048476A1
Application number: PCT/IB2004/003686
Authority: WO
Inventors: Michael John Camille Marsh
Original assignee: Trolley Scan (Proprietary) Limited
Priority date: 2003-11-14
Filing date: 2004-11-11
Publication date: 2005-05-26
Also published as: ZA200603210B

Abstract

A radio frequency transponder assembly comprises a shortened dipole antenna (34,36) and a transponder circuit (18). At least two impedance matching elements comprising a capacitor (38) and an inductor (42) and together effectively defining an impedance matching circuit between the antenna and the transponder circuit are formed integrally with the antenna. Both the antenna (34,36) and the impedance matching elements (38,42) are substantially planar and are formed integrally from a layer of conductive material on a common substrate. For example, the antenna (34,36) and matching elements (38,42) can be formed in a layer of conductive circuit board material by conventional etching processes, or printed on a substrate using conductive ink.

Description

Radio Frequency Transponder with Electrically Short UHF Antenna

BACKGROUND OF THE INVENTION

THIS invention relates to a radio frequency transponder and to an assembly incorporating the transponder.

In particular, the invention relates to a transponder assembly comprising a transponder and an associated antenna produced on a suitable substrate, and to the cost effective manufacture of an antenna system combined with a matching network to be used for low cost Radio Frequency Identification (RFID) transponders, where the antenna system has to be small compared to the ideal size to provide efficient operation.

As RFID transponders are typically used to label goods that are being identified, the physical size of the transponder needs to be small in those situations where the item being labeled is small. However to operate efficiently and collect as much energy as is practical, the transponder antenna's size is determined by the operating frequency of the transponder. For example, a half wave dipole antenna operating in the UHF (860- 930MHz) RFID band would typically be 16cm in length for efficient antenna operation.

To produce low cost transponder assemblies, designers need a simple antenna system to which is connected an integrated circuit containing all the transponder electronic components. Such components would typically comprise the rectifying diodes, storage capacitors, modulator, oscillator, memory storage and logic circuitry of the transponder.

In order to maximise the energy transfer from the energising field via the antenna to the rectifying circuitry, a matching network is needed between the RF electronic components (rectifiers, modulators, and energy storage), and the antenna. The matching network must convert the actual complex input impedance of the electronic components to the conjugate impedance of the antenna. In conventional designs this might be achieved using transmission lines, or combinations of inductors and capacitors. Due to the large values needed for the inductors and the capacitors and the poor manufacturing tolerances achieved with integrated circuit manufacture, as well as the effects of temperature on the nominal values of such components, it is not commercially viable to include these components inside the electronic integrated circuit. South African patent no. 2001/9659 entitled "Energy transfer in an Electronic Identification System" describes a transponder antenna incorporating such matching elements in its antenna structure to efficiently transfer the energy from the transponder antenna to an integrated circuit.

In order to produce a low cost transponder for passive RFID applications, it is important that the antenna system is simple, electrically efficient and yet easy to manufacture. SUMMARY OF THE INVENTION

According to the invention there is provided a radio frequency transponder assembly comprising: an antenna; a transponder circuit including a power supply circuit arranged to be fed with energy received by the antenna, a logic circuit, and a modulator circuit for generating data signals for transmission by the antenna; and at least two impedance matching elements comprising a capacitor and an inductor and together effectively defining an impedance matching circuit between the antenna and the transponder circuit, wherein the antenna and the impedance matching elements are substantially planar and are formed integrally from a layer of conductive material on a common substrate.

The transponder circuit may have a complex input impedance, and the antenna may be a. shortened dipole with first and second opposed limbs.

The matching circuit may comprise a capacitor connected in parallel with the antenna and an inductor connected in series between the antenna and the transponder circuit.

The capacitor may be connected between first and second terminals of the antenna in parallel with input terminals of the transponder circuit.

The capacitor may be defined between the first and second limbs of the antenna in a central region of the assembly. In a preferred embodiment, the capacitor comprises a plurality of interdigitated projections extending from respective inner ends of the first and second limbs.

The inductor may be connected in series with a first terminal of the antenna and an input terminal of the transponder circuit^'

In a preferred embodiment, the inductor comprises a loop of conductive material extending between the inner end of the first antenna limb, and a position adjacent the inner end of the second antenna limb.

The transponder circuit may have first and second terminals, one of which is connected to the inductor and the other of which is connected to the inner end of the second antenna limb.

The antenna and the impedance matching elements may be formed in a planar metal layer on a dielectric substrate.

The metal layer may comprise copper.

Preferably, the antenna and the impedance matching elements are formed by a photo-lithographic etching process.

Alternatively, the antenna and the impedance matching elements may be defined by a generally planar layer of conductive ink deposited on a dielectric substrate.

The substrate may comprise paper or board, or plastics.

The dimensions of at least one of the antenna and the matching elements may be varied according to the dielectric characteristics of the substrate and/or according to the dielectric properties of a label applied to the transponder assembly. The antenna may have an overall length substantially less than the theoretical optimum length thereof.

For example, the antenna may have an overall length of about half of its theoretical optimum length.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified schematic diagram showing the main components of a conventional reader and a passive RFID transponder;

Figure 2 is a simplified schematic diagram of an RFID transponder assembly including an antenna and a matching network with a rectifying circuit of the transponder;

Figure 3 shows the effective implementation of the matching network of Figure 2 using a lumped inductor and capacitor;

Figure 4 shows the physical configuration of an electrically short dipole antenna of the general kind suitable for use with a transponder of the invention;

Figure 5 shows a Smith Chart illustrating the transformation of the complex impedance of the rectifying circuit by the matching network components to the impedance of the antenna, and the locus of the conjugate complex impedance of a shortened dipole antenna with different stub widths;

Figure 6 shows a transponder assembly including a shortened dipole antenna and matching elements implemented in a single plane structure; Figure 7 shows the artwork used to produce the single plane antenna and matching elements of Figure 6; and

Figure 8 shows the artwork used to produce an alternative single plane antenna of irregular shape.

DESCRIPTION OF EMBODIMENTS

Figure 1 shows the typical arrangement of a transponder and reader system in which the transponder receives power from the energising field of the reader to power the transponder. This type of transponder is called "passive" as it contains no energy source and extracts power from the energising field to operate.

A radio frequency signal from an oscillator 10 operating at the designated transponder operating frequency or carrier frequency is boosted in a power amplifier 12 and radiated via a transmitter antenna 14. This creates an energising field 8. The transponder consists of an antenna 16 and an electronic circuit 18 attached to the antenna. The electronic circuit includes all the functional components of the transponder, such as a power extraction circuit or power rectifier 20 for deriving power from the energising field, a modulator 22 for sending data from the transponder back to the reader, and a logic circuit 24 including a low frequency internal oscillator, typically operating at 10kHz, memory, and a control logic circuit.

The transponder transmits its identification code or other data to the receiver 26 of the reader, which has an antenna 28, by modulating the impedance of its own antenna in a generally conventional backscatter modulation process.

Figure 2 shows a typical power rectification circuit used in a transponder to supply sufficient voltage and current to the equivalent load of the transponder. A matching network or circuit 30 converts the antenna impedance to the conjugate impedance of the equivalent load 32 as transformed by the impedances of the rectifying diodes D1 and D2, and an energy storage capacitor C.

The dimensions of an antenna are important for efficient operation and are determined primarily by the operating frequency of the system. The following table shows how the efficiency of energy collection drops off as a halfwave dipole antenna that is made to operate at a given frequency is reduced in length by shortening the physical length of the antenna.

Table 1

As can be seen, an antenna that is only 50% of the ideal length will only collect some 2.9% of the energy of a properly designed ideal length antenna. Much of this loss can be recovered by adding the correct matching network between the antenna and the electronic circuit of the transponder.

Figure 3 shows the effective lumped elements of the matching network and the antenna that need to be implemented in the antenna design to provide efficient energy transfer. For UHF transponders that operate in the 860Mhz to 930MHz bands, the length of a halfwave dipole implementation of an antenna, which is an efficient energy converter, would be 160 millimeters. Many users of RFID transponders find this length cumbersome and would prefer to have shorter antennas, which are closer in size to that of the typical object being labelled.

Whereas a properly designed halfwave dipole antenna would have an impedance of about 72 + jO ohms at resonance, shortening the antenna by 50% would make the antenna impedance change to 10 - j331 ohms, a reactive antenna that would be difficult to match using an inductor/capacitor combination for an integrated circuit having a typical input impedance of 14 - j 43 ohms.

Figure 4 shows the relative dimensions of the various sections of the shortened dipole antenna which are varied to determine their impact on the antenna impedance, as shown in the table below.

The following table shows the impact of increasing the width of the antenna while keeping the operating length to 50% of the ideal length of a halfwave dipole. Results are calculated using an operating frequency of 885MHz. Dimensions are in millimeters, impedance in ohms.

Table 2

As the width of the antenna is increased, the antenna impedance becomes more real and less reactive until a point is reached where a simple matching inductor/capacitor can be used to couple the energy from the shortened antenna to the integrated circuit.

Radio Frequency Engineers have been using for many years the graphical computation charts called "Smith (R) Charts" invented by Philip Smith prior to 1932 and sold via Analog Instruments of New Jersey USA. The use of these charts will be known to radio engineers who are taught its operation as part of their training.

The Smith chart graphically shows how impedances are converted from some complex source impedance to a complex load impedance. The chart works for all impedances. Generally a reference impedance is chosen and all values plotted on the chart are normalised to this impedance with all points being plotted as ratios.

Due to the parameters of the rectifying circuit which are dominant compared to the equivalent load, the impedance of the rectifying circuit as seen from the matching network will lie in the lower half of the chart in the "Negative Reactance sector" (see Figure 5). Matching occurs by moving this impedance to the "positive reactance component" sector by means of an inductor, and then translating it to the source impedance by means of a capacitor. This process requires two precise elements, the one to move it to the correct part of the sector, and the second to move it from the transition point to the load.

Also plotted on the Smith Chart are the conjugate antenna impedances for shortened dipoles with varying track dimensions. W1 refers to a shortened dipole of 1 millimeter stub width (C); and W10, W15, W20, W25 and W30 referring to impedances for stub widths of 10mm, 15mm, 20mm, 25mm and 30mm respectively. From the Smith Chart, it can be determined that with variations in the inductive and capacitive components, shortened dipole antennas with varying stub widths from 15mm to 30mm could be matched in a transponder assembly of the invention.

By shortening the antenna its impedance moves away from the nominal value for a half-wave dipole of 72 + jO, with a larger and larger imaginary component as it moves towards the edge of the chart. Widening the antenna brings the impedance closer to the centre again. Hence if one shortens the dipole one has to widen it to prevent it having a large imaginary impedance component.

The matching of a series inductor and a parallel capacitor across the antenna can transform the diode impedance inwards in the Smith chart. The inductor rotates the diode impedance from the bottom to the top, and fine adjustments to compensate for different materials and frequencies can be made be varying the length of the inductor primarily. This means that the starting point is the impedance of the diodes and chip, at a point a certain radius from the centre of the chart. This radius is effectively rotated by the inductor, and then it is necessary to move inside this rotation by widening the width of the dipole and fine tuning with a small capacitor.

A capacitor produced in a single plane is limited in value to a range of about 0.1 pF to 4pF, based on normal etching and printing methods. This limits how far in from the diode/chip impedance as translated by the inductor, one can move. Hence most of the adjustment has to happen via changing the dipole width.

Analysing the currents flowing in the dipole, one can cut away the material near the centre of the dipole without making a major negative impact on its performance as the current flow near the edge in the central portion is small. This creates space for inductive and capacitive matching elements in a single plane rectangular design. It is the purpose of this invention to implement the antenna and associated matching network of the transponder assembly as an integral structure defined in a single plane, such as in the copper cladding of a single sided printed circuit board, or by means of a layer of conductive ink applied to one side of a paper, board or plastics substrate, for example. This requirement limits the possible electrical size of the matching network capacitor and therefore the width of the stubs is chosen such that the conjugate impedance of the antenna is close to the impedance of the rectifying circuits as transformed by the inductor.

Figure 6 shows the design of the antenna and associated matching elements as implemented in a single plane from conducting material. The antenna comprises first and second limbs 34 and 36 designed as described above. A capacitor 38 is defined between the limbs 34 and 36 by interdigitated projections or fingers 40 of the respective limbs in a central region of the assembly. An inductor 42 comprising a loop of conductive material extends from the inner end of the antenna limb 36 to a position adjacent to the inner end of the limb 34, from which it is spaced by a small distance corresponding to the contact spacing of the transponder integrated circuit 18, which is soldered in position between the inductor 42 and the limb 34 of the antenna.

The value of the capacitor 38 can be varied by adjusting the length of and spacing between the interdigitated fingers 40, as well as the number of fingers. The value of the inductor 42 depends primarily on the length and width of the conductive loop defined by it, as well as the thickness of the conductive material.

Figure 7 shows an image from the photomask used to produce a printed circuit antenna/matching network according to the invention. Conventional photo-lithographic masking and etching techniques, as used in printed circuit manufacture, can be used. The version built using this mask needed just 200 microwatts of RF energy in its 134 cm² aperture to operate the transponder. This compares to the 55 milliwatts that is needed for a 5 volt benchmark transponder attached to a dipole without a matching network.

The methods of the invention can also be used for antennas with non- conventional shapes that have the required physical and electrical characteristics. Figure 8 shows an image from a photomask used to produce a printed circuit antenna/matching network having a required shape.

By shortening the antenna to just 80 millimeters overall length, that is, about half of its theoretical optimum length, the antenna is much more practical for labeling of goods to be identified. By widening the antenna and defining the matching network of an inductor and capacitor in the same plane as the antenna, in the same material and in the same manufacturing process, an efficient antenna can be produced that is simple to produce in high volume. Such antennas and matching networks can be produced using conventional etching processes, or can be manufactured by a printing machine using conductive inks on a variety of substrates. Examples of suitable conductive inks are Type XZ250 Touchkey high conductivity silver ink, Type XZ351 conductive carbon ink, and Type 26-8203 Touchkey conductive carbon ink, manufactured by Coates Circuit Products of Norton Hill, Bath, United Kingdom. In either case, the assembly operates efficiently allowing such transponder assemblies to have excellent operating range despite their smaller physical size.

The actual impedance of the antenna is affected by the materials immediately around the antenna limbs, such as the carrier/substrate or material upon which the antenna is made. This might be glass fibre material such as used in a printed circuit board, or paper, board and plastics, for example, when the transponder assemblies are manufactured using a printing machine. The outer labelling of the transponder, which is also in contact with the antenna elements, will also change the characteristics of the antenna limbs and the inductor and capacitor parts. These effects may require a variation of a few percent in the values of the capacitor and inductor to compensate.

Below is a table showing variation of the inductance for several different materials to maintain optimal matching.

Table 3

As the inductor impedance is frequency related, the adjustment needed can be measured by measuring the new resonant match frequency and adjusting the inductor value as per the table above.

The invention provides a transponder assembly with a combined antenna and matching network that can be printed in a single plane on paper/plastics packaging using conductive inks, or etched from a single copper layer of a printed circuit board.

This invention has particular applicability to the requirement for very low cost transponder assemblies, comprising a simple integrated circuit and an easily manufactured antenna and matching system made from low cost materials.

The present invention allows antennas together with their matching elements to be produced in a single physical plane with smaller dimensions than the ideal and yet provide good operating efficiency.

Claims

1. A radio frequency transponder assembly comprising: an antenna; a transponder circuit including a power supply circuit arranged to be fed with energy received by the antenna, a logic circuit, and a modulator circuit for generating data signals for transmission by the antenna; and at least two impedance matching elements comprising a capacitor and an inductor and together effectively defining an impedance matching circuit between the antenna and the transponder circuit, wherein the antenna and the impedance matching elements are substantially planar and are formed integrally from a layer of conductive material on a common substrate.

2. A transponder assembly according to claim 1 , wherein the transponder circuit has a complex input impedance and the antenna is a shortened dipole with first and second opposed limbs.

3. A transponder assembly according to claim 2, wherein the matching circuit comprises a capacitor connected in parallel with the antenna and an inductor connected in series between the antenna and the transponder circuit.

4. A transponder assembly according to claim 3, wherein the capacitor is connected between first and second terminals of the antenna in parallel with input terminals of the transponder circuit.

5. A transponder assembly according to claim 4, wherein the capacitor is defined between the first and second limbs of the antenna in a central region of the assembly.

6. A transponder assembly according to claim 5, wherein the capacitor comprises a plurality of interdigitated projections extending from respective inner ends of the first and second limbs.

7. A transponder assembly according to any one of claims 3 to 6, wherein the inductor is connected in series with a first terminal of the antenna and an input terminal of the transponder circuit.

8. A transponder assembly according to claim 7, wherein the inductor comprises a loop of conductive material extending between the inner end of the first antenna limb, and a position adjacent the inner end of the second antenna limb.

9. A transponder assembly according to claim 8, wherein the transponder circuit has first and second terminals, one of which is connected to the inductor and the other of which is connected to the inner end of the second antenna limb.

10. A transponder assembly according to any one of claims 1 to 9, wherein the antenna and the impedance matching elements are formed in a planar metal layer on a dielectric substrate.

11. A transponder assembly according to claim 10, wherein the metal layer comprises copper.

12. A transponder assembly according to claim 10 or claim 11 , wherein the antenna and the impedance matching elements are formed by a photo-lithographic etching process.

13. A transponder assembly according to any one of claims 1 to 9, wherein the antenna and the impedance matching elements are defined by a generally planar layer of conductive ink deposited on a dielectric substrate.

14. A transponder assembly according to claim 13, wherein the substrate comprises paper or board.

15. A transponder assembly according to claim 13, wherein the substrate comprises plastics.

16. A transponder assembly according to any one of claims 10 to 15, wherein the dimensions of at least one of the antenna and the matching elements are varied according to the dielectric characteristics of the substrate.

17. A transponder assembly according to claim 16, wherein the dimensions of at least one of the antenna and the matching elements are varied according to the dielectric properties of a label applied to the transponder assembly.

18. A transponder assembly according to any one of claims 1 to 17, wherein the antenna has an overall length substantially less than the theoretical optimum length thereof.

19. A transponder assembly according to claim 18, wherein the antenna has an overall length of about half of its theoretical optimum length.

20. A transponder assembly substantially as herein described with reference to the accompanying drawings.