WO2016090638A1

WO2016090638A1 - Dipole antenna for radio frequency identification (rfid) tag

Info

Publication number: WO2016090638A1
Application number: PCT/CN2014/093721
Authority: WO
Inventors: Jingtian XI; Tao Terry Ye
Original assignee: Hong Kong R&D Centre for Logistics and Supply Chain Management Enabling Technologies Limited
Priority date: 2014-12-12
Filing date: 2014-12-12
Publication date: 2016-06-16

Abstract

A dipole antenna arranged to couple with an integrated circuit chip of an RFID tag, wherein the dipole antenna has a dipole length of at least one wavelength λ so as to maximize directivity of the RFID tag.

Description

DIPOLE ANTENNA FOR RADIO FREQUENCY IDENTIFICATION (RFID) TAG

TECHNICAL FIELD

The present invention relates to a dipole antenna for a radio frequency identification (RFID) tag， and particularly， although not exclusively， to an RFID tag antenna utilizing dipole beyond one wavelength.

BACKGROUND

RFID tags have been widely utilized in applications such as commerce， transportation， logistics， human and animal identification， public transport， for identifying or tracking objects， or for storing data and information. One way of classifying RFID tags is to classify them based on the bands of fiequencies of electromagnetic waves with which they operate. Naturally， RFID tags with different operating frequencies would result in different operational and performance parameters， thus having significant effect on the range， costs， and deployment characteristics of the tags.

RFID tags that operate in the fiequency regime of 433， 840-960 MHz and tie 2.4 GHz range are called ultrahigh fiequency (UHF) RFID tags. Compared with RFID tags of other operating frequencies， UHF RFID tags have a relatively longer read range and a lower tag cost. In addition， UHF RFID tags can be updatcd with new data， and this enables them to be re-used. These advantageous characteristics make UHF RFID tag an ideal candidate for supply chain， inventory and asset management visibility applications.

The fundamental structure of an UHF RFID tag is an RFID inlay， which includes an IC chip coupled with an RFID antenna. In the simplest sense the IC chip is the component in which information to be retrieved is stored whilst the antenna is the component for enabling communication of the tag with an external RFID reader which reads or writes data on the IC chip.

Half-wave (half-wavelength) dipole is the most commonly used antenna type for UHF RFID tags. The utifization of half-wavelength is due to a traditional practical consideration that the RFID antenna form-factor should fit well within a normal label size (the label being separated from the object/product to be tracked) . As a result， dipole of other wavelengths/form-factors has been ignored.

Nonetheless as UHF RFID tags becomes more and more popular to be used in tracking and identification applications in the retail industry， it becomes increasing favourable to embed UHF RFID tags directly into the package during the package lamination process. By doing so， the form-factor of the RFID tag antenna is no longer limited by the label size， and the RFID tag antenna can utilize the whole dimension of the package. That said， it is not without technical challenges to adapt the tags to better utilize package dimensions.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention， there is provided a dipole antenna arranged to couple with an integrated circuit chip of an RFID tag， wherein the dipole antenna has a dipole length of at least one wavelength λ so as to maximize directivity of the RFID tag.

Preferably， the dipole antenna comprises an antenna portion arranged to receive and/or radiate RF signals； and an impedance matching portion arranged to match an impedance between the antenna portion and the integrated circuit chip.

Preferably， the impedance matching portion is arranged to match a resonance frequency of the RFID tag to a frequency at which the directivity substantially maximizes.

Preferably， both the elongated dipole segment and the loop segment are arranged on a substrate.

Preferably， the antenna portion includes an elongated dipole segment； and the impedance matching portion includes a loop segment coupled with the elongated dipole segment.

Preferably， the loop segment is connected with the elongated dipole segment at a first node and at a second node spaced apart from the first node； and a gap is arranged on the elongated dipole segment between the first node and the second node for receiving the integrated circuit chip.

Preferably， the elongated dipole segment is substantially straight.

Preferably， the substrate has a thickness of less than 10mm and a substrate permittivity of less than 4.

Preferably， the substrate comprises paper， plastic， and/or wood materials.

Preferably， the dipole antenna is substantially symmetric about its short axis.

Preferably， the radio frequency identification (RFID) tag is an ultra-high frequency (UHF) RFID tag.

Preferably， the radio frequency identification (RFID) tag is adapted to operate at around 800-1000MHz.

Preferably， the dipole length is at or larger than 1.269λ. The dipole length may also be around 1.269λ.

In accordance with a second aspect of the present invention， there is provided a radio frequency identification (RFID) tag comprising： an integrated circuit chip； and a dipole antenna in accordance with the first aspect of the present invention being coupled with the integrated circuit chip.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described， by way of example， with reference to the accompanying drawings in which：

Figure 1A is a graph showing the power pattern factor (F_p) as a function of dipole length for directivity at 90° (D_90deg) and the maximum directivity (D_max) based on theoretical analysis；

Figure 1B is a graph showing the radiation resistance (R_r) as a function of dipole length based on theoretical analysis；

Figure 1C is a graph showing the directivity (D) as a function of dipole length for directivity at 90° (D_90deg) and the maximum directivity (D_max) based on theoretical analysis；

Figure 2A is a graph showing the power pattern factor (F_p) as a function of dipole length for different substrates based on simulation；

Figure 2B a graph showing the radiation resistance (R_r) as a function of dipole length for different substrates based on simulation；

Figure 2C a graph showing the directivity (D) for directivity at 90° (D_90deg) as a function of dipole length for different substrates based on simulation；

Figure 3A is a graph showing the calculated and simulated resistances (R_in) as a function of frequencies for a long dipole antenna in accordance with one embodiment of the present invention；

Figure 3B is a graph showing the calculated and simulated impedances (X_in) as a function of frequencies for a long dipole antenna in accordance with one embodiment of the present invention；

Figure 4A is a graph showing the variation of the second series resonance fiequency (f_s2)， the first parallel resonance fiequency (f_pl) and the fiequency at which the directivity D maximizes (f_opt) as a tunction of the substrate permittivity (ε_r) based on simulation (with substrate thickness being kept at 3mm) .

Figure 4B is a graph showing the variation of the second series resonance fiequency (f_s2)， the first parallel resonance fiequency (f_pl) and the frequency at which the directivity D maximizes (f_opt) as a function of the substrate thickness based on simulation (with substrate permittivity being kept at 3) .

Figure 5A is a graph showing the variation of the difference between the second series resonance frequency and tie frequency at which the directivity D maximizes (f_s2-f_opt) as a function of the substratc pcrmittivity (ε_r) based on simulation.

Figure 5B is a graph showing the variation of the difference between the second series resonance frequency and the frequency at which the directivity D maximizes (f_s2-f_opt) as a function of the substrate thickness based on simulation.

Figure 6A shows a T-match network used for impedance matching in accordance with one embodiment of the present invention；

Figure 6B is a modified T-match network used for impedance matching in accordance with an alternative more preferred embodiment of the present invention；

Figure 7 is a photo showing a prototype of a long dipole RFID tag in accordance with one embodiment of the present invention；

Figure 8A shows a long dipole RFID tag design in accordance with one embodiment of the present invention；

Figure 8B shows a long dipole RFID tag design in accordance with another embodiment of the present invention；

Figure 9 is a graph showing the measured and simulated return losses of the tag prototype of Figure 7 as a function of operation fiequencies.

Figure 10A is a graph showing the measured and simulated E-plane radiation patterns (normalized) of the tag prototype of Figure 7；

Figure 10B is a graph showing the measured and simulated H-plane radiation patterns (normalized) of the tag prototype of Figure 7； and

Figure 11 is a graph showing the measured read range of the tag prototype of Figure 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a dipole antenna arranged to couple with an integrated circuit chip of an RFID tag， wherein the dipole antenna has a dipole length of at least one wavelength λ so as to maximize directivity of the RFID tag.

In this following description and the present invention in general， the term “long dipole” (or its variation) refers to dipole with a total length of more than 1λ， and preferably around 1.269λ.

DIPOLE DIRECTIVITY ANALYSIS

A.Theory of Dipole Directivity

In the theory of dipole directivity， the directivity D of an antenna can be calculated based on its radiation power density P：

where

and Z₀ is the intrinsic impedance of the propagation medium.

In the above Equations (1) - (3) ， P (θ， φ) is known as the power pattern and it can be expressed as

where B₀ is a constant， F_p is the power pattern tactor.

According to Equation (4) ， directivity D can be expressed as a function of F_p：

The electric field of dipoles has no E_φ component and is independent of φ. Thus for dipoles， Equation (5) can be simplified as：

Accordingly， the field pattern factor of dipoles can be expressed as

where L is the total length and k is the wave number. As indicated by Equation (2) ， the power pattern factor F_p of dipoles is

The integral in the denominator of Equation (6) ， which involves F_p as shown in Equation (8) ， requires extensive mathematical manipulations.

However， it owns a closed-form expression as follows.

where C is the Euler’s constant， C_i (x) and S_i (x) are the cosine and sine integrals. After comparing Equation (9) with the radiation resistance (R_r) derived according to the induced EMF method， it is found that

Substituting Equation (10) into Equation (6) ， the directivity D of dipoles becomes

This expression in Equation (11) explicitly shows the relationship between D and R_r.

Figures 1A-1C show respectively the variation of power pattern factor (F_p) ， radiation resistance (R_r) ， and directivity (D) with the dipole length for directivity at 90° (D_90deg) and the maximum directivity (D_max) ， calculated based on the above equations. It should be noted that in the above， the power pattern factor (F_p) is derived based on Equation (8) ， the radiation resistance (R_r) is determined by the induced EMF method， whereas directivity (D) is calculated based on Equation (11) using the values obtained for the power pattern factor (F_p) and radiation resistance (R_r) . Note that power pattern factor F_p and directivity D are functions of θ. Figures 1A-1C only show the directivity at θ＝90 deg (D_90deg， represented by the legend “90 deg” in the graphs) and the maximum directivity (D_max， represented by tie legend “max” in the graphs) as they are of particular interest.

According to Figures 1A-1C， it can be seen that F_pmax and R_r both increase with L in an oscillating way but their peaks do not happen at the same L， i.e. the peaks are slightly offset. As a result， D_max shows many ripples in an ascending trend. Figures 1A and 1C show that F_p990deg and D_90deg behave quite differently from their counterparts F_pmax and D_maz. As shown in Figure 1A， F_p90deg presents a constant peak value of 4 with a period of 2λ. As a result， D_90deg maximizes at its first peak where the decreasing rate of F_p90deg exceeds that of R_r. The data (not shown) of Figure 1C reveals that the maximum of D_90deg occurs at L＝1.269λ. It can be seen that the maximal D_90deg coincides with D_max at L＝1.269λ and this shows that the main beam remains at the plane of θ＝90 deg. From L＝1.44λ onwards， D_90deg deviates substantially from D_max.In Figure 1C， the maximum directivity of the 1.269λ dipole is 3.296 (i.e. 5.18 dBi) ， which is 3 dB larger than that of a half-wave dipole.

B.Influence of Thin Low-Permittivity Substrate on Dipole Directivity

For the ease of illustration， the dipole directivity theory presented above assumes that the dipoles are placed in air. In practice， RFID tag antennas， in particular UHD RFID tag antennas， are implemented in the form of printed dipoles which are planar dipoles fabricated on substrates. In other words， the substrate properties (e.g. permittivity， dimensions， etc. ) would have a key effect on the directivity performance of the long dipole.

Tie following discussion focuses on thin low-permittivity substrates. In particular， the thickness of the substrate is preferably within 10 mm so that the radiation efficiency of printed dipoles will not be significantly compromised due to high-order surface waves； and the permittivity of the substrate is no larger than 4. It should be noted that common packaging materials like paper， plastic and wood all falls within this range of permittivity.

In particular， two exemplary substrates can be used it the present invention：

Sub 1： ε_r＝2.5， thickness＝0.5 mm (slight dielectric-loading)

Sub 2： ε_r＝3， thickness＝4 mm (moderate dielectric-loading)

For the sake of simplicity in illustration， dielectric loss is ignored in this section.

In one example of the present invention， a Method-of-Moments simulator， IE3D， is used to study the influence of the properties of the substrate. In the exemplary simulation the dipole width is selected and fixed to be 1 mm. Also， in this simulation， Dis directly obtainedfrom the IE3D simulator； and R_r is indirectly derived based on the simulation results of thmaximum current and the radiated power. In this simulation， however， F_p cannot be determined based on Equation (11) using the values of D and R_r obtained since this method is only applicable to dipoles in air. For dipoles on substrate， Equations (10) and (11) are no longer applicable. Thus， in the present simulation， the inventor (s) of the present invention havedevised a new F_p estimation method to be used for printed dipoles. As F_p only depends on the current distribution and current distributions at corresponding resonance frequencies should be the same regardless of the substrate， F_p of dipoles on substrate at one frequency point should equal that of dipoles in air at another frequency point， provided that a mapping between these two frequency points is available. The inventor(s) of the present invention has devised that this kind of mapping can be approximated by a downward shift corresponding to the variation of a certain resonance frequency. In particular， the variation of the second series resonance frequency should be used since the frequency where D maximizes is in its vicinity. Therefore， in the present exemplary simulation， F_p of dipoles on substrate can be estimated by downward shifting that of dipoles in air in accordance with the offset of their second series resonance frequencies.

Simulation and calculation results of F_p90deg， R_r and D_90deg in air (“Air (Calc) ”， “Air” ) and on the two substrates (“Sub 1”， “Sub 2”) ， obtained using the method presented above， are shown in Figures 2A-2C. As shown in Figure 2A-2C， the small discrepancies between the simulation and the calculation in the “air” case is due to the fact that the dipole width adopted during the simulation is not infinitesimal.

Table. 1 Dipole length required for the maximized directivity

Air (calc)	Air	Sub	1	Sub 2
Air (calc)	Air	Sub	1	Sub 2	1.269λ	1.234λ	1.188λ	1.061λ

As shown in Figure 2C， the dipole length required for the maximum directivity D decreases with the dielectric loading. Table 1 above lists the optimum dipole lengths for all the cases shown in Figures 2A-2C. It can be observed that as the dielectric loading becomes more significant， the dipole length required for the maximized directivity approaches λ.

In one embodiment of the invention， for substrates whose permittivity or thickness is larger than that of Sub 2， the long dipole may require a length shorter than λ， but it sill deviates from a full-wave dipole in the sense of the guided wavelength. In other words， the long dipole of the present invention preferably is at least 1λ . In one preferred embodiment， the dipole has at least 1.269λ dipole. The significance of the long dipole of the present invention lies at its high-but still omni-directional radiation pattern.

IMPEDANCE MATCHING OF LONG-DIPOLE TAG

Based on the above analysis， it can be seen that a full-wave dipole is operable to offer a higher directivity than its half-wave counterpart. However， the high input impedance of the full-wave dipole greatly complicates the impedance matching.

Figures 3A-3B respectively shows the calculated and simulated resistances (R_in) and impedance (X_in) as a function of frequencies for a long dipole antenna in accordance with one embodiment of the present invention. It should be noted that the calculated and simulated results in Figures 3A-3B are calculated with the induced EMF method and simulated with IE3D respectively.

Figure 3B shows three resonance frequencies， namely the first and the second series resonance frequencies (f_s1， f_s2) and the first parallel resonance frequency (f_pl) . As shown in Figure 3B， the simulated results are consistent with the calculated results， except for the band near f_pl. This is due to the fact that the zero-current assumption of the induced EMF method breaks for the full-wave dipole around this frequency regime. Figure 3B also shows that the impedance behaviour around f_s2 are largely consistent with that around f_s1.

Figures 4A-4B respectively shows the variation of the second series resonance frequency (f_s2) ， the first parallel resonance frequency (f_pl) and the frequency at which the directivity D maximizes (f_opt) as a function of the substrate permittivity (ε_r) and substrate thickness based on simulations. For Figure 4A， the substrate thickness is fixed as 3 mm whilst for Figure 4B a substrate permittivity of 3 is used.

Figures 5A-5B respectively shows the variation of the difference between the second series resonance fiequency and the frequency at which the directivity D maximizes (f_s2-f_opt) as a function of the substrate permittivity (ε_r) and substrate thickness based on simulations. It is found from the simulation that f_opt is closer to f_s2 than to f_pl. Moreover， it is found that as the dielectric loading increases (irrespective of the increase in substrate permittivity or substrate thickness) ， the difference between f_opt and f_s2 becomes smaller.

The results in Figures 3A-5B show that the impedance around f_s2 and around f_s1 resembles each other and that f_opt is next to f_s2 and thus in one embodiment of the present invention it is feasible to apply matching techniques of half-wave dipole tags to long-dipole tags of the present invention.

Figure 6A shows a T-match network 600A utilized for impedance matching in one embodiment of the present invention. In Figure 6A， L_h and L_e represents the shunt inductance and the series inductance of the T-match network 600A respectively. Preferably， the introduction of the T-match network 600A in the present embodiment to the long dipole antenna does not disturb the directivity of the long dipole， duc to its electrically small size. However， the impedance matching of long-dipole tags also preferably requires the centre fiequency of the impedance bandwidth to coincide with f_opt so as to exploit the high directivity feature. It the present invention， for long dipoles on thin low-permittivity substrates， the difference between f_opt and f_s2 is up to 180 MHz as shown in Figures 5A-5B. A large difference between the centre frequency and the resonance fiequency (i.e. f_s1 of the half-wave dipole or f_s2 of the long dipole) requires a large shunt inductance in the T-match network. In other words， the difference between f_opt and f_s2 would not allow the application of the T-match network in the embodiment of Figure 6A to long-dipole tags as the T-match network in Figure 6A can only tolerate a fiequency difference about 30-60 MHz.

Figure 6B shows a T-match network 600B utilized for impedance matching in an alternative， more preferred， embodiment of the present invention. In Figure 6B， the T-match network 600B is arranged to provide a longer trace to facilitate implementing a larger L_h.

PROTOTYPE OF A LONG-DIPOLE TAG

Figure 7 shows a long-dipole RFID tag prototype 700 in one embodiment of the present invention. In this embodiment， the tag 700 is particularly adapted to operate in UHF frequency bands， although it is also possible for the tag to be adapted to operate in other frequency bands in some other embodiments. In the present embodiment， the long-dipole RFID tag 700 is implemented on a 130-μm-thick paper substrate and is encapsulated with 100- μm-thick plastic covers on top and bottom. The electric properties of the paper material used are： ε_r＝2.23， tanδ＝0.05. The electric properties of the plastic material used are： ε_r＝2.55， tanδ＝0.005. In this embodiment， the length of the long-dipole is fine tuned to be 388.1 mm (as example only) . Specifically， in the tag prototype 700 in the present embodiment， the long dipole antenna is matched to a RFIC integrated circuit chip with a T-match network of Figure 6B.Note that in this embodiment of Figure 7， the tag is a passive RFID tag without any battery/power means.

Figures 8A-8B respectively shows two different long dipole RFID tags 800A， 800B in accordance with one embodiment of the present invention. In these embodiment， both of the tags 800A， 800B have a similar structure to that shown in Figure 7. In particular， each of the tag includes a long dipole antenna portion， an impedance matching portion as well as an integrated circuit chip. The long dipole antenna portion is arranged to receive and/or radiate RF signals； the impedance matching portion is arranged to match an impedance between the antenna portion and the integrated circuit chip； and the an integrated circuit chip is used for information/data storage. Similar to that in Figure 7， the tags 800A， 800B in Figures 8A and 8B are preferably arranged on a substrate， which may be paper， plastic， glass or any other suitable material.

With reference to Figure 8A， the tag 800A includes an elongated dipole segment 802A which forms the antenna portion， and a loop segment 804A which forms the impedance matching portion. In this embodiment， the elongated dipole segment 802A is substantially straight. The integrated circuit chip 806A is arranged in a gap formed in the elongated dipole segment 802A， which can also be considered to be formed in a closed loop provided by the combination of the elongated dipole segment 802A and the loop segment 804A.

With reference to Figure 8B， the tag 800B includes an elongated dipole segment 802B which forms the antenna portion， and a loop segment 804B which forms the impedance matching portion. In this embodiment， the elongated dipole segment 802B is curved with a plurality of turns (forming a square wave like pattern) . The integrated circuit chip 806B is arranged in a gap formed in the elongated dipole segment 802B， which can also be considered to be formed in a closed loop provided by the combination of the elongated dipole segment 802B and the loop segment 804B.

In one embodiment， the tags 800A， 800B in Figures 8A and 8B are preferably made with substrate having less than 10mm thickness and permittivity less than 4. It is possible， however， to use thickness and permittivity out of this range in some other embodiment. The tags 800A， 800B are preferably， but not absolutely essential， to be substantially symmetric about its short axis. In addition， the shape of the antenna portion and the impedance matching portion could be different fiom that as shown， and could vary， for example， depending on the package of which the tags are to be attached to.

The long-dipole RFID tag prototype 700 of Figure 7 is characterized with both measurement and simulation， and the results are shown in Figure 9. The graph of Figure 9 shows the measured and simulated return losses of the tag prototype 700 of Figure 7 as a function of operation fiequency. In particular， in this graph of Figure 9， in addition to IE3D， a Finite-Element-Method simulator called HFSS is also used to verify the tag performance. In this example， the return losses of the long-dipole tag prototype are derived based on the impedance of the tag antenna and the tag chip， and the measured antenna impedance is obtained with the balanced-robe method. According to the measurement， the centre frequency of the impedance bandwidth (i.e. tag resonance frequency) is 909 MHz. 10-dB and 6-dB impedance bandwidths are found to be 27.6 MHz and 52.2 MHz respectively.

Radiation pattern and read range of the long-dipole tag prototype 700 of Figure 7 are measured with an RFID tester in an anechoic chamber (not shown) . During the radiation pattern measurement， the tag prototype 700 is placed on a rotation system and tested with a 1-deg angular step. Figures 10A-10B shows respectively the measured and simulated E-plane and H-plane radiation patterns of the tag prototype 700 of Figure 7. In Figures 10A-10B， the radiation patterns are normalized. The simulated radiation patterns， on the other hand， are from HFSS. The radiation patterns obtained from IE3D are almost the same and hence are omitted for brevity. The six-pedal E-plane pattern in Figure 10A shows that the tag antenna behaves as a long dipole. Also， fiom the measured E-plane pattern in Figure 10A， the half power beam width (HPBW) is measured to be 32 degrees， which is close to the 34 degrees HPBW of a standalone 1.269λ dipole.

Figure 11 shows the measured read range of the tag prototype 700 of Figure 7. The frequency step used for the read range measurement is 1 MHz. During the measurement， the alignment between the tag antenna (of tag prototype 700) and a tester antenna has been taken care of to minimize the polarization mismatch. The measured read range corresponds to an equivalent isotropically radiated power EIRP of 3.28 W. In this exemplary experiment it is also found that the tag prototype 700 in one embodiment of the present invention delivers a read range over 10m from 857 to 969 MHz， covering the whole UHF RFID band (i.e. 800～1000 MHz) . Figure 11 shows that the maximum read range of 18.4 m occurs at 910MHz. Based on the measured read range， the gain at 910 MHz is estimated to be 4.0 dBi with the Friis equation. Both IE3D and HFSS predict radiation efficiencies of around 84％. Combining the estimated gain and the simulated radiation efficiency in the above analysis， the directivity at 910 MHz is derived to be 4.8 dBi. In comparison， the directivities at 910 MHz obtained from IE3D and HFSS are 5.1 dBi and 4.7 dBi respectively. In a similar way， the maximum directivity of the tag prototype is found to be 4.9 dBi and it occurs at 916 MHz. It can be seen that the agreement between the measurement and the simulation is good. Table 2 below summates the measurement results of the tag prototype of Figure 7B.

Table 2 Measured results of the tag prototype

Tag resonance frequency (MHz)	909
Tag resonance frequency (MHz)	909	Frequency where D maximizes (MHz)	916
10-dB impedalice bandwidth (MHz)	27.6	Frequency where D maximizes (MHz)	916
10-dB impedalice bandwidth (MHz)	27.6	6-dB impedance bandwidth (MHz)	52.2
Half power beam width (deg)	32	6-dB impedance bandwidth (MHz)	52.2
Half power beam width (deg)	32	Maximum read range (m)	18.4
Maximum directivity (dBi)	4.9	Maximum read range (m)	18.4

The present invention provides embodiments of UHF RFID tag antennas in the form of a long dipole， and UHF RFID tag with such antennas. As shown above， long dipole provides the maximum directivity out of a single dipole element. At the same time long dipole is able to maintain its main beam at the plane of θ＝90 deg (assuming the dipole is oriented along the z axis) . A theoretical analysis of the directivity of dipoles in air provided above reveals that the directivity depends on not only the power pattern factor but also the radiation resistance. As a result， the directivity maximizes at a dipole length where the decrease of the power pattern factor overrides that of the radiation resistance. Dipole lengths required for the maximum directivity are found for both dipoles in air and those on typical thin low-permittivity substrates. It can be seen that as the dielectric loading increases， the optimum length of the long dipole approaches one free-space wavelength and even smaller.

Impedance matching is always critical for a successful implementation of tag antennas. Based on a study of the impedance behaviour of the long dipole across its four characteristic frequencies (i.e. the first and the second series resonance frequencies， the second parallel resonance frequency and the frequency where the directivity maximizes) ， the matching techniques for half-wave dipole tags are found to be valid for long-dipole tags. However， the impedance matching of long-dipole tags requires aligning the tag resonance frequency with the frequency where the directivity maximizes. This extra requirement is a big challenge， especially when there is little dielectric loading. The present invention is advantageous in that it substantially mitigates this problem by providing a T-match network arranged to match the impedance of long-dipole tags.

The antenna， the T-match network， as well as the tag provided by the present invention is particularly advantageous in that they enable RFID tags to be manufactured with a form factor that can fully take advantage of the dimension of the package and hence can provide additional design freedom to achieve better performance compare with traditional half-wave dipole. Other advantages of the present invention in terms of structure， function， manufacture， and costs will become apparent to the person skilled in the art who refers to the above description.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing fiom the spirit or scope of the invention as broadly described. The present embodiments are， therefore， to be considered in all respects as illustrative and not restrictive.

Any reference to prior art contained herein is not to be taken as an admission that the information is common general knowledge， unless otherwise indicated.

Claims

A dipole antenna arranged to couple with an integrated circuit chip of an RFID tag， wherein the dipole antenna has a dipole length of at least one wavelengthλso as to maximize directivity of the RFID tag.
A dipole antenna in accordance with claim 1， comprising：

an antenna portion arranged to receive and/or radiate RF signals； and

an impedance matching portion arranged to match an impedance between the antenna portion and the integrated circuit chip.
A dipole antenna in accordance with claim 2， wherein the impedance matching portion is arranged to match a resonance frequency of the RFID tag to a frequency at which the directivity substantially maximizes.
A dipole antenna in accordance with claim 2or 3， wherein both the elongated dipole segment and the loop segment are arranged on a substrate.
A dipole antenna in accordance with any one of claims 2-4， wherein the antenna portion includes an elongated dipole segment； and the impedance matching portion includes a loop segment coupled with the elongated dipole segment.
A dipole antenna in accordance with claim 5， wherein the loop segment is connected with the elongated dipole segment at a first node and at a second node spaced apart from the first node； and a gap is arranged on the elongated dipole segment between the first node and the second node for receiving the integrated circuit chip.
A dipole antenna in accordance with claim 5or 6， wherein the elongated dipole segment is substantially straight.
A dipole antenna in accordance with claim 4， wherein the substrate has a thickness of less than 10mm and a substrate permittivity of less than 4.
A dipole antenna in accordance with claim 4， wherein the substrate comprises paper， plastic， and/or wood materials.
A dipole antenna in accordance with any one of the preceding claims， wherein the dipole antenna is substantially symmetric about its short axis.
A dipole antenna in accordance with any one of the preceding claims， wherein the radio frequency identification (RFID) tag is an ultra-high frequency (UHF) RFID tag.
A dipole antenna in accordance with any one of the preceding claims， wherein the radio frequency identification (RFID) tag is adapted to operate at around 800-1000MHz.
A dipole antenna in accordance with any one of the preceding claims， wherein the dipole length is at or larger than 1.269λ.
A dipole antenna in accordance with any one of the preceding claims， wherein the dipole length is around 1.269λ.
A radio frequency identification (RFID) tag comprising：

an integrated circuit chip； and

a dipole antenna as claimed in any one of the preceding claims， being coupled with the integrated circuit chip.