Dual-band radio frequency identification tag antenna for metal objects

Abstract

A dual-band RFID tag antenna for installation on a metallic object includes a dielectric substrate having a first surface and a second surface opposite the first surface, a conductive layer provided on the first surface of the dielectric substrate and configured to be attached to the surface of the metallic object, an integrated circuit installed on the second surface of the dielectric substrate, an antenna structure formed on the second surface of the dielectric substrate and configured to have a first resonance in a first frequency band and a second resonance in a second frequency band, a loop feeding structure formed on the second surface of the dielectric substrate and configured such that the loop feeding structure electromagnetically couples the antenna structure to the integrated circuit, a first via pin passing through the dielectric substrate and electrically connecting a first point on the antenna structure to a corresponding first point on the conductive layer, and a second via pin passing through the dielectric substrate and electrically connecting a second point on the antenna structure to a corresponding second point on the conductive layer.

Description

Technical Field

[0001] The present disclosure relates to radio frequency identification (RFID) tags, and more particularly to RFID tag antennas for installation on metallic objects.

Background Art

[0002] Today, the rapid progress of radio frequency identification (RFID) technology has attracted much attention. Due to the long-distance identification ability of RFID tags in the ultra-high frequency (UHF) band, RFID tags have been widely implemented in supply chain management, inventory checking, asset tracking, gate automation, etc.

[0003] However, in many applications, it is necessary to install RFID tags on conductive metallic objects such as automobiles, cylinders, containers, weapons, and equipment. When a normal UHF tag is placed on a metallic surface, the reading range is significantly reduced due to impedance mismatch, reduced radiation efficiency, and deteriorated directivity. Therefore, so-called metal-resistant tags have to be specially designed to overcome these problems.

[0004] So far, tags for the UHF RFID band based on microstrip patch antennas with their own ground planes have been proposed. However, as a result, the size of the antenna becomes very large, the resonance length approaches a quarter wavelength, and it becomes difficult to install on small metallic objects. Many dipole-type and foldable-type tags have been designed for miniaturization, but the size of the tag antenna is still large, and this type of antenna is composed of multiple layers and has a fragile structure. For some special and compact metallic platforms such as metallic cylinders and bearings, it is necessary to design the tag antenna conformally. Due to the large size and unstable structure, the above design is difficult to implement in a conformal design. In addition, in the past few decades, dual-band UHF RFID metal-resistant tag antennas have rarely been reported.

Summary of the Invention

Means for Solving the Problems

[0005] According to one aspect of the present disclosure, a dual-band RFID tag antenna for installation on a metallic object is provided. The dual-band RFID tag antenna includes a dielectric substrate having a first surface and a second surface opposite the first surface, a conductive layer provided on the first surface of the dielectric substrate and configured to be attached to the surface of the metallic object, an integrated circuit installed on the second surface of the dielectric substrate, an antenna structure formed on the second surface of the dielectric substrate and configured to have a first resonance in a first frequency band and a second resonance in a second frequency band, a loop feeding structure formed on the second surface of the dielectric substrate and configured such that the loop feeding structure electromagnetically couples the antenna structure to the integrated circuit, a first via pin passing through the dielectric substrate and electrically connecting a first point on the antenna structure to a corresponding first point on the conductive layer, and a second via pin passing through the dielectric substrate and electrically connecting a second point on the antenna structure to a corresponding second point on the conductive layer.

[0006] Due to the first resonance and the second resonance, the dual-band RFID tag antenna can operate in different UHF band applications, such as both the US band of 902 - 928 MHz and the EU band of 865 - 868 MHz. Further, by providing the conductive layer on the first surface of the dielectric substrate, the dual-band RFID tag antenna can be installed on a metallic object.

[0007] In one embodiment, the dielectric substrate is formed from a flexible material. Thereby, the dual-band RFID tag antenna can be installed on both a planar metallic object and a metallic object with a curved surface, such as a cylindrical bearing.

[0008] In one embodiment, the first resonance is an omnidirectional resonance and the second resonance is a directional resonance.

[0009] In one embodiment, the antenna structure includes two antenna portions arranged in a dipole structure.

[0010] In one embodiment, each antenna unit includes a first leg portion and a second leg portion. The resonance length of each antenna unit may correspond to the length from the end of the first leg portion to the end of the second leg portion.

[0011] The end of the first leg portion of the first antenna unit may correspond to a first point on the antenna structure, and the end of the first leg portion of the second antenna unit may correspond to a second point on the antenna structure.

[0012] The second leg portion of each antenna unit may include a bend toward a non-peripheral portion of the second surface of the dielectric substrate. Thereby, the length of the second leg portion can be increased without increasing the overall dimensions of the dual-band RFID tag antenna.

[0013] In one embodiment, the dielectric substrate is rectangular. The first point on the antenna structure and the second point on the antenna structure may correspond to opposite corners on the diagonal of the dielectric substrate.

[0014] In one embodiment, the antenna structure includes a coupling portion disposed adjacent to the loop feeding structure. The coupling portion may have a straight edge on the side facing the loop feeding structure and / or a stepped edge on the side away from the loop feeding structure. The straight edge and the stepped edge may be introduced for impedance matching.

[0015] Hereinafter, embodiments of the present invention will be described as non-limiting examples with reference to the accompanying drawings.

Brief Description of the Drawings

[0016]

Figure 1A

Figure 1B

Figure 1C

Figure 2

Figure 3A

Figure 3B

Figure 4A

Figure 4B

Figure 5

Figure 6A

Figure 6B

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0017] The present disclosure provides an RFID tag antenna that can operate in two UHF band applications. For example, embodiments may be able to operate in both the EU band: 865 - 868 MHz and the US band: 902 - 928 MHz. In these applications, dual - band RFID tag antennas can be installed on metal platforms of different shapes and sizes, such as flat metal plates, metal cylinders, containers, bearings, etc.

[0018] Figures 1A - 1C show a dual - band RFID tag antenna according to an embodiment of the present invention. Figure 1A is a cross - sectional view, Figure 1B is a perspective view, and Figure 1C is a top view.

[0019] As shown in Figure 1A, the dual - band RFID tag antenna 100 includes a dielectric substrate 102. In order for the dual - band RFID tag antenna 100 to be able to operate on both flat metal platforms and curved metal platforms, the dielectric substrate 102 is formed from a flexible material. For example, a material such as Arlon AD430 can be selected. Any plastic material can be used for the dielectric substrate 102. In an exemplary embodiment, the dimensions of the dielectric substrate 102 are 20 mm × 30 mm × 1.5 mm.

[0020] The dual - band RFID tag antenna 100 is manufactured using printed circuit board (PCB) technology. A bottom conductive coating 104 is applied to the bottom surface of the dielectric substrate 102. The bottom conductive coating 104 covers the bottom surface of the dielectric substrate 102. A top conductive coating 106 is applied to the top surface of the dielectric substrate 102, and the top conductive coating forms an antenna structure and a loop feeding structure, which will be detailed below. The via pin 108 penetrates the dielectric substrate 102 to form an electrical connection between the bottom conductive coating 104 and the top conductive coating 106. The integrated circuit chip 110 is installed on the top surface of the dielectric substrate 102 and is connected to a part of the top conductive coating 106. The integrated circuit 110 is an RFID tag integrated circuit such as an Alien Higgs 9 (AH - 9) tag chip.

[0021] As shown in FIG. 1B, the integrated circuit chip 110 is coupled to a loop power supply unit 140 disposed at the center of the upper surface of the dielectric substrate 102. The dual-band RFID tag antenna 100 is rectangular, and the via pins 108 are disposed at opposite corners on the diagonal of the dual-band RFID tag antenna 100 (the upper left corner and the lower right corner in FIG. 1B). The via pins 108 connect the upper surface conductive coating 106 to the lower surface conductive coating 104.

[0022] The upper surface conductive coating 106 forms a first antenna portion (or first antenna part) 120A, a second antenna portion (or second antenna part) 120B, and a loop power supply unit 140. As shown in FIG. 1B, the first antenna portion 120A has a first leg portion 122A and a second leg portion 124A, and the second leg portion 124A forms an overlapping portion 123 that follows the inner path of the second antenna portion 120B. This overlapping arrangement can increase the effective length of the first antenna portion 120A without increasing the overall dimensions of the dual-band RFID tag antenna 100.

[0023] To reduce the size of the dual-band RFID tag antenna 100, two via pins 180 are used to connect the upper and lower conductive surfaces at the edge of the dielectric substrate, changing the radiation pattern of the tag from a dipole-like structure to a planar inverted-F antenna (PIFA).

[0024] FIG. 1C is a top view of the dual-band RFID tag antenna 100 showing the layout of the upper surface conductive coating 106. The upper surface conductive coating 106 forms a structure like a rotationally symmetric dipole around the loop power supply unit 140 at the center of the upper surface. The layout of the surface conductive coating 106 is composed of a first antenna portion 120A, a second antenna portion 120B, and a loop power supply unit 140. As described above, the first antenna portion 120A and the second antenna portion 120B are rotationally symmetric by 180 degrees around the loop power supply unit 140 at the center of the upper surface.

[0025] As shown in FIG. 1C, the first antenna portion 120A includes a first leg portion 122A that extends from the upper left corner of the upper surface connected to one of the via pins 108A, across the width of the upper surface, and to the lower left corner of the upper surface. The second leg portion 124A of the first antenna portion 120A is located near the right end of the upper surface but is separated from the right end of the upper surface by the second antenna portion 120B. The first leg portion 122A and the second leg portion 124A of the first antenna portion 120A are connected by a central portion 126A of the first antenna portion 120A that extends along the lower end of the upper surface. The coupling portion 130A of the first antenna portion 120A extends upward from the central portion 126A of the first antenna portion 120A.

[0026] The coupling portion 130A extends adjacent to the loop feeding portion 140. The coupling portion 130A has a stepped edge 132A facing away from the loop feeding portion 140 and a straight edge 134A facing the loop feeding portion 140. The stepped edge 132A and the straight edge 134A are introduced for impedance matching.

[0027] Similarly, the second antenna portion 120B includes a first leg portion 122B that extends from the lower right corner of the upper surface connected to the other of the via pins 108B. The first leg portion 122B of the second antenna portion 120B extends from the lower right corner to the upper right corner of the upper surface. The second leg portion 124B of the second antenna portion 120B is located near the left end of the upper surface but is separated from the end by the first leg portion 122A of the first antenna portion 120A. The first leg portion 122B and the second leg portion 124B of the second antenna portion 120B are connected by a central portion 126B of the second radiating portion 120B that extends along the upper edge of the upper surface. The coupling portion 130B of the second radiating portion 120B extends downward from the central portion 126B of the second radiating portion 120B. The coupling portion 130B extends adjacent to the loop feeding portion 140. The coupling portion 130B has a stepped edge 132B facing away from the loop feeding portion 140 and a straight edge 134B facing the loop feeding portion 140. The stepped edge 132B and the straight edge 134B are introduced for impedance matching.

[0028] To optimize the input impedance of the tag antenna, the shape of the openings formed between the coupling portions 130A and 130B and the second leg portions 124A and 124B may be changed. This provides an additional parameter for adjusting the impedance so that good impedance matching can be obtained.

[0029] The integrated circuit 110 is coupled to the loop power supply unit 140.

[0030] FIG. 2 shows the simulated power reflection coefficient of the dual-band RFID tag antenna according to an embodiment of the present invention. As shown in FIG. 2, the antenna can generate two resonance frequencies of 866 MHz and 920 MHz. The bandwidth of the power reflection coefficient (PRC) of the designed tag antenna < -3.0 dB is 911.2 to 915.8 MHz and 918.5 to 921.5 MHz.

[0031] FIGS. 3A and 3B show the simulated radiation patterns of the dual-band RFID tag antenna according to an embodiment of the present invention at 866 MHz and 920 MHz, respectively. The maximum gain is -2.73 dBi along the bore sight direction at 866 MHz, which means that this tag antenna has a directional radiation pattern at this frequency. The maximum gain occurs at a deviation of 45° from the normal direction, and the gain at 920 MHz is -6.8 dBi, but this tag antenna has an omnidirectional radiation pattern at this frequency.

[0032] FIGS. 4A and 4B show the simulated read ranges of the dual-band RFID tag antenna according to an embodiment of the present invention at 866 MHz and 920 MHz, respectively. When this tag is placed on a 15 cm × 15 cm planar ground, the maximum read range at 866 MHz is about 7.5 m, and the maximum read range at 920 MHz is about 6.0 m.

[0033] FIG. 5 shows the resonance length of the dual-band RFID tag antenna according to an embodiment of the present invention. As shown in FIG. 5, the resonance length is the length of the second antenna portion 120B (equal to the length of the first antenna portion 120A). The length extends from the end of the first leg portion 122B corresponding to one of the via pins 108 to the end of the second leg portion 124B.

[0034] Generally, the resonance length of an antenna is about half a wavelength. The short pin load means that by adding the via pin 108 to the PIFA antenna structure, the effective length of the antenna can be effectively shortened. By shortening the resonance length to about 1 / 4 wavelength, the antenna can achieve resonance at UHF frequencies. This approach helps to reduce the size of the entire tag while maintaining appropriate resonance. Therefore, the resonance length 500 shown in FIG. 5 corresponds to 1 / 4 of the resonance wavelength. Note that the overlap of the second leg portion 124B allows for further miniaturization of the dual-band RFID tag antenna.

[0035] FIGS. 6A and 6B show the current distributions at 866 MHz and 920 MHz, respectively, in the dual-band RFID tag antenna according to an embodiment of the present invention.

[0036] As shown in FIG. 6A, when operating at 866 MHz, the antenna has a directional radiation pattern. The basic principle of a directional radiation tag using an electric dipole current is to create a spatial distribution of the current along the antenna element. The reflector operates based on the principle of reflecting and focusing electromagnetic waves in a specific direction, thereby improving the directivity and gain of the tag. The reflector functions as a passive element that redirects and concentrates the radiated energy in the desired direction, improving the performance of the antenna.

[0037] As shown in FIG. 6B, when operating at 920 MHz, the antenna has an omnidirectional radiation pattern. The basic principle of an omnidirectional radiation tag is to use loop surface currents to create a balanced current distribution along the loop structure, thereby resulting in radiation in all directions. The loop antenna needs to be designed to resonate at the desired operating frequency. Resonance occurs when the electrical length of the loop corresponds to a half-wavelength or multiple half-wavelengths of the operating frequency. The loop tag is assumed to exhibit a balanced distribution of surface current along its circumference. This means that the magnitudes and phases of the currents flowing through different sections of the loop must be relatively equal.

[0038] FIG. 7 shows a loop feed matching network. As shown in FIG. 7, the RFID tag antenna can be regarded as a chip of an integrated circuit 110 coupled to a radiator.

[0039] Loop matching is an impedance matching network that includes a loop feed structure coupled to the feed line of the tag antenna. The component values can be determined using the following equations.

[0040]

Equation

[0041]

Equation

[0042] L is the inductor value, C is the capacitor value, C in and C out are the input capacitance and output capacitance of the tag antenna, respectively. Z in is the desired input impedance of the tag antenna, and f is the operating frequency. The actual component values may need to be adjusted or fine-tuned through simulation or experimental iteration.

[0043] FIG. 8 shows the impedance of the tag antenna and the tag chip. As shown in FIG. 8, the tag antenna has an impedance Z a and Z a =R a +jX a The tag chip has an impedance Z c . Z c =R c +jX c

[0044] For maximum power transfer, it is necessary to match the complex conjugate of the impedance. Z a =Z c *

[0045] FIG. 9 is a flowchart showing a method for determining design parameters of an RFID tag antenna according to an embodiment of the present invention.

[0046] First, in step 1202, a tag chip is selected, and from this, the input impedance of the tag chip can be known. In step 1204, the antenna type and the substrate material are selected.

[0047] Next, in step 1206, the parameters are investigated and optimized. In this step, it may involve adjusting the length of the loop feeding structure in order to adjust the input impedance of the antenna to match the impedance of the integrated circuit chip. Conjugate matching is employed to achieve maximum energy transfer between the tag and the chip. Therefore, when changing the chip, it is also necessary to adjust the input impedance of the antenna. As shown in FIG. 9, when the parameters are optimized, a check is made in step 1208 as to whether the impedance matching requirement is satisfied. If the requirement is satisfied, the design is finalized in step 1210. Otherwise, the method returns to step 1206 and further optimization is performed.

[0048] As described above, the present disclosure provides a highly miniaturized dual-band UHF RFID tag antenna for tagging small metal objects. In an exemplary embodiment, the total volume is only 20 mm × 30 mm × 1.5 mm. This tag can be used not only on a planar platform but also on a conformal required platform such as a metal cylinder and a bearing.

[0049] Furthermore, the present disclosure provides a very simple and compact foldable dipole structure designed for a metal-resistant UHF tag antenna. The proposed tag can achieve a large read distance of more than 4 m. The proposed tag antenna is very thin and can be easily made conformally flexible to several curved surfaces.

[0050] In the foregoing description, exemplary embodiments have been described, but those skilled in the art will understand that many variations of the embodiments are possible within the scope and spirit of the present invention.

Description of Reference Numerals

[0051] 100 Dual-band RFID tag antenna 102 Dielectric substrate 104 Lower surface conductive coating 106 Upper surface conductive coating 108 Via pin 110 Integrated circuit chip 120A First antenna portion 120B Second antenna portion 120B Second radiating portion 122A First leg portion 122B First leg portion 124A Second leg portion 124B Second leg portion 123 Overlapping section 126A Central portion 126B Central portion 130A Coupling portion 130B Coupling portion 132A Stepped edge Edge with 132B segment 134A straight edge 134B straight edge 140 Loop power supply section 180 via pin 500 Resonance length

Claims

1. A dual-band RFID tag antenna for installation on a metallic object, comprising: a dielectric substrate having a first surface and a second surface facing the first surface; a conductive layer provided on the first surface of the dielectric substrate and configured to be attached to the surface of the metallic object; an integrated circuit installed on the second surface of the dielectric substrate; an antenna structure formed on the second surface of the dielectric substrate and configured to have a first resonance in a first frequency band and a second resonance in a second frequency band; a loop feeding structure formed on the second surface of the dielectric substrate and configured to electromagnetically couple the antenna structure to the integrated circuit; a first via pin passing through the dielectric substrate and electrically connecting a first point on the antenna structure to a corresponding first point on the conductive layer; a second via pin passing through the dielectric substrate and electrically connecting a second point on the antenna structure to a corresponding second point on the conductive layer The dual-band RFID tag antenna.

2. The dual-band RFID tag antenna according to claim 1, wherein the dielectric substrate is formed of a flexible material.

3. The dual-band RFID tag antenna according to claim 1 or 2, wherein the first resonance is an omnidirectional resonance and the second resonance is a directional resonance.

4. The dual-band RFID tag antenna according to any one of claims 1 to 3, wherein the antenna structure includes two antenna portions arranged in a dipole structure.

5. The dual-band RFID tag antenna according to claim 4, wherein each antenna portion includes a first leg portion and a second leg portion.

6. The dual-band RFID tag antenna according to claim 5, wherein the resonance length of each antenna portion corresponds to the length from the end of the first leg portion to the end of the second leg portion.

7. The dual-band RFID tag antenna according to claim 6, wherein the end of the first leg portion of the first antenna portion corresponds to the first point on the antenna structure, and the end of the first leg portion of the second antenna portion corresponds to the second point on the antenna structure.

8. The dual-band RFID tag antenna according to any one of claims 5 to 7, wherein the second leg portion of each antenna portion includes a bend toward a non-peripheral portion of the second surface of the dielectric substrate.

9. The dual-band RFID tag antenna according to any one of claims 1 to 8, wherein the dielectric substrate is rectangular.

10. The dual-band RFID tag antenna according to claim 9, wherein the first point on the antenna structure and the second point on the antenna structure correspond to opposite corners on a diagonal line of the dielectric substrate.

11. The dual-band RFID tag antenna according to any one of claims 1 to 10, wherein the antenna structure includes a coupling portion disposed adjacent to the loop power supply structure.

12. The dual-band RFID tag antenna according to claim 11, wherein the coupling portion has a straight edge on a side facing the loop power supply structure.

13. The dual-band RFID tag antenna according to claim 11 or claim 12, wherein the coupling portion has a stepped edge on a side away from the loop power supply structure.

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