A helical antenna for UHF near-field RFID readers

Abstract

The present invention relates to a spiral antenna for a UHF frequency band near-field RFID reader / writer, comprising a substrate (1), a feeding connector (2), a coaxial line, and at least two groups of antenna load systems; wherein, based on each antenna (3) extending outward from the center position of the upper surface of the substrate (1) in a spiral line, a strong and uniform UHF frequency band near-field magnetic field with multiple polarizations is generated above the antenna (3) to excite the tag and perform reading and writing; each antenna (3) serves as both a radiation unit and a power divider, and cooperates with a corresponding load to obtain an impedance matching effect, and can avoid reflection of the feeding connector (2) when multiple reflections exist on each antenna (3) of the spiral line, thereby improving the magnetic field strength generated by the antenna; the feeding connector (2) is designed to deviate from the center position of the entire structure, so as to eliminate the zero point at the center of the structure, thereby comprehensively improving the actual reading and writing efficiency.

Description

Technical Field

[0001] The invention relates to a spiral antenna for a UHF frequency band near-field RFID reader / writer, belonging to the technical field of radio frequency antennas. Background Art

[0002] UHF RFID read / write systems can be categorized into two types: "far-field" and "near-field." Far-field RFID uses electromagnetic radiation, whose field strength is inversely proportional to the first power of the distance and decays slowly. This allows for longer read / write distances, up to 10 meters or more. However, its disadvantages include: 1) Electromagnetic radiation is significantly affected by media and conductors (such as water), making it inoperable in liquid environments. 2) The slow decay of field strength makes it difficult to control the read / write range, making it easy to misread distant tags that are not intended for read / write. Near-field RFID uses electric or magnetic field coupling, whose field strength is inversely proportional to the third power of the distance and decays quickly. This allows for shorter read / write distances and prevents misreading distant tags. Magnetic coupling, on the other hand, uses magnetic field excitation. Because magnetic fields are only affected by media with high magnetic permeability, and everyday materials generally lack high magnetic permeability, magnetic coupling can operate in complex environments without being affected by media (such as liquids).

[0003] A key technical requirement for magnetically coupled antennas is to ensure that the currents on the antennas are in phase. This is because the current's phase constantly changes during propagation. If special measures are not taken, the current will reverse phase after traveling half a wavelength, and the resulting magnetic fields will cancel each other out, affecting the sensitivity of the read / write system. The simplest and most direct way to ensure the currents are in phase with an electric loop antenna is to make the antenna small enough so that the current enters the load before it reverses phase, naturally avoiding the magnetic field cancellation caused by the current reversal. However, in practical applications, electric loop antennas are limited by their principle and cannot be manufactured in large sizes. The effective read / write area is generally less than 10x10cm.

[0004] A microstrip helical antenna is connected to a feed at one end and a load at the other, generating a traveling wave magnetic field. This antenna has a simple structure and is easy to design. However, due to the long length of the microstrip helix, the currents at different points in the antenna are out of phase, and the resulting magnetic fields cancel each other out. As a result, the magnetic field decays very quickly with distance, making it primarily suitable for reading and writing at very close ranges. However, in practice, the magnetic field decays very quickly with microstrip helical antennas, essentially limiting their use to applications where the tag is in close proximity to the reader / writer. Furthermore, due to the out-of-phase currents at different points in the antenna, the magnitude and direction of the generated magnetic field are highly complex, resulting in multiple, unpredictable magnetic field zero points along the aperture plane, reducing the success rate of reading and writing in practice.

[0005] A power divider can output signals with the same phase at each port, ensuring that the currents on each radiating element are in phase. Unlike electric loop antennas, this current phase is not limited by antenna size, allowing for both electrically small and larger antennas. However, the actual use of a power divider to excite multiple radiating elements requires a complex structure, resulting in high design and processing costs. Furthermore, the radiating elements are located around the antenna, leaving the center of the antenna unutilized, which negatively impacts magnetic field strength and uniformity.

[0006] Zero-order transmission lines incorporate series capacitance, using the phase shift created by the capacitance to offset the natural phase shift during propagation, ensuring that currents at all points are in phase. These lines are primarily used to create large antennas. However, in practice, the design of zero-order transmission lines is complex and requires high R&D costs. Furthermore, their principle is primarily applicable to large antennas; while its application to smaller antennas is feasible, it is not necessary. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a spiral antenna for a UHF band near-field RFID reader / writer. It adopts a new structural design to generate a strong and uniform UHF band near-field magnetic field above the antenna to stimulate the tag and perform reading and writing, which can effectively improve actual work efficiency.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions: The present invention designs a helical antenna for a UHF band near-field RFID reader, comprising a substrate, a feed connector, a coaxial line, and at least two sets of antenna loading systems;

[0009] The structures of the antenna load systems are identical, and each antenna load system includes an antenna, a conductive sheet, a conductive screw, and a load. The antenna, conductive sheet, conductive screw, and load in the same antenna load system correspond to each other. The antennas in each antenna load system are arranged on the upper surface of the substrate, and one end of each antenna is connected to the center position of the upper surface of the substrate, and the other end of each antenna extends outward from the center position of the upper surface of the substrate in a spiral manner. The layout of each antenna is centrally symmetrical relative to the center position of the upper surface of the substrate. The conductive sheets in each antenna load system are placed on the upper surface of the substrate via corresponding conductive screws, and each conductive sheet is in contact with the corresponding conductive screw, and each conductive screw is grounded. The two connecting ends of the load in each antenna load system are respectively connected to the corresponding conductive sheet and the corresponding conductive screw.

[0010] The feeding connector is arranged on the upper surface of the substrate at a preset distance greater than 0 from the center position of the surface, and the feeding connector is connected to the end of each antenna corresponding to the center position of the upper surface of the substrate. The end of the feeding connector facing the lower surface of the substrate passes through the substrate and is connected to the RFID reader via a coaxial cable, wherein the outer conductor of the coaxial cable is grounded and the inner conductor of the coaxial cable is connected to the feeding connector.

[0011] As a preferred technical solution of the present invention: it also includes a conductive ground plate arranged on the lower surface of the substrate, and the conductive ground plate upwardly covers the entire installation area of the antennas in each group of antenna load systems.

[0012] As a preferred technical solution of the present invention: the coaxial line connected to the feeding connector passes through the surface of the conductive grounding plate to connect to the RFID reader, and the outer conductor of the coaxial line connects to the conductive grounding plate to achieve grounding of the outer skin of the coaxial line.

[0013] As a preferred technical solution of the present invention: the conductive screws in each group of antenna load systems are respectively connected to the conductive grounding plate, so that each conductive screw is grounded separately.

[0014] As a preferred technical solution of the present invention: the feeding connector is a coaxial probe feeding connector.

[0015] As a preferred technical solution of the present invention: the loads in the antenna load systems of each group include one or more of resistors or capacitors.

[0016] As a preferred technical solution of the present invention: the antenna loading system consists of four groups.

[0017] As a preferred technical solution of the present invention, the other ends of the antennas in each antenna loading system extending outward from the center of the upper surface of the substrate in a spiral manner are respectively connected to the edge of the upper surface of the substrate.

[0018] The helical antenna for a UHF near-field RFID reader / writer described in the present invention has the following technical effects compared with the prior art by adopting the above technical solution:

[0019] The present invention discloses a helical antenna for a UHF near-field RFID reader / writer. The helical antenna adopts a novel structural design and includes a substrate, a feed connector, a coaxial cable, and at least two antenna load systems. Each antenna extends outward in a spiral pattern from the center of the substrate's upper surface. A strong, uniform UHF near-field magnetic field with multiple polarizations is generated above the antenna to excite the tag and perform reading and writing. Each antenna acts as both a radiating element and a power divider. Combined with the corresponding load element values and reasonable spiral dimensions, impedance matching is achieved. This prevents reflections from the feed connector in the presence of multiple reflections on each spiral antenna, thereby increasing the magnetic field strength generated by the antenna. In applications, the feed connector is designed to be offset from the center of the entire structure to eliminate the zero point at the center of the structure. Furthermore, the entire antenna area is designed to cover the surface on which it is located, thereby increasing the strength and uniformity of the magnetic field and comprehensively improving the efficiency of actual reading and writing. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a 3D structural diagram of the spiral antenna designed by the present invention for use in a UHF band near-field RFID reader / writer;

[0021] Figure 2 This is a top view schematic diagram of the spiral antenna designed by the present invention for a UHF frequency band near-field RFID reader;

[0022] Figure 3a is a near-field magnetic field intensity diagram showing the magnetic field intensity varying with the z direction (x=y=0) in an embodiment of the present invention;

[0023] Figure 3b is a near-field magnetic field intensity diagram showing the magnetic field intensity varying with the y direction (x=0) in an embodiment of the present invention;

[0024] Figure 4 1 is a schematic diagram of the S11 curve in an embodiment of the present invention;

[0025] Figure 5 3 is a plane schematic diagram of the far-field pattern ZOY in an embodiment of the present invention.

[0026] Among them, 1. substrate, 2. feed connector, 3. antenna, 4. conductive sheet, 5. conductive screw, 6. conductive ground plate. DETAILED DESCRIPTION

[0027] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0028] The present invention designs a spiral antenna for UHF frequency band near-field RFID reader / writer. In practical applications, such as Figure 1 、 2As shown, it specifically includes a substrate 1, a feeding connector 2, a coaxial line, and at least two groups of antenna loading systems.

[0029] Among them, the structures of each group of antenna load systems are the same. Each group of antenna load systems includes an antenna 3, a conductive sheet 4, a conductive screw 5, and a load. The antenna 3, conductive sheet 4, conductive screw 5, and load in the same group of antenna load systems correspond to each other. The antenna 3 in each group of antenna load systems is arranged on the upper surface of the substrate 1, and one end of each antenna 3 is connected to the center position of the upper surface of the substrate 1, and the other end of each antenna 3 extends outward from the center position of the upper surface of the substrate 1 in a spiral manner. The arrangement of each antenna 3 is centrally symmetrical relative to the center position of the upper surface of the substrate 1. The conductive sheet 4 in each group of antenna load systems is placed on the upper surface of the substrate 1 via the corresponding conductive screw 5, and each conductive sheet 4 is in contact with the corresponding conductive screw 5, and each conductive screw 5 is grounded. The two connecting ends of the load in each group of antenna load systems are connected to the corresponding conductive sheet 4 and the corresponding conductive screw 5. In actual applications, the load in each group of antenna load systems includes one or more types of resistors or capacitors. By adjusting the values of the resistors and capacitors, the operating frequency of the corresponding antenna 3 can be changed. Each conductive screw 5 is specifically selected from a metal screw. In specific implementation applications, according to Figure 1 、 Figure 2 In the four-antenna load system application shown, the load specifically uses a 260Ω resistor and a 0.9pF capacitor.

[0030] The feed connector 2 is arranged on the upper surface of the substrate 1 at a preset distance greater than 0 from the center position of the surface, and the feed connector 2 is connected to the end of each antenna 3 corresponding to the center position of the upper surface of the substrate 1. The end of the feed connector 2 facing the lower surface of the substrate 1 passes through the substrate 1 and is connected to the RFID reader via a coaxial cable, wherein the outer conductor of the coaxial cable is grounded and the inner conductor of the coaxial cable is connected to the feed connector 2. Here, the design of setting the feed connector 2 at a non-center position on the substrate 1 can weaken the zero point in the center of the overall structure.

[0031] Based on the basic technical solution of the spiral antenna designed for UHF band near-field RFID reader / writer, the following preferred technical solution is further designed: the design also includes a conductive ground plate 6 arranged on the lower surface of the substrate 1, and the conductive ground plate 6 covers the entire setting area of the antenna 3 in each group of antenna load systems upwards.

[0032] Based on the design and application of the conductive grounding plate 6, the coaxial line connected to the feeding connector 2 is further designed to pass through the surface of the conductive grounding plate 6 to connect to the RFID reader, and the outer conductor of the coaxial line is connected to the conductive grounding plate 6 to achieve grounding of the outer skin of the coaxial line; at the same time, the conductive screws 5 in each group of antenna load systems are respectively connected to the conductive grounding plate 6 to achieve grounding of each conductive screw 5.

[0033] When the above technical solution is applied in practice, the feed connector 2 is specifically designed, and a coaxial probe feed connector is used, whose impedance is a standard 50Ω. Regarding the application of the antenna loading system, four groups of antenna loading systems are specifically selected for implementation, and the other end of the antenna 3 in each group of antenna loading systems is designed to extend outward from the center position of the upper surface of the substrate 1 in a spiral routing manner and be respectively connected to the edge of the upper surface of the substrate 1.

[0034] The spiral antenna designed for UHF band near-field RFID reader is used to Figure 1 、 2 The quadrifilar helical antenna 3 shown is used in practical applications, such as W1=100 mm, W2=12.5 mm, D1=11.25 mm, and D2=9 mm. The specific implementation includes the following points.

[0035] 1) The principle of the four-arm spiral antenna as a radiating unit: When the spiral antenna is working, each antenna 3 will generate a standing wave current that propagates from the feed connector 2 to the corresponding load direction. The direction of the standing wave current is determined by the shape of the spiral trace of each antenna 3. In the part near the center of the substrate 1, the standing wave current is mainly radial (with Figure 1 The z-axis is the axial direction), which will excite an angular magnetic field above the antenna 3. In the part close to the periphery of each antenna 3, the standing wave current is mainly angular, which will excite an axial magnetic field (along the z-axis direction) above the antenna 3. The superposition of these magnetic fields forms the required near-field magnetic field radiation.

[0036] Different from the helical antenna in the general context, the antenna in the present invention basically only radiates near-field magnetic field and hardly generates far-field radiation. Figure 5 As shown, the far-field gain of the embodiment is less than -10dBi.

[0037] The above-mentioned helical antenna designed for UHF near-field RFID reader / writer improves the strength and uniformity of the magnetic field in two ways during implementation: the first way is to use multiple reflections generated by impedance mismatch to increase the magnetic field strength, such as Figure 1 and Figure 2As shown, in the embodiment, each arm of the four-arm helical antenna 3 is equivalent to a 65Ω microstrip line (in actual applications, this specific impedance value is not a limitation). After the 50Ω impedance of the feed connector 2 is split four ways, the impedance at the starting point of each arm is equivalent to 200Ω, significantly greater than 65Ω. Simultaneously, the impedance connected to the 260Ω + 0.9pF load is also significantly greater than 65Ω. This impedance mismatch causes the current to reflect multiple times between the feed connector 2 and each load, generating traveling standing waves. Each reflection contributes to the magnetic field strength, thus increasing the field strength. Simulations show that this multiple-reflection scheme can increase the magnetic field strength by more than twice that of a conventional non-reflection scheme.

[0038] In the second embodiment, the four-arm helical antenna 3 can evenly cover the entire surface of the substrate 1, which fully utilizes the aperture of the antenna 3 and improves the uniformity and intensity of the field.

[0039] 2) Antenna matching principle: As described in 1), there is a significant impedance mismatch between the four-arm helix and the corresponding load end and the feed connector 2 in the embodiment, but Figure 4 As shown, feed connector 2 exhibits no significant reflections. This is because the total length of each arm from feed connector 2 to the load is approximately half a wavelength. Transmission line theory indicates that impedance does not change across a half-wavelength transmission line. Therefore, the impedance seen from feed connector 2 is approximately equal to the load impedance, namely 260Ω + 0.9pF. This is roughly equivalent to the 200Ω impedance seen at feed connector 2 after the four-way power splitter. To compensate for the difference between the actual arm length and half a wavelength, the impedance at the load and feed ends are not identical. Therefore, despite multiple current reflections on quadrifilar helical antenna 3, these reflections do not manifest themselves at feed connector 2.

[0040] 3) Feed point bias eliminates the zero point at the center of the antenna, such as Figure 1 and Figure 2 As shown, feed connector 2 is not placed at the exact center of substrate 1, but slightly offset in the y-direction to eliminate the zero point at the center surface of the helical antenna. If feed connector 2 were placed at the exact center of the helical antenna, the angular magnetic fields generated by the radial currents would cancel each other out at the exact center, resulting in zero field strength at the center. Furthermore, the axial magnetic field generated by the angular currents would also have zero field strength at the center due to the boundary condition that "a normal magnetic field cannot exist on the surface of a good conductor." This means that a magnetic field zero point would exist at the exact center of the helical antenna, making it impossible to read tags located at the exact center surface of the helical antenna.

[0041] When feed connector 2 is biased, a current component (in the -y direction) flows from feed connector 2 toward the center of the helical antenna. This current generates a magnetic field in the -x direction above the antenna, filling the magnetic field zero point at the center of the helical antenna and improving the read / write success rate. As shown in the simulation results in Figure 3, this approach can increase this zero point from the theoretical -∞dBA / m to approximately -10dBA / m.

[0042] The helical antenna designed for a UHF band near-field RFID reader / writer according to the above technical solution adopts a brand-new structural design, including a substrate 1, a feed connector 2, a coaxial line, and at least two antenna load systems. Among them, based on each antenna 3 extending outward from the center position of the upper surface of the substrate 1 in a spiral pattern, a strong and uniform UHF band near-field magnetic field with multiple polarizations is generated above the antenna 3 to stimulate the tag and perform reading and writing. Each antenna 3 acts as both a radiation unit and a power divider, and in conjunction with the corresponding load element value and a reasonable spiral size, an impedance matching effect is achieved. When there are multiple reflections on each antenna 3 of the spiral pattern, reflections from the feed connector 2 can be avoided, thereby improving the magnetic field strength generated by the antenna. In application, the feed connector 2 is designed to deviate from the center position of the entire structure to eliminate the zero point at the center of the structure, and the entire antenna area is designed to cover the surface on which it is located, which can improve the strength and uniformity of the magnetic field, thereby comprehensively improving the actual reading and writing efficiency.

[0043] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. A helical antenna for a UHF near-field RFID reader / writer, characterized by: It comprises a substrate (1), a feed connector (2), a coaxial line, and at least two sets of antenna loading systems; The structures of the antenna load systems of each group are identical to each other, and each group of antenna load systems includes an antenna (3), a conductive sheet (4), a conductive screw (5), and a load; the antenna (3), the conductive sheet (4), the conductive screw (5), and the load in the same group of antenna load systems correspond to each other; the antenna (3) in each group of antenna load systems is arranged on the upper surface of the substrate (1), and one end of each antenna (3) is connected to the center position of the upper surface of the substrate (1), and the other end of each antenna (3) extends outward from the center position of the upper surface of the substrate (1) in a spiral arrangement, and the arrangement of each antenna (3) is centrally symmetrical relative to the center position of the upper surface of the substrate (1); the conductive sheet (4) in each group of antenna load systems is placed on the upper surface of the substrate (1) via the corresponding conductive screw (5), and each conductive sheet (4) is in contact with the corresponding conductive screw (5), and each conductive screw (5) is grounded; the two connecting ends of the load in each group of antenna load systems are connected to the corresponding conductive sheet (4) and the corresponding conductive screw (5); The feed connector (2) is arranged on the upper surface of the substrate (1) at a position at a preset distance greater than 0 from the center position of the surface, and the feed connector (2) is connected to the end portion of each antenna (3) corresponding to the center position of the upper surface of the substrate (1), and the end portion of the feed connector (2) facing the lower surface of the substrate (1) passes through the substrate (1) and is connected to the RFID reader via a coaxial line, wherein the outer conductor of the coaxial line is grounded, and the inner conductor of the coaxial line is connected to the feed connector (2); It also includes a conductive grounding plate (6) arranged on the lower surface of the substrate (1), and the conductive grounding plate (6) upwardly covers the entire setting area of the antenna (3) in each group of antenna loading systems; The coaxial line connected to the feed connector (2) passes through the surface of the conductive grounding plate (6) to connect to the RFID reader, and the outer conductor of the coaxial line connects to the conductive grounding plate (6), thereby achieving outer skin grounding of the coaxial line.

2. The helical antenna for a UHF near-field RFID reader according to claim 1, characterized in that: The conductive screws (5) in each group of antenna load systems are respectively connected to the conductive grounding plate (6), so that each conductive screw (5) is grounded.

3. The helical antenna for a UHF near-field RFID reader according to claim 1 or 2, characterized in that: The feeding connector (2) is a coaxial probe feeding connector.

4. The helical antenna for a UHF near-field RFID reader according to claim 1 or 2, characterized in that: The loads in the antenna load systems of each group include one or more of resistors and capacitors.

5. The helical antenna for a UHF near-field RFID reader according to claim 1 or 2, characterized in that: The antenna loading system consists of four groups.

6. The helical antenna for a UHF near-field RFID reader according to claim 1 or 2, characterized in that: In each group of antenna loading systems, the other ends of the antennas (3) extending outward from the center of the upper surface of the substrate (1) in a spiral routing manner are respectively connected to the edges of the upper surface of the substrate (1).

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