by on May 25, 2022
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 1. Introduction

RFID is a non-contact automatic identification technology using RF technology. It has the advantages of fast transmission rate, anti-collision, mass reading, movement process reading and so on. Therefore, RFID technology has great application potential in logistics and supply chain management, production management and control, anti-counterfeiting and safety control, traffic management and control and other fields. At present, the working frequency band of RFID technology includes low frequency, high frequency, ultra-high frequency and microwave band, among which high frequency and ultra-high frequency are the most widely used.

2. RFID technology principle

RFID system is mainly composed of reader / writer (target), transponder (RFID tag) and background computer. The reader / writer realizes the data reading, writing and storage of the tag, which is composed of control unit, high-frequency communication module and antenna. The tag is mainly composed of an integrated circuit chip and external antenna. The circuit chip usually includes RF front-end, logic control, memory and other circuits. According to the principle of power supply, tags can be divided into active tags, semi active tags and passive tags. Passive tags are favored because of their low cost and small volume.

The basic working principle of RFID system is: after the tag enters the RF field emitted by the reader / writer, the induced current obtained by the antenna is used as the power supply of the chip through the boost circuit. At the same time, the induced current with information is transformed into a digital signal through the RF front-end circuit and sent to the logic control circuit for processing. The information to be replied is sent from the tag memory and sent back to the RF front-end circuit through the logic control circuit, Finally, it is sent back to the reader through the antenna.

3. Antenna in RFID system

From the principle of RFID technology, the key to the performance of RFID tag lies in the characteristics and performance of RFID tag antenna. The antenna plays a key role in the data communication between the tag and the reader. On the one hand, the chip start-up circuit of the tag starts to work, which needs to obtain enough energy in the electromagnetic field generated by the reader through the antenna; On the other hand, the antenna determines the communication channel and communication mode between the tag and the reader. Therefore, the research of antenna, especially the antenna inside the tag, has become the focus.

3.1 category of RFID system antenna

According to the power supply mode of RFID tag chip, RFID tag antenna can be divided into active antenna and passive antenna. The performance requirements of active antenna are lower than those of passive antenna, but its performance is greatly affected by the battery life: passive antenna can overcome the limitation of active antenna by battery, but it has high requirements for the performance of antenna. At present, the research focus of passive antenna is RFID. According to the working frequency band of RFID system, for RFID system operating in LF and HF segments f (such as 6.78mhz and 13.56MHz), the transmission of electromagnetic energy is completed in the induction field area (quasi stable field), also known as induction coupling system; For the system in UHF section (such as 915MHz and 2400mttz), the transmission of electromagnetic energy is completed in the far-field area (radiation field), also known as microwave radiation system. Due to the different energy generation and transmission modes of the two systems, the corresponding RFID tag antenna and front-end part have their own particularity. Therefore, the tag antenna is divided into near-field induction coil antenna and far-field radiation antenna. The inductive coupling system uses a near-field inductive coil antenna, which is composed of multi turn inductive coils. The inductive coils and their parallel capacitors form a parallel resonant loop to couple the maximum RF energy; The types of far-field radiation antenna used in microwave radiation system are mainly dipole antenna and slot antenna. The far-field radiation antenna is usually resonant and generally takes half wavelength. The shape and size of the antenna determine the frequency range and other properties it can capture. The higher the frequency, the more sensitive the antenna is and the less area it occupies. Higher operating frequency can have smaller label size. Compared with near-field induction antenna, far-field radiation antenna has higher radiation efficiency.

3.2 design requirements of RFID tag antenna

The design requirements of RFID tag antenna mainly include: the physical size of the antenna is small enough to meet the needs of tag miniaturization; Having omnidirectional or hemispherical coverage directionality; A chip with high gain that can provide the largest signal to the label; Good impedance matching. No matter what direction the tag is in, the polarization of the tag antenna can match the signal of the reader / writer; It has tenacity and low cost. When selecting the antenna, we mainly consider: the type of antenna, the impedance of the antenna, the RF performance applied to the item, and the RF performance when other items surround the labeled item.

4. Category and research status of RFID tag antenna

Tag antennas are mainly divided into three categories: coil type, dipole type and slot type (including microstrip patch). Coil antenna is to coil the metal wire into a plane or wind the metal wire on the magnetic core; The dipole antenna consists of two straight wires of the same thickness and equal length arranged in a straight line. The signal is fed from the two ends in the middle, and the length of the antenna determines the frequency range; The slot antenna is composed of grooves cut out from the metal surface. The microstrip patch antenna is composed of a circuit board with a rectangle at the end. The length and width of the rectangle determine the frequency range.

RFID antenna for medium and low frequency short-range application system with identification distance less than 1m generally adopts coil antenna with simple process and low cost; The long-distance application system of high frequency or microwave band above 1i1 needs to adopt dipole and slot antenna.

4.1 coil antenna

When the tag coil antenna enters the alternating magnetic field generated by the reader, the interaction between the tag antenna and the reader antenna is similar to that of a transformer. Their coils are equivalent to the primary coil and secondary coil of the transformer.

The carrier frequency used in the two-way communication between the tag and the reader is that when the shape of the tag antenna coil is required to be very small, that is, the area is small, and a certain working distance is required, Mutual inductance of antenna coil between RFID tag and reader (obviously, it can not meet the actual needs. Ferrite materials with high permeability can be inserted into the tag antenna coil to increase the mutual inductance and compensate for the problem of small coil cross-section). At present, the implementation technology of coil antenna has been very mature and is widely used in RFID systems such as body identification and cargo tags. However, for RFID applications with high frequency, large amount of information and uncertain working distance and direction, wire is adopted The loop antenna is difficult to achieve the corresponding performance index.

4.2 dipole antenna

Dipole antenna has the advantages of good radiation ability, simple structure and high efficiency. It can be designed as an RFID system suitable for omni-directional communication. It is widely used in the design of RFID tag antenna, especially in long-distance RFID system.

The biggest problem of traditional half wave dipole antenna is the influence on the label size, such as 915MHz half wave dipole. The research shows that the terminated, inclined and folded dipole antenna can obtain the required input impedance by selecting appropriate geometric parameters. It has the advantages of high gain, wide frequency coverage and low noise. Its performance is very excellent, and its size is much smaller than that of the traditional half wave dipole antenna. If it is combined with brazed electrical terminals and unbalanced transformer, it can also maximize the gain, impedance matching and bandwidth. It is known that increasing the bending times of the antenna is conducive to reducing the size of the antenna without reducing the efficiency of the antenna. Then, how to "bend" in a limited space? What are the specific parameters of "bending" affecting the resonant frequency and input impedance of the tag antenna? How can the RF efficiency of "bending" be the highest?

We know. Objects with fractal structure generally have the characteristics of proportional self similarity and space filling. When applied to antenna design, multi band characteristics and size reduction characteristics of antenna can be realized. A lot of research work has been done on the antenna with fractal structure at home and abroad, which proves that the antenna with fractal structure has good size reduction characteristics and can greatly improve the antenna network efficiency in a limited space.

Hilbert fractal transform is used for different positions and dimensions of half wave oscillator, and Hilbert tag antenna is simulated by moment method. The simulation results of resonant frequency and input impedance of tag antenna with different fractal dimension and order can be obtained. The antenna gain and efficiency in the results are analyzed to judge which dimension and order of tag antenna best meets the design requirements of actual tag antenna, and the solid antenna is further manufactured, And test the RF recognition distance, which is a common research method.

4.3 slot (including microstrip patch) antenna

Slot antenna has the characteristics of low profile, light weight, simple processing, easy conformal with objects, mass production, diversified electrical performance, broadband and active devices and circuit integration into a unified component. It is suitable for large-scale production and can simplify the production and debugging of the whole machine, so as to greatly reduce the cost.

Microstrip patch antenna is composed of a radiation patch conductor attached to a dielectric substrate with a metal base plate. According to the radiation characteristics of the antenna, the patch conductor can be designed into various shapes. It is widely used in low profile structures with frequencies higher than 100MHz. It is usually composed of a rectangular or square metal patch placed on the surface of a thin layer of dielectric (called substrate) on the grounding plane. The patch can be manufactured by lithography process, which makes it low cost and easy to mass produce.

As mentioned earlier, the bent antenna is conducive to reducing the physical size of the tag antenna and meeting the design requirements of tag miniaturization. For slot antennas, the concept of bending can also be used. In fact, the folded slot antenna is suitable for RFID tags in high-frequency microwave band, which can effectively reduce the size of the antenna and has excellent performance. It has broad market prospects. The research method is similar to the bending dipole antenna. The influence of the number, height, position, width of slot bending and the size of slot antenna flat plate on the resonant characteristics of rectangular antenna is studied by the method of moment.

The plane size of the folded slot antenna is LXW, the bending width and height of the slot are s and h respectively, and the distance between the slot and the center of the feeding point is. The effects of the changes of these parameters on the resonant characteristics, reflection coefficient and antenna efficiency of the slot antenna are discussed below.

Based on the influence of bending parameters on the performance of slot antenna, the slot antenna for UHF RFID tag can be designed according to the actual needs, and the specific physical antenna can be made. It can be predicted that the folded slot antenna will be a promising development direction in the field of UHF tag antenna design.

5. Hot issues of RFID tag antenna

In the design of RFID tag antenna, in addition to the problem of reducing physical size, which has always been paid great attention to, further improve the bandwidth and gain characteristics of miniaturized antenna to expand its practical application range, and analyze the cross polarization characteristics of miniaturized antenna to clarify its polarization purity is also an important research direction. In addition, the design of composite antenna covering various frequencies and the optimal distribution technology of multi tag antenna, Reader / writer smart beam scanning antenna array technology, design simulation software and platform, tag antenna and attachment medium matching technology, consistency, anti-interference, safety and reliability technology are all worthy of further research.

Among them, on-chip antenna technology is a hot issue in recent research. With the continuous expansion of the application field of RFID technology, the requirements of passive RFID tags for miniaturization, lightweight, multi-function, low power consumption and low cost are also increasing. However, at present, passive RFID tags still use off-chip independent antenna, which has the advantages of high Q (quality factor) value, easy manufacture and moderate cost. The disadvantage is that it is large and easy to break, and it is not competent for anti-counterfeiting or implanting into animals in the form of biological labels. If the antenna can be integrated on the label chip, it can work without any external devices, which will make the whole label smaller and more convenient to use, which leads to the research of on-chip antenna technology.

Integrating the antenna into the chip not only simplifies the original label production process, reduces the cost, but also improves the reliability. As an energy receiver and signal sensor, on-chip antenna determines the performance of the whole system. Its basic starting point is to use Faraday electromagnetic induction principle. The external magnetic field energy is transformed into the on-chip power supply voltage as the working power supply of the whole chip. At the same time, the on-chip current or voltage change caused by the change of electromagnetic field is used to identify the received signal. The signal is transmitted to the receiving end by changing the external magnetic field caused by its own output impedance. So far, the silicon-based integrated spiral inductor is still the main structure of the on-chip antenna realized in the standard CMOS process.

In addition to the internal design of passive RFID tags, research in fields such as RFID smart table antenna has also attracted more and more attention.

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