RFID radio frequency identification technology (Radio Frequency IdentificaTIon, RFID) has a long history of application, and it can be traced back to the aircraft identification system used by British Air Force aircraft during World War II. Recently, RFID radio frequency identification technology has been widely used in item management, vehicle positioning, and underground personnel positioning. This technology is a non-contact automatic identification technology that uses wireless radio frequency signals to achieve non-contact information transmission through spatial coupling (alternating magnetic field or electromagnetic field) and achieve automatic identification through the transmitted information.
1 Overview of RFID radio frequency technology1.1 The basic composition of RFID wireless identification system
The RFID wireless identification system is mainly composed of RFID electronic tags, RFID readers, antennas and upper computer management system. RFID electronic tags and RFID readers transmit information wirelessly, so there are wireless transceiver modules and antennas (induction coils) between them. The effect diagram is shown in Figure 1.
(1) RFID electronic tag (Tag): RFID electronic tag is the data carrier of the radio frequency identification system. Composed of coupling elements and chips, each RFID electronic tag has a unique EPC (Electr ctronic ProductCode) electronic code, which is attached to the object to identify the target object. Compared with traditional barcodes, EPC codes can not only reflect a certain type of product, but can also be specific to a certain product.
(2) RFID reader (Reader): The reader is a device that can read or write electronic tag information. Its basic function is to transmit data with the tag. It can be designed as a handheld reader or a fixed reader.
(3) Antenna: Transmit radio frequency signals between the tag and the reader.
1.2 The working principle of RFID system
After the RFID electronic tag enters the magnetic field emitted by the RFID reader, it receives the radio frequency signal from the reader, and uses the energy obtained by the induced current to send out the product information stored in the chip (Passive Tag, passive tag or passive tag), or The tag actively sends a signal of a certain frequency (AcTIve Tag, active tag or active tag), and the deduplicator extracts and decodes the information, and then sends it to the central information system for relevant data processing. The schematic diagram of the radio frequency identification process is shown in Figure 2.
From the identification process of the RFID system, it is not difficult to see that in the process of RFID reader sensing RFID electronic tags, the antenna plays an important role in transmitting radio frequency signals between the RFID electronic tag and the RFID reader. The RFID reader antenna, The performance of the RFID electronic tag antenna is of great significance to improve the performance of the entire identification system. Since the RFID electronic tag is attached to the identified object, the RFID electronic tag antenna will be affected by the shape and physical characteristics of the identified object. Influencing factors include the material of the identified object, the working environment of the identified object, and so on. In addition, in RFID radio frequency devices, when the operating frequency is increased to the microwave area, the matching problem between the antenna and the RFID electronic tag chip becomes more severe. These factors put forward higher requirements for the design of RFID electronic tag antennas, but also brought huge challenges.
An antenna is a device that receives or radiates the front-end radio frequency signal power in the form of electromagnetic waves. It is a device that connects the circuit and the space, and is used to convert the energy of the guided traveling wave and the free space wave. The current RFID radio frequency systems are mainly concentrated in the low frequency, high frequency, ultra high frequency, and microwave frequency bands. The principles and designs of RFID system antennas in different working frequency bands are fundamentally different:
(1) Directional characteristics
The radiation of the antenna is directional. The relationship curve between the amplitude of the radiation field and the direction is called the pattern, which is actually the relationship curve between the field strength and the direction at a point in any direction of the far field. Directional pattern A stock index normalized pattern, that is, the relationship curve of the ratio of the field strength at a certain point in any direction of the far field to the maximum field at the same distance in the same direction. Define the pattern function as:
Where EM is the maximum value of |E(α, β)|.
(2) Directivity coefficient
The directivity coefficient is a parameter used to indicate the degree to which the antenna concentrates and radiates electromagnetic waves in a certain direction. The directivity coefficient of any directional antenna refers to the ratio of the total radiated power of the non-directional antenna to the total radiated power of the directional antenna under the condition of equal electric field strength at the receiving point. According to this definition, due to the unequal radiation intensity of the directional antenna in various directions, the directivity coefficient of the antenna also varies with the position of the observation point. The directivity coefficient is also the largest in the direction where the radiated electric field is the largest. In general, the directivity coefficient of a directional antenna is the directivity coefficient of the maximum radiation direction, that is, at a certain distance from the antenna, the radiation power flow density Smax of the antenna in the maximum radiation direction is the same as the ideal non-directional antenna with the same radiation power. The ratio of the radiant power flow density So at the same distance is denoted as D, namely:
(3) Antenna efficiency
Antenna efficiency is an index used to measure the effectiveness of antenna conversion energy. The antenna efficiencies are all less than 1, which means that part of the antenna input power is converted into radiated power and part is lost power. Antenna efficiency is defined as the ratio of antenna radiation power to input power, denoted as ηA, namely:
Where: Pi is the radiated power; Pj is the power loss.
(4) Antenna gain
The antenna coefficient only reflects the most concentrated degree of the antenna's radiation energy, and the antenna gain not only reflects the antenna's radiation ability, but also considers the antenna's loss factor. Under the same input power conditions, the ratio of the radiated power density S(θ, φ) of a directional antenna in a certain direction (θ, φ) in space to the radiated power density So of a lossless point source antenna in that direction is called the antenna’s The gain is denoted as G(θ,φ). which is:
The gain coefficient is a parameter that comprehensively measures the energy conversion and directional characteristics of the large line. It is the product of the directional coefficient and the antenna efficiency, denoted as G, that is:
G=D·ηA
For the RFID radio frequency identification system whose frequency band is UHF and microwave, the gain of the antenna is also limited due to the small area of ​​the RFID electronic tag antenna. The magnitude of the gain depends on the type of antenna radiation pattern.
(5) Impedance characteristics
The input impedance of the antenna can be expressed by the ratio of the voltage to the current at the antenna feed point, which is usually a function of frequency. The impedance of the RFID antenna should be designed to be 50 Ω or 70 Ω in order to achieve impedance matching with the conventional feeder. The RFID antenna is equivalent to the terminal load of the reader and the output end of the electronic tag, and the input impedance Zin is defined as the ratio of the antenna input voltage to the input current Io. which is:
Where: Rin and Xin are the real and imaginary parts of the input impedance, respectively.
The radiation power P∑ of the RFID antenna is equivalent to the loss produced by an equivalent impedance. This equivalent impedance is called the radiation impedance Z∑, namely:
Where: I is the reference current; R∑, X∑ are the real and imaginary parts of the radiation impedance, respectively.
3 ConclusionWith the continuous clarification of the application requirements of RFID radio frequency technology and the continuous expansion of application fields, the design and research of the antenna as a key component of the RFID system has become very important and urgent. Antenna technology is one of the key technologies of the RFID system, which has theoretical significance and practical value for the maturity and wide application of RFID technology.
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