Research on UHF RFID Antenna Design Technology

0 Preface

The application of RFID radio frequency identification technology (Radio Frequency Identification, RFID) has a long history. It can be traced back to the aircraft identification system used by the British Air Force aircraft during the Second World War. 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, which uses radio frequency signals to achieve non-contact information transmission through spatial coupling (alternating magnetic field or electromagnetic field) and achieves the purpose of automatic identification through the transmitted information.

1 Overview of RFID radio frequency technology

1.1 Basic composition of RFID wireless identification system

The RFID wireless identification system is mainly composed of RFID electronic tags, RFID Readers, antennas and host computer management systems. The information between the RFID electronic tag and the RFID reader is transmitted wirelessly, so there are wireless transceiver modules and antennas (induction coils) between them. The effect diagram is shown in Figure 1.

Research on UHF RFID Antenna Design Technology

(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 (Electronic 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 also be specific to a certain product.

(2) RFID reader (Reader): The reader is a device capable of reading or writing 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 (Antenna): transmit radio frequency signals between the tag and the reader.

1.2 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 sent by the reader, and sends out the product information (Passive Tag, passive tag or passive tag) Stored in the chip by virtue of the energy obtained by the induced current, or The tag actively sends a signal of a certain frequency (Active Tag, active tag or active tag), and the decoder reads 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.

2 RFID tag antenna performance index

It is not difficult to see from the identification process of the RFID system that the antenna plays an important role as a bridge for the RFID reader to transmit radio frequency signals between the RFID electronic tag and the RFID reader in the process of sensing the RFID electronic tag. 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 marked object, the RFID electronic tag antenna will be affected by the shape and physical characteristics of the marked object. Influencing factors include the material of the marked object, the working environment of the marked item, etc. In addition, in the RFID radio frequency device, when the operating frequency increases to the microwave region, the matching problem between the antenna and the RFID electronic tag chip becomes more severe. These factors have put forward higher requirements for the design of RFID electronic tag antennas, but also brought great challenges.

The antenna is a device that receives or radiates the power of the front-end radio frequency signal in the form of electromagnetic waves. It is a device at the interface between the circuit and the space, and is used to realize the energy conversion between the guided wave and the free space wave. The current RFID wireless radio frequency systems are mainly concentrated in low frequency, high frequency, ultra high frequency, and microwave frequency bands. The principles and designs of RFID system antennas in different operating frequency bands are fundamentally different:

(1) Directional characteristics

Antenna radiation is directional. The relationship curve between the amplitude and direction of the radiation field is called the direction diagram, which is actually the relationship curve of the field strength at a point in any direction of the far field field in the same direction. The direction diagram generally refers to the normalized direction diagram, that is, the relationship curve in the same direction as the ratio of the field strength at a point in any direction of the far-field field to the maximum field at the same distance.

(2) Directivity coefficient

The directivity coefficient is a parameter used to indicate the degree to which the antenna radiates electromagnetic waves in a certain direction. The directivity coefficient of any directional antenna refers to the ratio of the total radiation power of the non-directional antenna to the total radiation power of the directional antenna under the condition of equal electric field strength at the receiving point. According to this definition, since the radiation intensity of the directional antenna varies in all directions, the directivity coefficient of the antenna also varies with the position of the observation point. In the direction where the radiation electric field is the largest, the directivity coefficient is also 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 flux density Smax of the antenna in the maximum radiation direction is the same as that of an ideal non-directional antenna with the same radiation power The ratio of the radiation power flux density So at the same distance is denoted as D.

(3) Antenna efficiency

Antenna efficiency is an index used to measure the effectiveness of an antenna in converting energy. The antenna efficiencies are all less than 1, which means that part of the input power of the antenna is converted into radiated power, and part of it is lost power. Antenna efficiency is defined as the ratio of antenna radiation power to input power, denoted as ηA.

(4) Antenna gain

The antenna coefficient only reflects the most concentrated degree of the antenna radiation energy, and the antenna gain not only reflects the radiation capability of the antenna, but also considers the loss factor of the antenna. Under the condition of the same input power, the ratio of the radiated power density S(θ, φ) of the directional antenna in a certain direction (θ, φ) in space to the radiated power density So of the lossless point source antenna in this direction is called the antenna’s Gain, denoted as G(θ, φ).

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 directivity coefficient and the antenna efficiency, which is denoted as G, namely:

G=D·ηA

For UHF and microwave RFID radio frequency identification systems, the gain of the antenna is limited due to the small area of the RFID electronic tag antenna. The amount of gain depends on the type of antenna radiation pattern.

(5) Impedance characteristics

The input impedance of an antenna can be expressed as the ratio of voltage to current at the antenna feed point, usually as 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 of the electronic tag, and the input impedance Zin is defined as the ratio of the antenna input voltage to the input current Io.

The radiated power P∑ of the RFID antenna is equivalent to the loss in an equivalent impedance. This equivalent impedance is called the radiation impedance Z∑,

3 Conclusion

With the continuous clarification of the application requirements of RFID wireless radio frequency technology and the continuous expansion of the application field, the design and research of the antenna as a key component of the RFID system has become very urgent and urgent. Antenna technology is one of the key technologies of the RFID system, and it has theoretical significance and practical value for the maturity and wide application of RFID technology.