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RFID technology uses wireless radio frequency for non-contact two-way communication, which can achieve the purpose of identifying and exchanging data. Unlike contact identification technologies such as magnetic cards and IC cards, the identification can be completed without physical contact between the electronic tag and the reader of the RFID system, which is a non-contact identification. RFID technology has some unique advantages, it can be more widely used in transportation, medical and anti-counterfeiting fields.
With the rapid development of my country’s economy, the Ministry of Railways has invested a large amount of money in the construction of an automatic identification system for the whole road. The goal is to install electronic tags on all locomotives, The station is equipped with a ground read-write device to accurately identify the running train and vehicle information. The railway radio frequency car number automatic identification system has become an important part of the railway informatization construction. The TKCG-08 RFID automatic train identification system is developed under this background. It uses microwave radio frequency communication technology to realize the automatic identification of train numbers.
Data transmission is an important part of RFID system operation. The radio frequency signal realizes data transmission through the spatial coupling (alternating magnetic field or electromagnetic field) between the reader antenna and the tag antenna. Therefore, the antenna plays an important role in the entire RFID system. On the one hand, the quality of the antenna determines the communication quality of the system. On the one hand, the antenna determines the communication distance of the system.
Depending on the working frequency band, different types of antennas are used in RFID products, and there are many types of antennas to choose from. When choosing an antenna, antenna size, cost, and performance are all very important factors. The three most common short-range antenna devices are PCB microstrip antennas, chip antennas, and whip antennas with a connector. The application of TKCG-08 subway train identification system is mainly to install the reader in the middle of the line track, and install the radio frequency card in the corresponding position of the lower center of the car body, as shown in Figure 2. When the train passes the reader at a certain speed, the reader recognizes the corresponding card number of the radio frequency card, and then obtains the train number information. Due to the limited volume of the radio frequency card and the need to control the cost, the microstrip antenna has the advantages of low cost, high performance, and small size. Therefore, the microstrip antenna is selected as the antenna form used in this system.
Microstrip antenna
The microstrip antenna is an antenna formed by attaching a conductive sheet to a dielectric substrate with a conductive ground plate. It uses feeder lines such as microstrip line and coaxial line to excite a radio frequency electromagnetic field between the conductor patch and the ground plate, and radiates outward through the gap between the patch and the ground plate, so the microstrip antenna can be regarded as It is a slot antenna, as shown in Figure 3. Compared with commonly used microwave antennas, it has the following advantages: small size, light weight, low cost, the feed network can be made together with the antenna structure, suitable for mass production using printed circuit technology, and can be combined with active devices and The circuit is integrated into a single module, it is easy to obtain circular polarization, and it is easy to realize dual-frequency and multi-frequency operation.
According to the needs of antenna radiation characteristics, patch conductors can be designed in various shapes. Generally, the distance between the radiation conductor of the patch antenna and the metal base plate is a few tenths of a wavelength. Assuming that the radiation electric field does not change along the horizontal and vertical directions of the conductor, but only changes along the length of the conductor, which is about half the wavelength, the radiation of the microstrip patch antenna is basically caused by the fringe field at the open edge of the patch conductor. The radiation direction is basically determined.
The microstrip antenna is conducive to selecting a suitable feeding position to make the radiating element and the feeding line well matched, and has a small size and low profile, excellent electrical performance, and realizes one-dimensional miniaturization. Based on this, the miniaturized antenna adopts the form of a microstrip antenna. The main power feeding methods for microstrip antennas to achieve circular polarization are: double feed point feed and single feed point feed. Each of the power feeding methods can use multiple power feeding methods such as direct feeding, slot coupling feeding, and probe feeding. The single-feed point method of direct feeding can realize circularly polarized radiation without designing any complicated phase shifting network and power distribution. It is a simple method to realize circular polarization, so the single-feed point direct feeding method is generally used for feeding .
Microstrip antenna design
According to the microstrip antenna theory, the size of the patch unit width W directly affects the directivity function, radiation resistance and input impedance of the microstrip antenna. Taking into account the need to take into account the radiation efficiency and avoid the generation of high-order modes, the width W is usually required to meet the requirements of formula (1):
Among them, h and w are the height and width of the antenna, respectively. But in actual design, the influence of fringe field should be considered, and the value of L should be corrected. Therefore, L is determined by formula (4):
Because the electric field is unevenly distributed along the L direction, the impedance of the feed point can be changed by changing the position of the coaxial feed point in the L direction to achieve the signal matching state. The admittance of the feed point position is shown in formula (6):
Where Z0 is the characteristic impedance when the antenna is regarded as a transmission line; Y0 is its corresponding admittance; ZW is the wall impedance; β is the phase constant in the medium; L1 and L2 are the feed points along the L direction to the edge of the radiation electrode, respectively distance. The input impedance can meet the requirements of impedance matching by changing the size of L1 and L2.
In the design, the length and width L and W of the radiating electrode must also meet the necessary conditions for circular polarization of the rectangular patch antenna, namely:
In the formula, Qr, Qd, and Qe correspond to the Q values of radiation, dielectric and conductor losses, respectively.
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