Abstract: As a key component in the design of radio frequency identification system, the antenna directly affects the performance of the system. U2270B is a typical contactless IC card radio frequency base station chip with a transmission frequency of 125kHz. Based on the introduction of the basic principles of radio frequency identification system, this article explains the importance of antenna design; focuses on the key parts and specific steps of U2270B base station chip antenna design, and further illustrates them through examples. Keywords: Radio frequency identification system, resonant frequency, magnetic field coupling factor, antenna design In recent years, automatic identification methods have been rapidly developed in service fields, goods sales, logistics distribution, commerce, production enterprises and material circulation, and radio frequency identification technology has developed rapidly and has gradually become an independent interdisciplinary professional field, mainly including high frequency technology, semiconductor technology, electromagnetic compatibility technology, data security and confidentiality technology, telecommunications and manufacturing technology, etc. As a key component in the design of radio frequency identification system, the antenna directly affects the performance of the system. 1 Principle of Radio Frequency Identification System Radio frequency identification system (RFID) generally consists of two parts: reader (PCD) and transponder (PICC). A typical reader includes a high frequency module (transmitter and receiver), a control unit and a coupling element connected to the transponder [1]. The transponder is the real data carrier of the radio frequency identification system. Usually, the transponder is composed of a coupling element and a microelectronic chip. The transponder does not have its own independent power supply, but only receives the radio frequency power from the reader within the response range of the reader. The energy required for the transponder to work, like clock pulses and data, is obtained through non-contact transmission through the coupling unit [2]. Therefore, the element that realizes the coupling—the antenna—plays a key role in this system. The design of the antenna is directly related to the communication distance and data transmission reliability of the system. The antenna design of the radio frequency identification system is mainly discussed below, taking the radio frequency base station chip U2270B as an example. There are two LC circuits in the RFID system: the LRCR circuit composed of the base station coil and the connecting capacitor and the LTCT circuit composed of the transponder coil and the connecting capacitor. In a single coil system, the two LC circuits are required to be tuned to the same resonant frequency. If the resonant frequencies of the base station and the transponder are mismatched, zero modulation will occur, thereby reducing the performance of the system. After the system design is completed, the inductance of the antenna is fixed. Therefore, to change the resonant frequency of the LC circuit, the capacitance in the circuit must be adjusted. The reader base station antenna is a series resonant circuit composed of an inductor, a capacitor and a resistor, as shown in Figure 1. Its characteristics are represented by the resonant frequency fo and the Q factor [3]. fo is the operating frequency of the RFID system, which is determined by the inductance and capacitance of the antenna and can be calculated by equation (1): Generally, the reader is designed to operate in a single frequency mode. For U2270B, fo = 125kHz can be taken. The relationship between the Q factor (QR) and the bandwidth B of the antenna and the resonant frequency fo is B = fo / QR. A high QR value will result in a higher reader antenna voltage, which can increase the energy transmitted to the transponder. The disadvantage of a high QR value is that it reduces the antenna bandwidth, which in turn reduces the data signal voltage sensed by the transponder when the transponder frequency shifts, thus making it difficult to demodulate the RFID card [4] and preventing it from working properly. The coupling factor is the coupling between the electromagnetic field generating coil of the reader base station and the transponder coil. The coupling factor depends on the structural parameters of the system and directly affects the reading distance between the reader and the transponder. Optimizing the coupling factor will benefit the energy transmission channel and the signal transmission channel. To determine the coupling factor, the test transponder coil (TTC) and circuit provided by Temic can be used for testing. The value of QR should be controlled within the range of 5 to 15. Generally, QR=12 is suitable for most application requirements. If the inductance of the antenna is determined, the QR factor can be adjusted by RR through equation (2): 2 Antenna Design Steps Antenna design mainly involves determining the mechanical dimensions, number of coil turns, inductance, and capacitance of the equivalent circuit based on actual requirements, so as to maximize the antenna's working efficiency. The general steps of antenna design are introduced below. 2.1 Optimize the magnetic field coupling factor The coupling factor is only related to the mechanical dimensions of the coil arrangement (such as coil diameter, reading distance, coil azimuth angle) and the material near the coil in the magnetic field, and is not related to the inductance of the reader antenna or transponder antenna. In order to improve the coupling factor, the transmission distance should be as small as possible, and the antenna axes of the reader and transponder should be parallel. If the reading distance is determined, the reader antenna coil diameter and magnetic field coupling factor k can be optimized based on this specific distance. The magnetic field strength can be calculated by equation (3): According to equation (3), the magnetic field strength is directly related to the antenna structure, and the magnetic field coupling factor k also depends on the structural dimensions of the coil arrangement, so the magnetic field strength and k are proportional. Optimizing the coupling factor means determining the relationship between the antenna radius and the reading distance when the antenna efficiency is highest. Figure 2 shows the change of magnetic field strength with the coil radius under certain conditions. The measurement conditions in Figure 2 are: fo=125kHz, LR=737μH, r=5～55mm, d=20 mm. It can be seen from Figure 2 that if the reading distance d is constant, the field strength decreases proportionally when rd. Therefore, it can be concluded that the optimal radius of the antenna coil is r≈d. 2.2 Determining the coupling factor of the magnetic field To determine the coupling factor, the test transponder coil (TTC) and circuit provided by Temic can be used for testing. The test principle is shown in Figure 3. The TTC can be placed at the actual transponder position. When the reader antenna works under the excitation of the signal generator, the voltage UT through the TTC can be measured. Figure 4 is the equivalent circuit model of the TTC and the measuring equipment. Cpara is the internal parasitic capacitance of the coil, and Ccable and Cprobe are the cable capacitance and load capacitance of the measuring device, respectively. These capacitances all affect the measured voltage. To make the measurement more accurate, a correction factor Ak is introduced, calculated as follows: Figure 5 shows the results of the measured coupling factor under different reading distances. 2.3 How to meet the actual frequency tolerance Figure 6 is the curve of the total antenna tolerance frequency deviation as a function of the magnetic field coupling factor k when the operating frequency is fixed and the reader inductance is of different values. As can be seen from Figure 6, the overall tolerance frequency deviation increases with the increase of k and decreases with the increase of the reader coil inductance. It is worth noting that the antenna inductance is inversely proportional to the current flowing through the antenna. For U2270B, the maximum antenna current (IRpp) is limited to 400mA. Considering the voltage of the reader antenna coil, the antenna inductance LR cannot be less than 413μH. In Figure 6, the vertical axis represents the total allowable frequency deviation of the antenna, and the horizontal axis represents the magnetic field coupling factor. Any inductance value greater than 413/μH and less than the inductance corresponding to the nearest curve above that point can be selected. After determining LR, with a fixed operating frequency, the antenna capacitance can be calculated using equation (6): where fo≈125kHz. The number of turns of the antenna coil can be calculated using equation (7): 3 Antenna Design Example Assume the following conditions: The allowable frequency deviation of the reader coil is ±3%; the allowable frequency deviation of the transponder coil is ±4%; the nominal reading distance is 20mm. Step 1: To achieve the best magnetic field coupling effect, select a reader coil radius of r=20mm. Step 2: According to Figure 5, the coupling factor k=1.2%. Step 3: Calculate the total allowable frequency deviation as the sum of ±3% and ±4% ±7%. As shown in Figure 6, only the curve with LR=1.24mH is below the point (k=1.2%, ±7%), so LR can take any value between 413μH and 850μH. Here, LR=737μH is chosen. The number of coil turns N=97 can be calculated using equation (7), and CR=2.2nF can be calculated using equation (6). Conclusion This article mainly analyzes the general steps of antenna design for the U2270B RFID system. External interference and other factors may also bring some special problems to the design process. This article only hopes to provide some inspiration for the research of RFID systems. Editor: He Shiping