Design analysis based on RFID antenna antenna

With the rapid development of radio frequency identification (RFID) technology, radio frequency identification systems have become more and more widely used. Due to the high read rate and long read distance of the decimeter wave band (UHF) RFID system, more and more studies on UHF band RFID systems have been made in recent years. Passive RFID tags usually consist of an RFID tag chip and an RFID tag antenna. The design of the RFID tag antenna plays an important role in the RFID system. Recently, more and more researches have been done on the RFID tag antenna, especially for the 900 MHz band RFID tag antenna. Some dipole-based tag antennas have been successfully used in many RFID systems, but in some special applications, the impedance characteristics of the tag antenna are greatly affected when the tag is near a metal surface or is attached to a metal surface. The effect is that the reading distance is greatly reduced, and even the work cannot be performed. This problem has attracted the attention of many researchers. In some articles, RFID tag antennas that can be placed on metal surfaces have been proposed. These antennas themselves have a metallic ground structure and therefore can work well when placed on a metal surface. However, the real value of the impedance of the RFID tag chip matched by these antennas is relatively small, generally less than 20. The impedance real part of some existing RFID tag chips is close to 40. The real part of the impedance of the RFID tag chip used in the RFID tag antenna proposed in this paper is 42.

On the other hand, with the development of the personal electronic communication field, wearable antennas have also been studied more and more. In the field of RFID, wearable RFID tags also have great development prospects. In addition to changing the antenna characteristics when the RFID tag antenna is close to the metal, the general RFID tag antenna also makes the impedance characteristics greatly change in the case of being close to the human body, so that the reading distance is greatly shortened.

This paper proposes an RFID tag antenna that can be worn on the wrist. Firstly, the planar structure of the proposed antenna is studied. Then, the situation when the antenna is worn on the wrist is simulated and analyzed, and the actual measurement of the antenna is made. .

The antenna designed in this paper is designed for an RFID tag chip working at 915MHz. The input impedance of this chip is 42-j157Ω. By adjusting the parameters, the antenna proposed in this paper can be easily matched with chips with other input impedance values. The planar structure of the antenna is shown in Figure 1.

Figure 1 RFID tag antenna plane structure

The antenna is mainly composed of three parts, a floor, a main radiation patch and a coupled feed patch. The main radiation patch is two symmetrical patches, and the interval between the two patches is 1 mm. The outer end of the patch is connected to the floor by a shorting film. Between the main radiation patch and the ground is a foam layer support. The main radiating patch has a pair of symmetrical coupling feeding patches, which are separated by 1 mm. The label chip feeds symmetrically to the coupling feeding patch, and the capacitive coupling between the coupling patch and the radiation patch transmits energy. The FR4 dielectric plate is placed between the main radiation patch and the coupling patch, er=4.4. By adjusting the length L1 of the primary radiating patch, the resonant frequency of the antenna can be adjusted, and the input impedance of the antenna is adjusted by the parameters W1, Lc. The antenna size designed in this paper is shown in Table 1.

Table 1 RFID tag antenna size parameters (unit: mm)

LL1LcW1WHH1

14211330203022.5

Because when the antenna is worn on the wrist, the shape of the antenna will change greatly. Therefore, in addition to considering the characteristics of the antenna when it is in a planar structure, the antenna design needs further consideration when the antenna is worn on the wrist. Deformed properties. In the simulation study, the ring structure shown in Figure 2 was used to simulate the situation when the antenna was worn on the wrist. The return loss of the antenna in different shapes is shown in Figure 3.

Figure 2 The simulated structure of the antenna after deformation

Figure 3 Return Loss Simulation Results

From the simulation results in Figure 3, it can be seen that when the antenna is deformed, its impedance characteristics will be affected. However, it can be seen that the design proposed in this paper is still able to maintain a good return loss characteristic when the antenna is bent into a wrist shape. Antenna pattern simulation results shown in Figure 4

Figure 4 Antenna pattern simulation results

In order to verify the working effect of designing the antenna, based on the simulation, a deformable RFID antenna was fabricated according to the size given in Table 1. The antenna uses foam double-sided tape as a support between the radiating patch and the floor so that the antenna can be bent and can be worn on the wrist for measurement. Figure 4 shows a photograph of the manufactured RFID antenna.

Figure 5 Photo of the manufactured RFID antenna

The CSL461 reader was used to verify the RFID tag antenna. The transmit power of the reader was set to 30dBm, and the gain of the antenna connected to the reader was 6dBi. When the antenna was kept flat, the maximum reading distance was measured. It is 2.5m. When the antenna is deformed into a ring structure and carried on the wrist, the maximum reading distance is 1.5m. The measurement results show that the antenna structure proposed in this paper can effectively reduce the impact of the human body on the performance of the antenna. The antenna still has acceptable reading distance when worn on the wrist, so it can be used in hand-worn RFID tags. Get applied. In addition, since the RFID antenna proposed herein also has a metal floor structure, it also has excellent characteristics that can avoid the influence of the metal surface.

This paper proposes and designs an RFID tag antenna that can be adapted to hand wear applications. From the simulation results, it is found that when the designed antenna is deformed and in a ring shape, it will have a certain influence on its impedance characteristics, but it can still better match the input impedance of the tag chip, that is, it has a good return loss characteristic. In order to verify the working effect of the designed antenna, the actual measurement of the manufactured antenna with a tagged chip was performed. The measurement results show that the antenna can achieve a maximum read distance of 2.5m when there is no deformation. When deformed and worn on the wrist, the maximum reading distance can be kept at 1.5m, and it remains within an acceptable reading range. Starting from the actual verification, the antenna structure proposed in this paper can effectively reduce the human body. The effect of antenna performance can be applied to hand-worn RFID tags.