This paper presents two novel UHF RFID near-field reader antennas with uniform vertical electric field distribution. The two antennas have the following common characteristics. First, the radiating parts of the two antennas are simulated and fabricated by the microstrip lines and work using the leakage wave principle of microstrip lines. Second, the end of microstrip lines match the load to form a traveling wave mode of operation, so the two antennas have broadband characteristics. Third, both antennas are fed in a coaxial manner at the center of the antenna. The simulation and measurement results can show that the proposed three-branch antenna and four-branch antenna achieve good impedance matching in the range of 883–960 MHz and 870–960 MHz, respectively, and achieve uniform distribution of the vertical electric field component in a certain area. The reading areas of the three-branch antenna and the four-branch antenna are 70 mm × 70 mm × 90 mm and 100 mm × 100 mm × 120 mm (length × width × height), respectively. Due to the introduction of the ground plate, the antenna gain is low, which meets the design requirements of near-field antennas.
Radio frequency identification (RFID) is a wireless noncontact communication technology that uses radio frequency signals to identify specific targets and complete data exchange. Because of its excellent characteristics such as no contact, reliable communication, automatic identification of moving targets, and fast reading and writing, RFID technology has received the attention and rapid development since its inception. RFID has become an indispensable part of our lives in today’s era [
In recent years, domestic and foreign experts and scholars have invested in the research boom of near-field RFID antennas [
After all, the near-field field distribution is complicated. In order to achieve 100% reading of tags placed randomly in the reading area, the capacitively coupling RFID reader antenna needs to consider the uniformity of the electric field three-dimensional component. However, the current research mainly focuses on the uniformity of the total electric field strength. In this paper, two novel capacitively coupling RFID reader antennas are proposed. The vertical electric field components excited by two antennas are uniformly distributed in the reading area. And two antennas can achieve 100% reading of tags placed vertically in the reading area. Both antennas are traveling wave antennas with good impedance matching characteristics. At present, few scholars discuss and design the vertical electric field component. This paper considers the uniform distribution of this component separately, so it can provide an important reference value for the research of near-field reader antenna with uniform electric field distribution in the three-dimensional direction.
Figures
Configuration of the three-branch antenna. (a) Schematic diagram of structure. (b) Physical processing diagram.
Configuration of the four-branch antenna. (a) Schematic diagram of structure. (b) Physical processing diagram.
Optimized three-branch antenna parameters (unit: mm).
Parameter | L | L1 | L2 | W0 | h |
---|---|---|---|---|---|
Value | 134.5 | 51.75 | 38.2 | 3.5 | 2 |
Optimized four-branch antenna parameters (unit: mm).
Parameter | L | La | Lb | Lc | W0 | h |
---|---|---|---|---|---|---|
Value | 165 | 49.5 | 49.5 | 38.5 | 1.3 | 2 |
The radiating parts of the two antennas are center-symmetrical, and the coaxial feed is used at the center of the two antennas. The current between the inner and outer conductors of the coaxial structure generates a vertical electric field. Current leaks along the microstrip while radiating. Leakage waves include surface waves in the horizontal direction and space waves in the vertical direction. We use the vertical space waves leaked by the microstrip lines to achieve uniform vertical electric field distribution of the two antennas.
Figure
Current distribution. (a) Three-branch antenna. (b) Four-branch antenna.
To fully evaluate the working performance of the two antennas, they were simulated and measured separately. In this part, we mainly focused on the reflection coefficient, the distribution of the vertical electric field component, and the tag 100% reading area. In the analysis process, we assume that two antennas are placed in the XOY plane, where Ez represents the vertical electric field component.
Figures
Simulated and measured reflection coefficients of the three-branch antenna.
Simulated and measured reflection coefficients of the four-branch antenna.
Different from the far-field RFID antenna, the near-field RFID reader antenna is required to have low gain characteristics, so as to avoid misreading of tags outside the reading area and achieve a controllable reading range. Figure
Simulated far-field gains of three-branch and four-branch antennas.
Figures
Simulated Ez distributions of the three-branch antenna at (a)
Simulated Ez distributions of the four-branch antenna at (a)
We used the proposed two antennas, Impinj Speedway R420 reader, application software, Alien A9662 electrical tag, RF cable, and foam board to build an RFID near-field measurement system in a relatively large space. RF cable was used to connect the reader and antenna, and TCP/IP connections were used to communicate between application software and the reader. The size of the tag used by the measurement system was 70 mm × 17 mm. The foam board was divided into grids, and the dimensions of each grid was 35 mm × 35 mm for three-branch antenna, and 33.3 mm × 33.3 mm for four-branch antenna. To measure the reading performance of the two antennas on vertically placed tags, we set up two measurement systems. Figure
Four-branch antenna measurement setup.
Figures
Three-branch antenna measurement results.
Four-branch antenna measurement results.
The thickness and the dielectric constant of the substrate are the key parameters affecting the space wave leakage of the microstrip lines. These two parameters were simulated, discussed, and analyzed. We took the three-branch antenna as an example for analysis.
In order to analyze the influence of the substrate thickness on the antenna performance, we simulated the reflection coefficient and vertical electric field distribution when the substrate thickness of the three-branch antenna is 0.5 mm, 1.5 mm, 2.5 mm, and 3.5 mm while keeping other parameters in Table
Figure
Simulated reflection coefficients of the three-branch antenna with different substrate thicknesses.
Figures
Simulated Ez distributions of the three-branch antenna at different heights when the substrate thickness
Simulated Ez distributions of the three-branch antenna at different heights when the substrate thickness
Simulated Ez distributions of the three-branch antenna at different heights when the substrate thickness
Simulated Ez distributions of the three-branch antenna at different heights when the substrate thickness
In order to analyze the influence of dielectric constant on the antenna performance, we simulated the reflection coefficient and vertical electric field distribution when the dielectric constant of the three-branch antenna is 1, 3, and 5 while keeping other parameters in Table
Figure
Simulated reflection coefficients of the three-branch antenna with different dielectric constant.
Figures
Simulated Ez distributions of the three-branch antenna at different heights when the dielectric constant is 1. (a)
Simulated Ez distributions of the three-branch antenna at different heights when the dielectric constant is 3. (a)
Simulated Ez distributions of the three-branch antenna at different heights when the dielectric constant is 5. (a)
This article designs two UHF capacitively coupling RFID near-field reader antennas with uniform vertical electric field distribution. Both antennas have broadband characteristics. The vertical electric field components excited by the two antennas are uniformly distributed without zeros, and the tags placed in the vertical direction in the reading area can be read 100%. The two antennas can meet the needs of near-field applications.
The dimension parameters of proposed two antennas’ data used to support the findings of this study are included in Tables
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was funded by the China Scholarship Council and K. C. Wong Foundation.