RFID applications are increasing, and market size is also expanding, and in terms of technical requirements are also diverse. This paper proposes an ultra-small 433 MHz PCB antenna, a gain to -17 dB, and reaches the application requirements of the RFID system. The antenna radius is 14 mm semicircular regions, small size, and meets two aspects of label miniaturization and antenna performance.
Active RF Identity Positioning system is now widely used in various positioning scenarios. In response to the demand of electronic tag miniaturization in the actual scene, the miniaturization design of the label PCB antenna is realized in the half circle of the radius of 14 mm, and the gain reaches -17 dB. Based on the collective component circuit, the antenna achieved 433 MHz The resonance characteristics, and the label antenna and the label chip have achieved 50 Ω impedance matching.
That is, RFIo Frequency Identification, RFID, is a non-contact automatic identification technology, and the non-contact bidirectional data communication is performed by wireless radio frequency mode, and the target is identified and acquired, which has low cost and high positioning accuracy. advantage. Active RFID positioning system has been widely used in various positioning scenarios. The antenna is an important part of the RFID system, which has the development of RFID technology in many scenarios, so the research and design of RFID antennas is very urgent. The current RFID system is mainly concentrated in low frequency, high frequency, ultra-high frequency and microwave bands, and has a great difference in antenna design on these bands. This paper system is designed to communicate with ultra-high frequency sections, and is currently in the UHF band, and there is a multi-pole and its deformed structure, such as a bending line antenna, a reticulated dipole antenna. The label miniaturization antenna of the ultra high frequency band 433 MHz is designed to meet the requirements of the label miniaturization and antenna performance.
1 label antenna design idea
1.1 Label Antenna Radiation Gain Determination
The antenna gain G is defined as: Under the same conditions as the input power, the power flux density of a certain point in the maximum radiation direction and the power flux density at the same point source antenna at the same point. General RFID system maximum transmission distance is expressed as
The label antenna transmit signal, the reader antenna receives the signal. In the corresponding (1), PTH is the minimum received power of the antenna specified by the reader RF chip, and -110 dBm · w; Pt is the antenna transmit power, adjustable, -20 ~ 3 dBm · W; GT is the label antenna gain to be designed; GT is an existing reader antenna gain, 3 dBi; λ is the working wavelength; R is the working distance design target between the label and the reader, 50 m; operating frequency is 433MHz. The reader antenna in the RFID system is a circularly polarized antenna, the labeling antenna is a line polarized antenna, and the two will generate the polarization loss loss of 3 dB. According to the computational formula and actual scene, finally determine the label antenna gain is -19 dB, which can meet the application needs.
1.2 Label Antenna Impedance Matching
The label antenna area design goal is a semicircular region having a radius of 14 mm. The 433 MHz labeling antenna is used in the predetermined area of the antenna structure in the specified area. The number of bent lines, bending angles, bending height, and bending line width in the bending line antenna are largely affected the resonance characteristics of the antenna. With the conditions of the antenna shape, the resonant frequency of the bending line is decreased as the number of bent line antennas, the resonance impedance decreases, but the trend of the decline is slow, and the resonance frequency of the antenna is basically the result when the number of bending is continued. constant. Also under the conditions of the antenna size, change the bending height and the bending angle, the result is similar, all as the start of the bend height and the bending angle increase, the antenna resonant frequency and the impedance have decreased significantly, then trend gradually Success, the final basics will not change. Since the antenna area is small, the use of the bending line in a limited area cannot reach the resonant frequency of 433 MHz, so it is necessary to increase the corresponding matching circuit when designing.
In addition, the electronic label board is designed in the radio circuit and the feeder, the impedance between the feed line and the antenna should achieve conjugate matching, the antenna can obtain the maximum power transmission, the maximum energy of the radiation. When the RF circuit is mounted with antenna impedance, the energy of the RF circuit will not be emitted by the antenna radiation, and the energy of this part reflects the radiofrequency circuit, producing a standing wave, and a large damage to the circuit. Therefore, in order to make the signal and energy efficiently, the circuit must be operated in an impedance matching state, that is, the chip impedance connected to the antenna must match the antenna impedance conjugate. The chip impedance is adjusted to 50 Ω and the antenna input impedance is adjusted to 50 Ω, thereby achieving impedance matching.
2 label antenna structure design
The electronic label employs the FR4-epoxy circuit medium substrate (FR4_expoy) thickness of 0.8 mm; dielectric constant is 4.4; the line width is 0.5 mm. After a long-term winding, the antenna geometry is obtained as shown in Figure 1.
Figure 1 Design antenna structure diagram
This structure takes advantage of the electronic tag shape, the gain of the antenna is shown in Figure 2.
Figure 2 Antenna gain map
As can be seen from Fig. 2, the antenna gain is -15.7 dB, all-to-axis radiation. The current distribution on the antenna is shown in Figure 3.
Figure 3 Surface current density
The antenna resonant image is shown in Figure 4.
In Fig. 4, the antenna is exactly, and there is no resonance in 433 MHz, and it is necessary to increase the corresponding matching circuit.
Figure 4 Design Antenna S11 Curve
3 radio frequency circuit impedance matching
The antenna 433 MHz is resoned to match the circuit tuning accordingly. Here, the SMITH circle is tuning and impedance matching, first of all, the impedance value of the structural antenna is at 433 MHz. The impedance of the antenna at each frequency point can be found in the HFSS simulation results, as shown in Figure 5.
Figure 5 Impedance of 400 ~ 460 MHz
Figure 5 is the impedance value corresponding to the 433 MHz frequency point, ZT = 2.832 004-I2222.484 839, the antenna is small, and the accumulation resistance is present. Here, the ADS is used to impedance matching. The design principle is that after antenna increases the matching circuit, it makes up a new circuit structure, and the entire circuit resonates at 433 MHz, and the impedance reaches 50 Ω, thereby achieving impedance matching. The ADS schematic represents the impedance of the antenna in the aggregation element, the specific design method is the antenna equivalent to a resistor and capacitance, with a resistor R, and the capacitance is C.
Calculate, R = 2.832 004 Ω, C = 1.65 pf.
The aggregate components, smith chart match circles are connected to the ADS, and the specific structure is shown in Figure 6.
Figure 6 ADS NAI Smith matches Smith Chart match
Figure 6 The circuit structure is matched by Smith Chart, and four matching structures are provided in ADS, as shown in Figure 7.
Figure 7 4 Matching Network Structure Diagram
According to the L-type matching circuit, it is designed to use the inductance of inductive inductance in the upper right corner. After connecting the matching circuit to the antenna, the resonant frequency and bandwidth of the antenna at this time, as shown in FIG.
Figure 8 Antenna resonance S11 graph
Figure 8 shows that the simulated antenna resonance point is 433 MHz, and the antenna bandwidth is 2 MHz specific ranges of 432 to 434 MHz, and the results conform to the design requirements of communication frequencies in the active RFID system. However, the ADS is automatically adjusted by the input value due to the presence of accuracy, so it only uses the matching circuit structure diagram thereof, and the specific component value needs to be actually debugged. The actual commission is commissioned in the same matching circuit structure with the vector network analyzer. Prepare an antenna plate with a matching circuit inductor. Try the matching circuit according to the following steps: (1) Start the vector network analyzer. (2) Connect the coaxial transmission line outer conductor in the vector network analysis to the ground terminal, connect the inner conductor to the circuit feeder end. (3) One of the inductors in the welding matching circuit, adjust the other inductive value according to the SMITH chart in the vector network analysis until the resonant frequency is 433 MHz, and the debug is completed. After debugging, L3 is 30 NH, L2 is 12 NH during the debugging process. The final result is shown in Figure 9.
Figure 9 Matching circuit after actual debugging
The gain of the antenna in the actual debug is -17 dB, which is reduced when simulating. When the antenna is resonant at 433.92 MHz, the bandwidth of the antenna is narrow, and the results are reduced by 400 kHz than the simulation results. Moreover, the inductance value in the matching circuit changes a large change, because the ADS performs the default impedance real part of the SMITH circle to a minimum of 5.3, while the actual antenna is only 2.8, and a large error occurs. Even so, the antennas designed in the article also meet the application requirements of the active RFID positioning system. The real object is shown in Figure 10.
Fig. 10 Introduction
4 Conclusion
Antenna interface
RFID radio frequency identification
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