In recent years, automatic identification methods have seen rapid development across various fields such as service, goods sales, logistics distribution, commerce, manufacturing, and material circulation. Among these, radio frequency identification (RFID) technology has experienced significant growth and has evolved into an independent interdisciplinary field. Key professional areas include high-frequency technology, semiconductor technology, electromagnetic compatibility, data security, telecommunications, and manufacturing. As a critical component of RFID systems, the antenna directly influences the system's performance.
The principle of an RFID system typically involves a reader (PCD) and a transponder (PICC). A standard reader comprises a high-frequency module (transmitter and receiver), a control unit, and a coupling element connected to the transponder. The transponder serves as the actual data carrier in the system and is composed of a coupling element and a microelectronic chip. Unlike the reader, the transponder does not have its own power source; it draws RF power from the reader within its response range. The energy required for the transponder’s operation—such as clock pulses and data—is transmitted non-contact through the coupling unit. Therefore, the antenna, which acts as the coupling element, plays a vital role in the system. The design of the antenna significantly affects the communication distance and the reliability of data transmission. This article will focus on the antenna design for the U2270B RFID base station chip.
An RFID system includes two LC circuits: one is an LRCR circuit formed by the base station coil and a connecting capacitor, and the other is an LTCT circuit formed by the transponder coil and a connecting capacitor. In a single-coil system, both LC circuits must be tuned to the same resonant frequency. If the frequencies do not match, zero modulation can occur, degrading system performance. Once the system is built, the antenna’s inductance is fixed, so adjusting the capacitance in the loop is the only way to change the resonant frequency of the LC circuit.
The reader’s antenna is a series resonant circuit made up of an inductor, a capacitor, and a resistor, as shown in the diagram. Its characteristics are defined by the resonant frequency (fo) and the Q factor. The operating frequency of the RFID system, fo, is determined by the inductance and capacitance of the antenna and can be calculated using the formula:
$$ f_o = \frac{1}{2\pi\sqrt{LC}} $$
Most designs operate the reader at a single frequency, such as 125 kHz for the U2270B. The relationship between the Q factor (QR) and the bandwidth (B) of the antenna is given by $ B = \frac{f_o}{Q_R} $. A higher QR value increases the voltage of the reader antenna, thus enhancing the energy delivered to the transponder. However, a high QR also reduces the antenna's bandwidth, potentially making it harder to demodulate the signal when the transponder frequency shifts, leading to potential operational issues. The coupling factor, which measures the interaction between the reader’s coil and the transponder’s coil, is influenced by the system’s structural parameters and directly affects the reading distance. Optimizing this factor improves both energy transfer and signal transmission. Testing the coupling factor can be done using a test response coil (TTC) and associated circuitry provided by Temic. The QR value should generally be kept between 5 and 15, with 12 being a common choice for most applications. If the antenna’s inductance is known, the QR factor can be adjusted using the following equation:
$$ Q_R = \frac{\sqrt{L R}}{R} $$
Designing an antenna involves determining its mechanical size, number of turns, inductance, and capacitance to achieve optimal working efficiency. The general steps of the design process are outlined below.
**Optimizing the magnetic field coupling factor**
The coupling factor depends solely on the mechanical dimensions of the coil arrangement (e.g., diameter, reading distance, and coil orientation) and the materials near the coil in the magnetic field, regardless of the reader or transponder antenna inductance. To increase the coupling factor, the reading distance should be minimized, and the axes of the reader and transponder antennas should be aligned. When the reading distance is fixed, the reader antenna coil diameter and the magnetic field coupling factor k can be optimized accordingly. The magnetic field strength can be calculated using the following equation:
$$ B = \frac{\mu_0 N I}{2r} $$
Where μ₀ is the permeability of free space, N is the number of turns, I is the current, and r is the radius of the coil. The magnetic field coupling factor k is proportional to the magnetic field strength, and optimizing k involves finding the best balance between the coil radius and the reading distance. As shown in Figure 2, the magnetic field strength decreases proportionally with increasing coil radius when the reading distance is constant. It can be concluded that the optimal coil radius is approximately equal to the reading distance.
**Determining the magnetic field coupling factor**
To measure the coupling factor, the test response coil (TTC) and associated circuitry provided by Temic can be used. The test setup is illustrated in Figure 3, where the TTC is placed in the position of the actual transponder. When the reader antenna is activated, the voltage UT through the TTC can be measured. An equivalent circuit model of the TTC connected to the measuring device is shown in Figure 4. Cpara represents the internal parasitic capacitance of the coil, while Ccable and Cprobe are the cable and load capacitances of the measuring device. These capacitances affect the measured voltage, so a correction factor Ak is introduced to improve measurement accuracy.
**Meeting actual frequency tolerance**
Figure 6 illustrates the allowable frequency offset of the antenna as a function of the magnetic field coupling factor k when the operating frequency is fixed and the reader inductance varies. As shown, the total allowable frequency offset increases with higher k values but decreases as the reader inductance increases. For the U2270B, the maximum antenna current (IRpp) is limited to 400 mA, and the minimum inductance LR must be at least 413 μH. After determining LR, the antenna capacitance can be calculated using the following formula:
$$ C = \frac{1}{(2\pi f_o)^2 L} $$
Where fo ≈ 125 kHz. The number of turns of the antenna coil can be calculated using the formula:
$$ N = \frac{L}{\mu_0 \cdot r^2} $$
**Antenna Design Example**
Assuming the following conditions:
- Allowable frequency offset of the reader coil: ±3%
- Transmissible frequency offset of the transponder coil: ±4%
- Nominal reading distance: 20 mm
Step 1: Choose a reader coil radius of r = 20 mm to optimize the magnetic field coupling effect.
Step 2: From Figure 5, determine the coupling factor k = 1.2%.
Step 3: Calculate the total allowable frequency offset as ±7% (sum of ±3% and ±4%). Based on Figure 6, select LR = 737 μH and CR = 2.2 nF. The number of turns N is calculated to be 97.
**Conclusion**
This paper outlines the general steps for designing antennas in RFID systems using the U2270B chip. While external disturbances may introduce additional challenges, this article aims to provide useful insights for further research in RFID system design.
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