Design of RFID-based tag antenna

In modern society, products are becoming more and more abundant, and the demand for data management is also increasing. People need to identify, manage and locate a variety of items in the process of production, sales and circulation. Using traditional barcodes to identify items will bring a series of inconveniences: it is impossible to identify at a long distance, manual intervention is required, many items cannot be identified, etc. On the contrary, since the radio frequency identification fRFID1 system uses penetrating electromagnetic waves for identification, it can identify at a long distance without manual intervention, and can identify a variety of items.

Radio frequency identification technology is a contactless automatic identification technology. It is a short-range wireless communication system composed of electronic tags (Tag/Transponder), readers/Interrogators and middleware. The tag in radio frequency identification is a combination of radio frequency identification tag chip and tag antenna. Tags are divided into active tags and passive tags according to their working modes. Active tags carry batteries to provide them with the energy required for reader communication: passive tags use inductive coupling or backscattering working mode, that is, the energy is activated by the electromagnetic field or electromagnetic waves emitted by the tag antenna from the reader, and the matching degree between the radio frequency identification tag chip and the tag antenna is adjusted to feed back the information stored in the tag chip to the reader. Therefore, the impedance of the radio frequency identification tag antenna must be conjugate matched with the input impedance of the tag chip so that the tag chip can maximize the electromagnetic energy emitted by the radio frequency identification reader. In addition, the application of the electronic tag must be considered when designing the tag antenna. For example, there are great differences in the structure and material selection of the tag antenna used on the surface of metal objects and the tag antenna used on the surface of ordinary objects. The low-cost, multi-purpose tag antenna suitable for a variety of chips is one of the key technologies for the widespread popularization of radio frequency identification in my country.

RFID System and Antenna Classification

For the radio frequency identification system using passive tags, there are two working modes according to the working frequency band. One is the inductive coupling mode, which is also called the near-field working mode and is mainly applicable to low-frequency and high-frequency RFID systems. The other is the backscattering mode, which is also called the far-field working mode and is mainly applicable to ultra-high frequency and microwave RFID systems.

Inductive coupling mode mainly refers to the reader antenna and tag antenna both adopt coil form. When the reader reads the tag, it sends out an unmodulated signal. After the electronic tag antenna in the near field of the reader antenna receives the signal and activates the tag chip, the tag chip controls the current in the tag antenna according to the globally unique identification number (ID) stored internally. The magnitude of this current further enhances or reduces the magnetic field emitted by the reader antenna. At this time, the near-field component of the reader shows the characteristics of being modulated, and the internal circuit of the reader detects the modulation amount generated by the tag and demodulates it to obtain the tag information.

In the backscattering T mode, electromagnetic waves are used to transmit information between the reader and the electronic tag. When the reader reads and identifies the tag, it first sends out an unmodulated electromagnetic wave. At this time, the electronic tag antenna located in the far field receives the electromagnetic wave signal and generates an induced voltage on the antenna. The internal circuit of the electronic tag rectifies and amplifies this induced voltage to activate the tag chip. After the tag chip is activated, the impedance of the tag chip is changed with its own global unique identification number. When the impedance of the electronic tag chip and the impedance between the tag chips are well matched, the signal is basically not reflected, and when the impedance matching is not good, almost all the signals are reflected. In this way, the reflected signal has an amplitude change, which is similar to the amplitude modulation processing of the reflected signal. The reader determines the identification number of the electronic tag and identifies it by receiving the modulated reflected signal. This type of antenna mainly includes microstrip antennas, planar dipole antennas and loop antennas. Figure 2 is a UHF electronic tag antenna developed by us that can work in a variety of identification environments.

Design and testing of electronic tag antenna

As mentioned above, the RFID systems operating at low and high frequencies use inductive coupling mode for communication, so the readers and electronic tags operating at these two frequency bands all use coil antennas. The RFID systems operating at these two frequency bands are restricted by the range of the near field, resulting in a shorter recognition distance. According to the current situation, the maximum recognition distance of the RFID system using near field communication is less than 1 meter.

Since the low-frequency and high-frequency RFID systems use electromagnetic field coupling mode, the antennas in the system are all in coil form. The main reasons for using this form are as follows:

1. The coupling of electromagnetic fields is relatively tight between coils:

2. The antenna adopts the form of coil, which further reduces the volume of the antenna and thus reduces the volume of the tag:

3. The characteristics of the tag chip require that the tag antenna has a certain reactance.

In the UHF and microwave bands, the communication between the electronic tag and the reader adopts the backscattering working mode. At this time, the bridge between the electronic tag and the reader is no longer the near magnetic field but the electromagnetic wave. At this time, the passive electronic tag is in the far field of the electromagnetic wave of the reader. According to the wavelength of the frequency band and the caliber of the antenna, the distance between the far field of the RFID system and the reader in the frequency band can be calculated. Generally speaking, the working distance of passive tags in the UHF range can reach about 10 meters, according to existing data. The working distance of passive tags working in the microwave band (mainly 2.45GHz) is only about 1 meter. Therefore, the RFID system currently using the backscattering working mode mainly uses the UHF band located at 860~960MHz.

In the RFID system composed of passive tag antennas, the tag needs to obtain energy from the electromagnetic field or electromagnetic waves generated by the writer to activate the tag chip. Therefore, there is a part of the circuit in the electronic tag that is specifically used to detect the induced electromotive force or induced voltage on the tag antenna, and rectify it through the diode circuit and amplify the voltage through other circuits. These circuits are integrated inside the tag chip. When the chip is packaged, some distributed capacitance is usually introduced. However, the antenna design itself does not need to know the specific circuit in the chip, but only needs to master the chip and the chip impedance after packaging, and use the law of maximum energy transfer to design the input impedance of the antenna.

Since the output impedance of the electronic tag chip has a reactance component, in order to achieve the maximum energy transfer, the input impedance of the antenna needs to be designed as the conjugate of the tag chip impedance. Generally speaking, the input impedance of the electronic tag chip is in the form of Z=R_X. In order to obtain the conjugate impedance, the impedance of the electronic tag antenna should be in the form of Z=R+iX.

As mentioned above, the passive tag antenna working in the low-frequency and high-frequency RFID system adopts a coil form, which can introduce inductive reactance to offset the capacitive reactance in the equivalent circuit to achieve maximum energy transfer between the tag chip and the antenna.

For the tag antennas operating in the UHF and microwave bands, in order to introduce inductive reactance to offset the capacitive reactance of the chip, it is necessary to add a ring structure for inductive feeding or a T-type matching structure in the antenna design. In addition, in order to obtain a longer reading distance under the specified equivalent isotropic radiated power (EIRP), in addition to requiring the electronic tag antenna to have high gain, it is also required that there is sufficient matching between the electronic tag antenna and the tag chip.

After the tag antenna is designed and simulated and the ideal results are obtained, the antenna needs to be processed and tested to verify the correctness of the design and simulation. Because the tag antenna introduced in the previous article has the characteristic of complex impedance, its test method is different from the test method of the antenna with real impedance. In addition, in the test process of the same tag antenna, the test method is also different according to the required data. Usually, it is not necessary to specifically test the input impedance of the antenna during the antenna test. However, the impedance of the tag antenna is a negative impedance, and the ratio of its imaginary part to its real part is large (usually X/R>10). Such an impedance curve is close to the short-circuit circle in the Smith circle diagram, and it is not easy to observe the impedance bandwidth of the antenna through the Smith network diagram. In order to obtain the input impedance of the tag antenna, the output port of the test equipment can be directly connected to the input port of the antenna. Since this method does not take into account the characteristic that the tag antenna itself has complex impedance, there is no conjugate matching between the antenna and the test equipment. At this time, only the impedance parameters of the antenna can be obtained, and circuit parameters such as scattering matrix parameters and standing wave ratio, which are commonly used to measure the antenna, cannot be directly obtained.

In order to obtain circuit parameters such as scattering parameters and standing wave ratio, so as to evaluate the impedance bandwidth characteristics of the antenna, the measured impedance parameters can be substituted into the relevant formula for calculation or the impedance matching method can be used to add a matching circuit between the test equipment and the antenna. The matching circuit can be constructed in two ways, one is to use discrete components with higher operating frequencies, and the other is to use microwave circuits. It should be noted that the matching circuit should be close enough to the antenna port. In this way, a larger bandwidth can be obtained and the negative impact of the connection line between the antenna and the matching circuit can be avoided.

The circuit is used to test the tag antenna. However, the matching circuit has some disadvantages:

1. Regardless of whether discrete components or microwave circuits are used to construct impedance matching circuits, their bandwidth is always limited. When the actual bandwidth of the antenna is greater than the bandwidth of the matching circuit, the measured bandwidth will no longer be accurate;

2. Because there is always loss in the distribution circuit, the parameters such as bandwidth and return loss value obtained by testing are slightly different from the actual antenna parameters;

3. There is always a distance between the introduced distribution circuit and the antenna, which causes a certain error in the test.

In addition to obtaining parameters such as bandwidth and co-wave loss with a certain degree of accuracy, the above-mentioned scheme of using matching circuits for testing is also necessary for testing the radiation characteristics of the antenna, such as the directivity pattern and gain. Only through impedance matching circuits can most of the energy received by the antenna be transmitted to the test system without reflection, thereby testing the corresponding radiation parameters.

Conclusion

As the application of radio frequency identification technology continues to expand, more and more occasions require the use of radio frequency identification systems. As an indispensable part of the radio frequency identification system, the design, production, and testing of electronic tag antennas are one of the main contents of future research. Due to the inherent characteristics of electromagnetic waves, the performance of radio frequency identification systems will be greatly reduced in environments such as near metals and liquids. In such an environment, in addition to improving the performance of the reader, it is more important to improve the performance of the electronic tag antenna. We are currently conducting research on the application of electronic tag antennas in these complex environments. In addition, the performance of flexible electronic tags will also deteriorate when attached to non-flat surfaces. How to avoid the impact of flexible tags applied to non-flat surfaces is also another focus of our current research.