Application of absorbing materials in RFID tags

- Apr 25, 2018-

RFID industry in China after more than ten years of development, now the technology is relatively mature, especially in the past two years, with continuous advance of various factors on the Internet of things, it has maintained a steady rise in the development of the situation.

The industry also reached consensus on the frequency standard of RFID. The current international comparison is 13.56MHz. The high frequency of 13.56 MHz RFID technology with stable performance and reasonable price, the read distance range and actual application scope, so the RFID technology is widely used in public transportation card, mobile payment, especially in South Korea, Japan.

RFID tags are often used in metal environments, and when RFID tags are close to metal, they are strongly reflective of electromagnetic waves. When electromagnetic interference is serious, RFID tag reads will fail. The current general solution is to attach a magnetically absorbing material to the back of the RFID tag.

The absorbing material has many applications in electronic equipment noise reduction, absorbing wave and EMC. The experts have also made many models to explain the working principle, which has formed a lot of theoretical knowledge. But the downside is that these theories are complicated, and it's hard to understand.


The composition of the RFID system

RFID systems are devices that place an electronic tag or a non-contact smart card that is placed on an identified object, and a device for sending instructions and collecting signals, also known as an RFID reader or reader. As shown in figure 1, in order to allow other devices to display or use these data, usually can read and write on the external interface with RS232 protocol, so that you can communication with external devices.


 Figure 1

Because RFID is a passive electronic tag. So electronic tag chip and memory work in the energy needed to require provided by reading and writing, speaking, reading and writing device with electronic tag communication is done by electromagnetic coupling principle, the energy of the electronic tag antenna coil by reading and writing by electromagnetic coupling.

The high frequency electromagnetic field is generated by an antenna coil of the reader, and then the magnetic field passes through the cross section of the coil and the space around the coil. According to label the use frequency of 13.56 MHz, its wavelength is 22.1 m, of electronic tag antenna and the distance is far greater than to read and write device, so you can read and write device to the antenna distance between electromagnetic field as a simple alternating magnetic field to deal with.

Figure 2 provides an energy model for the electronic tag for the reader.


Figure 2

By adjusting the RFID tag antenna coil and capacitor resonant circuit, tuned to read and write device specifies the transmission frequency of 13.56 MHz, according to the resonance of the loop, so in the tag coil inductance on the voltage peak. And legibility of the antenna coil and label the power transfer efficiency between them and the label in the area of the number of turns, surrounded by a coil of wire coil, the place is directly proportional to the relative Angle and the distance between each other, this is also the RFID tag card read distance have a certain limit.

For the RFID tags used in 13.56MHz, the maximum read and write distance is usually about 10cm, and the current consumption of the chip is approximately 1 milliamp. As the frequency increases, the inductance of the required label coil is reduced by the winding number of turns, usually at the frequency of 3 ~ 10 turns.

RFID tag read card distance is not only related to itself, but also has a great relationship with the environment. When using an inductively coupled radio frequency identification system, it is often required to install the reader or tag antenna directly on the metal surface. However, it is impossible to install a magnetic antenna directly on a metal surface.

Because antenna flux through the metal surface will produce eddy current sensor, according to lenz's law, the eddy current be adversely affected for the field antenna, and makes the metal on the surface of the magnetic field decay quickly, so that between the read/write device and label data read distance will be affected by severe, may even be misinterpreted or read failure. The result is the same regardless of the magnetic field generated by the coil itself on the metal surface, or the field near the metal plate from the outside (the RFID tag is on the metal surface).


The working principle of absorbing materials in RFID

The absorbing material is a magnetic functional material with high permeability. It is usually filled with some absorbent in the polymer material and made by special process. Compared with the traditional absorbing material, this class is different in performance characterization and operation principle for 13.56MHz high performance absorbing materials.

Traditional absorbing material, is the main application object on the military confrontation, to cover up, confused each other some of radar reconnaissance aircraft, warships and armored tanks, with the use of high frequency microwave segment, and analytical and far field model.

Absorbing material mentioned in this paper, mainly for civilian electronic equipment inside for providing magnetic path magnetizer, is in the use of high frequency magnetic permeability, low magnetic loss, while higher than when using frequency, loss will increase wait for a characteristic, is the nature of the low-pass filter. However, due to its advantages of flexibility and convenient installation, more and more research and development engineers have been attracted to it.

As shown in figure 3, figure 3 (a) a non-metallic and non-magnetic object basic not affected to the transmission of electromagnetic field, or in accordance with the original direction, the equivalent of electromagnetic wave propagation in free space, so the energy of the electromagnetic field and the direction is not disturbed. FIG. 3(b) is a metal plate with good conductive properties on the basis of FIG. 3(a), and it can be clearly seen that the direction of the magnetic force line has changed greatly. The main performance of the metal plate before and after the magnetic field changes, this is known as the shield phenomenon.

Behind the metal plate without magnetic field, and also to face the direction of the incident electromagnetic field because produced eddy current lead to produce a metal plate with the incident electromagnetic field in the opposite direction of electromagnetic field, thereby weakening magnetic field, or even completely offset the original magnetic field. The problem is shown in figure 3 (c), from the solution, namely in the face of the incident electromagnetic field direction metal plate surface with absorbing material (film), can effectively provide effective path for magnetic transmission, so as a result of the existence of absorbing material, effectively avoid the metal plate of eddy current effect.


 figure 3

Similarly, in the RFID tag near the sheet metal, seen as shown in figure 4 (a), the same will happen over a similar effect, and the resonance frequency of the coil fr can change, fr will move in the direction of low frequency, at this point, the communication capability of electronic tag drops greatly, card read distance severely disrupted.

Through between the coil and the metal surface insert the high permeability of magnetic material, as shown in figure 4 (b), will be able to avoid the generation of eddy current, to a great extent to label also can be safely used on metal surface. When install the antenna on the magnetic sheet should be paid attention to: back to the inductance of the coil antenna due to the high permeability of magnetic material will be increased, so need to adjust the resonant frequency or together with the matching network (within the read/write device needs to be determined).


 figure 4

After years of development, absorbing materials have made great progress, but as the demand for electromagnetic wave shielding is higher and higher, the absorbing materials will become more and more important. The absorbing materials will also be developed in the direction of thin thickness, high performance and light weight.