Application of absorbing materials in RFID tags

Absorbing materials are widely used in various aspects such as noise reduction, wave absorption and EMC of electronic equipment, and experts have also made many models to explain the working principle of them, forming a lot of theoretical knowledge, but the disadvantage is that these theories It is more complicated and difficult for some readers who are not in the field to understand.

The RFID system consists of an electronic tag or non-contact smart card (such as a smart phone with swiping function) placed on an identified object and a device that issues instructions to the electronic tag and collects feedback information from the electronic tag. This device is also called It consists of two parts: RFID reader or reader. In order to allow other devices to display or use these data, it is generally possible to have an external RS232 protocol interface on the reader, so that information can be transferred with external devices.

Because it is a passive electronic tag, the energy required by the chip and memory in the electronic tag needs to be provided by the reader. The communication between the reader and the electronic tag is realized through the principle of electromagnetic coupling. The energy of the electronic tag Produced by the reader coil antenna through electromagnetic coupling.

The high-frequency electromagnetic field is generated by the 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 the tag's operating frequency of 13.56MHz, its wavelength is 22.1m, which is much longer than the distance between the reader's antenna and the electronic tag, so the electromagnetic field between the reader's and the antenna can be treated as a simple alternating magnetic field.

By adjusting the antenna coil and capacitor of the electronic tag to form a resonant circuit, it is tuned to the transmission frequency of 13.56MHz specified by the reader, so that according to the resonance of the circuit, the voltage generated by the coil inductance in the tag reaches the maximum value. The power transmission efficiency between the antenna coil of the reader and the electronic tag is proportional to the number of turns of the coil in the tag, the area enclosed by the coil, the relative angle between the two placed and the distance between each other, which is also The reason why the RFID tag reading distance has a certain limit.

For the RFID electronic tag used at 13.56MHz, its maximum reading and writing distance is usually about 10 cm, and the current consumption of the chip is roughly 1 mA. Because as the frequency increases, the required inductance of the electronic tag coil is shown as a decrease in the number of turns of the coil, usually at this frequency, the typical number of turns is 3-10 turns.

The RFID tag reading distance is not only related to itself, but also to the environment in which it is located. When using an inductively coupled radio frequency identification system, such a requirement is often put forward: The antenna of the reader or electronic tag is directly mounted on a metal surface. However, it is impossible to mount the magnetic antenna directly on the metal surface.

Because the antenna magnetic flux passing through the metal surface will produce induced eddy currents. According to Lenz’s law, the eddy current will react to the antenna’s field and rapidly attenuate the magnetic field on the metal surface, so that the data between the reader and the electronic tag The reading distance will be severely affected, and even misreading or reading failure may occur. Regardless of the magnetic field generated by the coil itself installed on the metal surface or the field approaching the metal plate from the outside (the electronic tag is on the metal surface), the result is the same.
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