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In the RFID system, the working distance between the reader and the electronic tag is an important indicator of RFID application.
This distance is closely related to the cooperation between the RFID tag and the reader. According to the distance of the system, the coupling between the tag antenna and the reader can be divided into three types: close coupling system, remote coupling system and long-distance system.
1) Close coupling system
The close coupling system is a radio frequency identification system with a very small working distance, and its typical working distance range is 0 to 1 cm. The close coupling system works by using the inductive coupling (closed magnetic circuit) between the radio frequency tag and the reactive near field area of the reader antenna to form a contactless spatial information transmission radio frequency channel. The operating frequency of the close coupling system is generally limited to frequencies below 30MHz.
Close coupling has less electromagnetic leakage and large coupling energy, and is suitable for applications with high security and no requirements for working distance: electronic gating.
2) Remote coupling system
The typical working distance of the remote coupling system can reach 1m. All remote coupling systems are inductive (magnetic) coupling between the reader and the tag. The transmission frequency of the remote coupling system usually uses frequencies below 135KHz, or uses 6.75MHz, 13.56MHz and 27.125MHz.
The remote coupling system can be further divided into a close coupling system (typical working distance is 15cm) and a sparse coupling system (typical working distance is 1m).
3) Long-distance system
The typical working distance of long-distance system is 1 to 10 meters, and some systems have a longer working distance.
All long-distance systems work by using the electromagnetic field coupling between the tag and the reader antenna radiation far field (emission and reflection of electromagnetic waves, also known as backscatter coupling) to form a contactless spatial information transmission channel. The typical operating frequencies of long-distance systems are 915MHz (which is not allowed in Europe), 2.45GHz and 5.8GHz. In addition, there are some other frequencies, such as 433MHZ.
For ease of memorization, the above content is simplified below.
1. According to the distance from the observation point to the antenna and the wavelength of the electromagnetic wave, the electromagnetic field area is divided into the near field area and the far field area
2. The communication principle in the near field is similar to the inverse conversion of electric field and magnetic field in a transformer; the far field transmits energy and information through the radiation of the electric field.
(The energy coupling method is inductive coupling. The RFID reader transmits electromagnetic variable field signals with overlapping energy and information through the antenna (coil), and the RFID electronic tag obtains the electromagnetic variable field signal through the antenna (coil) to generate induced current and read the signal; the electromagnetic field in the far field breaks away from the constraints of the antenna and enters the space, transmitting energy and information through the radiation of the electric field. The energy coupling method is capacitive coupling.)
3. Information transmission in the low-frequency and high-frequency bands is carried out in the near-field area, and in the ultra-high-frequency and microwave bands is carried out in the far-field area.
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