Radio Frequency of Identificatio (RFID) is a non-contact automatic
identification technology that uses radio frequency technology. It has the
advantages of fast transmission rate, anti-collision, mass reading, and movement
process reading. Therefore, RFID technology is used in logistics and Supply
chain management, production management and control, anti-counterfeiting and
security control, traffic management and control and other fields have
significant application potential. At present, the working frequency band of
radio frequency identification technology includes low frequency, high
frequency, ultra high frequency and microwave range, among which high frequency
and ultra high frequency are the most widely used.
The RFID system is mainly composed of a reader (target), a transponder
(RFID tag) and a background computer. Among them, the reader implements the
reading and writing and storage of data on the tag. It is composed of a control
unit, a high-frequency communication module and an antenna. It is mainly
composed of an integrated circuit chip and an external antenna. The circuit chip
usually includes circuits such as radio frequency front-end, logic control, and
memory. According to the power supply principle, tags can be divided into active
(acTIve) tags, semi-active (semiacTIve) tags and passive tags. Passive tags are
popular because of their low cost and small size.
The basic working principle of the RFID system is: after the tag enters the
reader to transmit the radio frequency field, the induced current obtained by
the antenna is used as the power supply of the chip after the booster circuit,
and the induced current with information is converted into a digital signal
through the radio frequency front-end circuit. Enter the logic control circuit
for processing, and the information that needs to be replied is sent from the
tag memory, sent back to the RF front-end circuit via the logic control circuit,
and finally sent back to the reader through the antenna.
The goal of the antenna is to transmit the maximum energy in and out of the
tag chip. This requires careful design of the matching between the antenna and
the free space and the tag chip connected to it. When the operating frequency
increases to the microwave area, the matching problem between the antenna and
the tag chip becomes It's more severe. For a long time, the development of tag
antennas is based on 50 or 75 ohm input impedance. In RFID applications, the
input impedance of the chip may be any value, and it is difficult to accurately
test under working conditions, lack of accurate parameters, and antenna design.
It is difficult to achieve the best.
In recent years, the application of RFID technology has become more
widespread, and it has also received much attention. Especially the UHF
frequency band RFID system, due to its long transmission distance and high
transmission rate, has received more attention. A typical RFID system consists
of two parts: an RFID reader and a tag. RFID passive tags rely on
electromagnetic signals emitted by the RFID reader to supply power, and
communicate with the reader through reflection-modulated electromagnetic
signals. Therefore, the pros and cons of the RFID tag antenna design have a key
impact on the performance of the system.
Common RFID reader antennas include folded dipole antennas, fractal
antennas, microstrip antennas and axial mode helical antennas. Since folded
dipole antennas and fractal antennas are generally linear polarized antennas, it
is difficult to meet the reader's identification requirements for electronic
tags in various directions, so they are not suitable for many occasions; and
microstrip antennas are used in miniaturized due to their large area and size.
The use of reader handsets is restricted; the axial mode helical antenna is also
restricted in actual use due to the high axial height. Therefore, how to design
a small-size, low-profile, high-performance circularly polarized radio frequency
identification antenna has become the focus of attention. (How to make a
shortwave antenna)
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