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• 日期: 2018-09-05
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The antenna structure determines antenna directivity, impedance characteristic, standing wave ratio, antenna gain, polarization direction and working frequency.Antenna characteristics are also affected by the shape and physical characteristics of the attached object.For example, magnetic fields cannot penetrate magnetic materials such as metals, the shape of the magnetic lines of force around a metal will change, and, because of the magnetic field, they can cause eddies on the metal surface.According to lenz's law, the eddy current produces magnetic flux resistant to excitation, which leads to a great attenuation of magnetic flux on metal surface.The energy emitted by the reader antenna is absorbed by the metal and the reading and writing distance is greatly reduced.In addition, the liquid will absorb electromagnetic signals, the elastic base will cause label and antenna deformation, and the wide-band signal source (such as engine, water pump, generator) will generate electromagnetic interference, etc., all of which must be carefully considered when we design the antenna.At present, a variety of solutions have been proposed in the research field according to the above characteristics of antennas. For example, tortuous antennas are used to solve the size limit, and inverted F antennas are used to solve the problem of reflection on metal surface.

The goal of antenna is to transmit the maximum energy in and out circuit, which requires careful design of antenna and free space and its circuit matching. The higher the antenna matching, the better the antenna radiation performance.As the operating frequency increases to the uhf region, the problem of matching between antenna and label chip becomes more serious.In traditional antenna design, the antenna size and structure can be controlled by using an impedance matching converter to match the input impedance with the feeder.Generally, the development of antenna is based on 50 or 75 ohm impedance, while in the RFID system, the input impedance of the chip may be arbitrary, and it is difficult to accurately test in the working state, so the antenna design is difficult to achieve the best.

For close distance RFID applications, the antenna is generally integrated with the reader. For remote RFID systems, the RFID reader antenna and reader are usually separated and connected by an impedance matching coaxial cable.Generally speaking, directional antenna is more suitable for label application due to its low echo loss.Because the label placement direction is not controllable, the reader antenna generally adopts circular polarization mode.Reader antennas require low profile miniaturization and multiband coverage.For separate readers, the design of antenna arrays will also be involved.Foreign countries have begun to study the application of smart beam scanning antenna array in the reader. The reader can open and close different antennas according to a certain processing sequence, so that the system can sense the labels of different antenna coverage areas and increase the system coverage.