RFID antenna structure and environmental factors have a significant impact on the performance of the antenna. Determines the antenna structure of the antenna pattern, impedance, VSWR, antenna gain, polarization direction and frequency characteristics. Antenna characteristics are also affected by the post attached to the object shape and physical characteristics. For example, the magnetic field can not penetrate metal material, a metal material near the magnetic field lines will change shape, and, because the magnetic field to cause eddy currents in the metal surface. Seen by the Lenz's law, eddy current generates a magnetic flux resistance to excitation, leading to the metal surface magnetic flux greatly attenuated. Reader antenna metal absorbed energy is emitted, write distance will be greatly reduced. In addition, the liquid absorption of electromagnetic signals, the elastic grass will cause the tag and antenna distortion, wide-band signal source (such as engines, pumps, generators) will produce electromagnetic interference and so on, these are our design must be carefully considered when the antenna where . Currently, the research areas above the antenna characteristics under a variety of solutions proposed, such as using zigzag antenna solution size constraints, the use of inverted-F antenna solution metal surface reflection problems. The goal is the transmission antenna is the maximum energy out of the circuit, which requires careful design antenna and free space, and the circuit, the antenna matching degree is higher, the better the performance of the antenna radiation. When the operating frequency to UHF area, the antenna and the tag match between the chip problem becomes more severe. In the conventional antenna design, we can control the antenna size and structure of the impedance converter is used to match the input impedance of the feeder. General development based on the antenna 50 or 75 ohm impedance, and in the RFID system, the input impedance of the chip may be any value, and difficult to accurately test the working state, the antenna design will be difficult to achieve the best. For close range RFID applications, antennas and readers generally integrate RFID systems for long-range, reader antenna and reader antenna separation structure generally taken by the impedance matching of the coaxial cable. In general, the directional antenna return loss because with fewer, more suitable for label applications; Because the label orientation is not controllable, the reader antenna generally use circular polarization. Reader antenna requires low profile, compact and multi-band coverage. For separate reader, will also involve the antenna array design problems. Foreign countries have begun to study the reader application intelligence beam scanning antenna array, the reader can follow the processing order of a "smart" opening and closing different antennas, the system can sense the different regions of the tag antenna coverage, increasing the system coverage. |
