Study On UHF And ISM Frequency RFID Tag Antenna

Radio frequency identification(RFID) technology, also known as electronic tags, is a kind of communication technology, which can identify specific targets and read and write data through radio signals without the need of mechanical or optical contact between the recognition system and a specific targets. RFID technology has been widely used in the commercial automation,industrial automation, transportation management and many other fields. As one of the important parts of RFID system, the research on RFID tag is becoming increasingly important. Up to new, the development of low frequency(LF) and high frequency(HF) RFID tag has been relative mature. Hence we mainly focus on the passive RFID research in the ultrahigh frequency(UHF) and microwave frequency band(2.45GHz). The research on UHF and 2.45 GHz tag antenna covering the bandwidth all over the world could provide guarantee for the global promotion and widely application of RFID technology. Therefore, UHF and 2.45 GHz tag antenna were designed in this thesis.The purpose of this thesis is to design a miniaturized 2.45 GHz tag antenna and a broadband UHF tag antenna, both of which could cover the global bandwidth. The global bandwidth for 2.45 GHz is 2.4GHz-2.484 GHz, while the global bandwidth for UHF is 840MHz-960 MHz.In order to achieve miniaturization of the tag antenna, the proposed tag antenna consists of two radiating elements. One of radiating elements is composed of first-order Hilbert fractal structure and the folded dipole, while the other consists of the folded dipole. The interaction between two radiating parts makes the proposed antenna a 150 MHz frequency band covering 2.37GHz-2.52 GHz. Due to introduction of Hilbert fractal structure, the proposed antenna size is greatly reduced, and the final whole size is 30mm×7.3mm. Moreover, the gain of the proposed antenna is 2d B across the frequency band. Hence the proposed antenna is a tag antenna with minimized size, wide frequency, and good gain.According to the design of 2.45 GHz tag antenna, the design of UHF tag antenna utilizes firstorder Hilbert fractal structure and the helical structure. These two structures produce two close resonant frequencies to broaden the bandwidth. Meanwhile, A T- matching network is implemented in design of the UHF tag antenna. By changing the length and width of T- matching network, we can adjust the input impedance of the antenna conveniently. Finally the antenna can operate from 820 MHz to 1.01 GHz, a 190 MHz bandwidth with the gain of over 2d B.This article also describes methods of measurement of the tag antenna performance parameters, highlighting a dual-port method to measure the bandwidth of tag antenna, which is relatively simple and practical. This dual-port method is used to measure the bandwidth of two abovementioned antennas, and the measured results are consistent with the simulated ones, which proved the accuracy and practicality of this measurement method.