Design Of Tag Antenna In Long Distance RFID System

As a typical representative of rapidly developed automatic identification technology, radiofrequency identification technology has been widely applied in society, economy, national defenseetc., which brought a lot of convenience to people’s lives, especially the RFID system with theelectromagnetic backscatter modulation method. This system can be utilized to accomplish longdistance data communication, which is highly prized by people. The RFID system includes twoparts, the reader and the tag, and the former is mainly composed of a tag antenna and a chip. As apart of tag, the tag antenna determines the choice of channel and mode of communication betweenreader and tag. Therefore, tag antenna plays an important role in the RFID system.Tag antenna in long distance RFID system works in the mode of electromagnetic backscattermodulation based on the modulation of radar cross section (RCS). Δ RCSexpresses the RCSchange of tag antenna in the backscatter modulation state, which can react the Modulation Ratio ofmodulated digital. When Δ RCSreaches a maximum, the most sensible discernibility degree of0and1is achieved, while the lowest communication error rate and the highest correct datatransmission rate can be got. In order to gain the ΔR CSmaximum, two points should be met inthis paper：First, resonant phenomenon occurs in the915MHz frequency when the load of theantenna is short-circuited, so the antenna will have a maximum scattered wave. Second, a minimumantenna scattered wave will be reached when the antenna matches with the load.Two antennas are introduced in this thesis, the T-match tag antenna and the dual L-type loadtag antenna. First of all, these two antennas not only meet the impedance requirement of Monza4,but also achieve the maximum ΔR CSin the Backscatter modulation state. Secondly, research onthe impedance and Radar Cross Section affected by antenna structure parameters is accomplished.Finally, each physical mode of these two antennas is designed, after that related parameters aretested.For design of T-match tag antenna, antenna has a final size of58mm×30mm×1mm with1.25dB Gain and7.5dB ΔR CSin the backscatter modulation state. When the frequency is915MHz,7.62+j141.98input impedance is obtained. In860MHz-960MHz band, the input resistanceranges from2.9to30.9, while the input reactance ranges from115.3to190.7. Antenna canmeet the impedance requirement of Monza4in part of the input mode very well. According to theISO/IEC18000-6, the longest recognition distance of the RFID system by using this tag antenna is10m.For design of dual L-type load tag antenna, antenna has a final size of42mm×42mm× 0.35mm with-0.63dB Gain and7.3dB Δ RCSin the backscatter modulation state. When thefrequency is915MHz,8.9977+j143.98input impedance is obtained. In860MHz-960MHz band,the input resistance ranges from5.7-14.9, while the input reactance ranges from58.4-249.3.Antenna can meet the impedance requirement of Monza4in part of the input mode very well.According to the ISO/IEC18000-6, the longest recognition distance of the RFID system by usingthis tag antenna is8.2m.