Reseach On Magneto-dielectric Materials And Reader Antenna Applied In UHF Frequency Band

In recent years, the application of the Internet of things technology has deep into various fields. Radio frequency identification as the key technology of the Internet of things obtained the rapid development, the internal hardware size of the system became smaller and smaller. Hence, how to realize the RFID reader antenna miniaturization became crucial importance. In addition to structural adjustment, the introduction of low loss of magneto-dielectric materials to replace the traditional dielectric substrates could well realize the antenna miniaturization, and at the same time introducing magnetic loss would also be able to broaden the antenna impedance bandwidth. Experiments showed that using magneto-dielectric materials gained by the impedance bandwidth was better than that of high dielectric constant substrates under the condition of same miniaturization. Therefore, in this paper we have systematically investigated a low loss of magneto-dielectric material used for ultra-high frequency RFID spectrum(840 ~ 960 MHz), designed and fabricated two different structures of reader antennas based on the material, which was to verify the feasibility of the experimental scheme.On the one hand, from viewpoint of materials, we studied the Co2 Z ferrite and analysed the effects of the lack of iron, aluminum substituted iron and doping BBSZ glass, etc on the properties of materials step by step. Experiments showed that when the lack of two iron atom, the materials had good electromagnetic performance. Then, we studied the experimental project of aluminum substituted iron. It showed that the replacement of Fe3+ ions by Al3+ ions suppressed the appearance of Fe2+ effectively, which could reduce the magneto-dielectric loss of materials. With the increasing of Al2O3 contents, the magneto-dielectric loss of materials presented the trend of firstly decreases and then increases. When the replace quantity of aluminum was 1.6 and the sintering temperature was 980℃, the materials loss obtained minimum. Finally, based on the aluminum substituted iron experiments we studied the experiments of doping BBSZ glass, which was to improve the sintering density. Experiments showed that the permeability and permittivity have improved obviously with the increasing of BBSZ content. However, the excessive BBSZ content would make the permeability slightly decreased. When the BBSZ content was 3.0 wt% and the sintering temperature was 950℃, the materials had optimal performance.On the other hand, we have designed and fabricated two different types of RFID reader antennas based on the homemade materials. The first antenna used the traditional microstrip antenna structure-circular radiation patch, which was slotted on the inner ring deviating from the feed of 45° to realize circular polarized radiation. The center frequency of the antenna was about 914 MHz, and the impedance bandwidth reached 48 MHz of S11 less than-10 dB, but its gain was low. According to the gain low problem of the first antenna, we suspended a windowed metallic layer above the antenna as the second antenna, it was fed with the open microstrip line. The test results showed that the antenna gain reached 3.6 dBi at the center frequency of 915 MHz, and the impedance bandwidth reached 29 MHz, its radiation pattern was good. These two kinds of design schemes combined well the technology requirement of the RFID reader antenna in miniaturization and wide-band. The second scheme also made the design improvements on the structure according to the gain low problem of the first antenna.