Low-loss Magneto-dielectric Materials And The Applications In Antennas

As the mobile wireless communication devices are becoming smaller, thesignificance of antenna miniaturization is highlighted. High dielectric constantsubstrates are widely used in antennas for the benefits of reducing the antenna physicaldimensions. But there are drawbacks with the use of such substrate materials. Recently,there has been some work aimed at overcoming the drawbacks by usingmagneto-dielectric substrates. In this paper, two kinds of magneto-dielectric materialswere prepared and thoroughly studied. In addition, these materials were used in thedesign and fabrication of two different antennas.In the first part of the thesis, the micro structures and performance of WO₃dopedCo₂Z were studied with different sintering temperatures and different contents of WO₃.Equal permittivity and dielectric constant can be reached by sintering the0.2wt%WO₃doped Co₂Z at1200℃. The loss of material was further reduced by using the two-stepsintering method. When f=200MHz, the magnet loss of two-step sintered sample is0.047, which is42%lower than the traditional sintered sample. And the dielectric lossof two-step sintered sample can be reached to0.0063at200MHz, which is62%lowerthan the traditional sintered sample. To obtain a higher application frequency, thedoping effect of low melting point material, H₃BO₃, was also studied. The resultsindicate that, by slightly decreasing the permittivity, the higher application frequency isexpected to be realized. And the optimized performance can be obtained by sintering4wt%H₃BO₃doped Co₂Z at900℃.A T-DMB cellphone antenna and RFID reader antenna were designed in the secondpart of the thesis. The WO₃doped Co₂Z material was used in the design and fabricationof the T-DMB antenna. The measured center frequency was187MHz where the S11reached-40dB and the bandwidth was29MHz （VSWR=3）. Based on the H₃BO₃dopedlow loss material, the RFID reader antenna was designed. The radio performance of thisantenna was also simulated. The center frequency of the antenna was916MHz, and80MHz bandwidth can be reached when S11was less than-10dB. With axial ratio （AR）of1.07dB in highest gain direction, excellent circular polarization on the antenna was exhibited. The study on the application of these two kinds of materials providespotential candidates for the portable wireless communication devices.