絕緣天線在有損介質中之阻抗特性與加熱性能分析

Abstract

絕緣同軸槽狀天線和絕緣偶極天線等兩種不同結構天線 經常運用於 微波醫療天線的設計上. 然而Zhang等人對絕緣同軸槽狀結構天線的輸入 阻抗的分析仍沿用絕緣偶極天線的分析模式,而忽略了此種天線結構在天 線第一臂的多餘路徑效應在此研究中,吾人設計一組天線長度等長的均勻 絕緣共軸天線和偶極天線,且各加裝四分之一波長的抗流槽使其輸入阻抗 不隨插入深度而變,經由比較結果分析,驗證了絕緣同軸槽狀天線的多餘路 徑效應.並運用此一效應,在不破壞天線的阻抗匹配下,將一均勻絕緣同軸 槽狀天線的長度縮短,另外,非均勻絕緣結構的天線雖有較佳的頂端加熱效 果,但其第二段之波數太小,以致使其相對共振長度增加,電流分布也隨之 延伸,影響了其比吸收率(SAR)之軸向分布的集中. 加熱性能結果顯示，利 用多餘路徑效應縮短的均勻絕緣同軸槽狀天線其比吸收率的軸向集中與均 勻性會比非均勻絕緣同軸槽狀天線好. Two different types of insulated antenna,namely,an insulated coaxial slot antenna(ICSA) and an insulated dipole antenna(IDA) are often emplyed in microwave hyperthermia. However,Zhang et al. followed the impedance model which is only applicable to IDA to analyze ICSA while ignoring the effect of the excess current path inherent in an ICSA. In this sthdy, a suit of uniformly insulated coaxial slot antennas(U-ICSAs) and uniformly insulated dipole an-tenna(IDA),which have the same antenna length,were investigated. In all cases,with a quarterwavelength choke mounted,the impedance of the antennas remained constant regardless of thevaring insertion depth into the phantom. From the comparison of the U-IDA and U-ICSAs,theeffect of the excess current path among ICSAs was identified. Moreover,based on the obtained result, the effect of the excess current path was also applied to shorten the length of oneU-ICSA without degrading its impedance matching performance. The nonuniformly insulated coax- ial slot antenna(NU-ICSA) has the improved tip heating performance. However,the wave number inthe second arm is so small that the resonance length becomes much longer than the uniformlyinsulated antenna. With the antenna length increased, the current and SAR distributions stretch toward the generator. Experimentally measured results show that the U-ICSA,with its length sho-tened by excess cureent path,exhibits more uniform and concentrated axial SAR distribution thanthe NU-ICSA