嵌入於損耗性介質之絕緣式同軸狹縫天線的分析與設計

Abstract

本論文提出一系統性設計方法用以探討植入型微波熱療系統中之絕緣式天線的設計問題。首先,利用平移對稱特性(translational symmetry property)配合細線幅射積分(thin-wire radiation integrals)來提昇計算效率之快速演算法則將被詳細介紹。此一演算法則將可決定嵌入於損耗介質中的絕緣式偶極天線的近場特性。從我們的分析結果顯示,基本型的細線幅射積分比傳統的King-Casey方法快了380倍。而利用平移對稱特性配合細線幅射積分的分析方法又比單純的細線幅射積分方法快了7倍。接著,由King等人所發展的類傳輸線輸入阻抗模型用以計算嵌入於損耗介質中的絕緣式偶極天線的輸入阻抗,將被擴展到絕緣式同軸狹縫天線的分析。從實際結構觀察, 絕緣式同軸狹縫天線在結構上多了存在於三軸同軸電纜內部之電流路徑。此一內部電流路徑將導致天線共振長度的縮短。此一效應將在我們所提出的阻抗模型中加以探討。同時,簡單的輸入阻抗匹配最佳化的方法亦加以描述。最後,利用非對稱饋入絕緣式同軸狹縫天線設計出同時具有優異的阻抗匹配及增強式頭端加熱(enhanced tip-heating)特性的植入型微波熱療隔離式天線。理論與實驗數據的一致性將對此一設計分析方法提出最佳佐證。

In this dissertation, a systematic method of designing insulated applicators for interstitial microwave hyperthermia is presented. First of all, a fast algorithm that exploits the translational symmetry properties associated with a thin-wire radiation integral to improve its computational efficiency for determining the near-field characteristics of an insulated dipole antenna (IDA) embedded in a homogeneous dissipative medium is described. In one case investigated, the basic thin-wire approach that uses no symmetry property is found to yield accurate results in approximately 380 times less CPU time than the traditional King-Casey approach. In another case, use of symmetry property further reduces the CPU time by a factor of 7; additional reduction in CPU time is also possible by taking into account the near-field nature of the problem. Subsequently, transmission-line-approximation type of input impedance model originally developed by King et al. [4] for the interstitial dipole antenna embedded in a conductive medium is extended to the case of insulated coaxial slot antenna (ICSA). Physical construction of the later indicates the presence of additional current path(s) inside the feed line of the triaxial cable which shall lead to the shortening of its resonance length. This effect is taken into account in the impedance model and verified to be accurate by experiments. Furthermore, a simple strategy for optimizing the applicator’s impedance matching performance is also described. Excellent agreements observed between theoretical and measured data indicate that these models can be relied upon when designing efficient applicator for interstitial microwave hyperthermia. Finally, a novel use of asymmetrically-fed ICSA type of applicator for interstitial microwave hyperthermia that simultaneously exhibits good impedance matching and enhanced tip-heating performances is presented. Theoretical analysis reveals that by making the distal arm much shorter than the proximal arm of the antenna, charge densities distributed over the distal arm of the antenna increase significantly. This in turn can result in the radial electric field component becoming the dominant contributor to the specific absorption rate (SAR) over the distal arm side of the heating region and, therefore, the achievement of enhanced tip-heating performance. With the length of the longer proximal arm chosen to be slightly longer than a quarter wavelength, good impedance matching and enhanced tip-heating performances are achieved when the length of the shorter distal arm is reduced to no more than 25% that of the longer proximal arm. Good agreements observed between theoretical and measured SAR patterns for two ICSAs designed for operation at 915- and 433-MHz, respectively, confirm the validity of the design method.