改良式單根絕緣偶極天線應用於微波熱療時之輸入阻抗及加熱性能之分析

Abstract

絕緣偶極天線在微波熱療法的使用已被廣泛的討論。然而，先前的 天線設計仍然面臨預測反射係數及單位質量能量吸收率 (SAR) 的難題， 再加上其單位質量能量吸收率之分佈延著天線往訊號源方向延伸，更降低 了在醫療上的使用。雖然非均勻絕緣偶極天線對天線前端的單位質量能量 吸收率分佈較典型的均勻絕緣偶極天線有較好的控制，兩種設計都缺乏饋 飼端和訊號源之間對單位質量能量吸收率的控制機制。這個缺陷也影響了 天線在阻抗匹配上的表現。本文中，我們提出一個改進的天線設計。從饋 飼端一段距離處，利用四分之一波長的扼流裝置，往訊號源方向的電流可 以準確的控制住，反射係數也可因此得到最佳化。結合了扼流裝置和增胖 的天線前端，延著天線的單位質量能量吸收率分佈可以限制住，而且更加 強了前端的加熱效果。在我們的設計中，天線的長度 (從頂端到扼流裝置 的前端開口) 約8公分。實驗結果顯示，延著天線的單位質量能量吸收率 有較均勻的分佈，幾乎延伸到整個天線，且在天線的兩端明顯的下降。理 論預測經由實驗結果驗證，極佳的吻合度證明了其正確性。 The use of interstitial dipole antenna (IDA) for microwave hyperthermia has been widely discussed. However, previous designs of the antenna still face the problems on both the prediction of the reflection coefficient and the Specific Absorption Rate (SAR) distribution. By the way, the axial SAR distribution extending along the antenna toward the generator degrades its clinical use. Although the nonuniformly insulated antenna has a better control on SAR at the tip portion than the conventional uniformly insulated type, both designs lack the mechanism to control the SAR distribution between the feed gap and the power source. This deficiency will also influence the impedance matching performance of the applicator. In this paper, an improved antenna is investigated. The remedy may be found by installing a quarter wavelength choke at a certain distance away from the feed gap to definitely control the current toward the generator. The reflection coefficient can thus be optimized. Design of IDAs that incorporate both the choke and an enlarged section at the tip can restrict the SAR distribution along the antenna and enhance the heating effect at the tip. In our design, the over length of the dipole (from tip to the open end of the choke) is about 8cm. Experimentally measured results are found to exhibit more uniform axial SAR distributions extending over almost the entire length of the antenna and sharp falloffs near both ends. Theory are verified through measured data and excellent agreement proves its correctness.