Design and analysis of insulated coaxial slot antenna embedded in dissipative medium

• Main Authors: David Wen-Fong Su, 蘇紋楓
• Other Authors: Lin-Kun Wu
• Format: Others
• Language: en_US
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/20996532730712189633

博士 === 國立交通大學 === 電信工程系 === 87 === 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.