Design optimization of interstitial antennas.

To improve the performance of interstitial antennas for microwave hyperthermia, parameters such as the uniformity of the heating pattern, the depth of penetration, and the impedance matching properties must be optimized. We examined analytically and experimentally the radiation characteristics of multisection insulated antennas in conductive tissue. The effects of varying the diameters and lengths of the center conductors in the various sections of the antenna and the diameter and type of the insulation on the electromagnetic power deposition pattern and input impedance characteristics were examined. A new approximate numerical model which calculates the current distribution and the radiation characteristics of multisection insulated antennas was developed. The numerical predictions were verified in a qualitative way experimentally by mapping the various near- and far-field components of these antennas. Based on the obtained results, design tradeoffs are identified and quantified, and guidelines for optimum designs are specified. In particular, it is shown that an insulation-to-center-conductor diameter ratio between 1.5 to 2.0 is optimum for a uniform Teflon insulation, and that a multisection arrangement with the thinnest insulation near the antenna tip has superior performance compared with the uniform insulation or other multisection designs.