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The use of 2450-MHz microwave energy applied via a miniature coaxial cable-mounted helical coil antenna was investigated as a means to increase the treated volume of cardiac tissue in a saline-perfused, tissue-equivalent manner during ablation. Using an array of fiber-optic temperature probes implanted in a saline-perfused, tissue-equivalent gel phantom model designed to simulate the myocardium during ablation, the heating pattern from the microwave antenna was characterized and compared to that induced by a commercial RF electrode catheter at 550 kHz. Effects of variable contact angle between the heat source and heart wall were assessed. Heating patterns from the RF electrodes dropped off much more abruptly both radially and axially than the microwave antenna. The volume of effectively heated tissue was more than ten times larger for the microwave antenna when the heat sources were well-coupled to the tissue, and more than four times larger for the microwave antenna when the sources were angled 30 degrees away from the tissue surface.