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We apply a new triaxial antenna for microwave ablation procedures to an ex vivo bovine liver. The antenna consists of a coaxial monopole inserted through a biopsy needle positioned one quarter-wavelength from the antenna base. The insertion needle creates a triaxial structure, which enhances return loss more than 10 dB, maximizing energy transfer to the tissue while minimizing feed cable heating and invasiveness. Numerical electromagnetic and thermal simulations are used to optimize the antenna design and predict heating patterns. Numerical and ex vivo experimental results show that the lesion size depends strongly on ablation time and average input power, but not on peak power. Pulsing algorithms are also explored. We were able to measure a 3.8-cm lesion using 50 W for 7 min, which we believe to be the largest lesion reported thus far using a 17-gauge insertion needle.