Abstract: A microwave ablation instrument including a transmission line having a first conductor and a second conductor suitable for the transmission of microwave energy. A horn antenna device is mounted to the end of the transmission line, and include an outer conductor and an inner conductor. The outer conductor of the horn antenna is electrically coupled to the second conductor of the transmission line, and includes a peripherally extending interior wall flaring outwardly to define a recess therein. The inner conductor of the horn antenna is electrically coupled to the first conductor of the transmission line and terminates in the outer conductor recess. The inner conductor and the outer conductor cooperate to emit an electromagnetic field sufficiently strong to cause tissue ablation in a direction generally away from the flared interior wall of the outer conductor.