An ablation instrument having an ergonomic handle that includes an actuator adapted to selectively extend and retract a blade that is pivotably mounted within an aperture assembly coupled to the housing by a shaft. The shaft extends distally from the handle and includes a coaxial feedline, a wire conduit disposed along a longitudinal axis of the shaft, and a pull wire disposed within the wire conduit and having a proximal and a distal end, wherein a proximal end of the pull wire is operably coupled to the actuator. The aperture assembly is coupled to a distal end of the shaft and includes a reflector having a closed upper portion, and an open lower portion from which the blade may be extended for use. Also disclosed is an ablation system that includes the above-described ablation instrument, a source of ablation energy, and optionally, a source of coolant for cooling the shaft and aperture assembly.