An electrosurgical device for directing energy to a target volume of tissue includes a coaxial feedline having an inner conductor, an outer conductor and a dielectric material disposed therebetween. A proximal cylindrical dielectric sleeve is coupled to the inner conductor at a distal end of the coaxial feedline. A distal cylindrical dielectric sleeve is coupled to the inner conductor. First and second dielectric segments are coupled to the inner conductor and disposed between the proximal cylindrical dielectric sleeve and the distal cylindrical dielectric sleeve. The device also includes an elongated shaft overlying the proximal cylindrical dielectric sleeve, the first dielectric segment, the second dielectric segment and the distal cylindrical dielectric sleeve. The elongated shaft includes an opening defined therethrough, wherein the opening is at least partially aligned with the first dielectric segment. A balun structure is disposed on the elongated shaft, at least partially overlying the opening in the elongated shaft.