An electrosurgical device for directing energy to a target volume of tissue, comprising: a coaxial feedline having an inner conductor (220; 1420), an outer conductor coaxially disposed around the inner conductor (220; 1420), and a dielectric material disposed therebetween; and an antenna assembly, wherein a portion of the inner conductor (220; 1420) extends distally beyond the outer conductor and the dielectric material at a distal end of the coaxial feedline into the antenna assembly; a proximal cylindrical dielectric sleeve coupled to the inner conductor (220, 1420) and disposed at the distal end of the coaxial feedline, surrounding a proximal portion of the portion of the inner conductor (220; 1420) that extends distally beyond the outer conductor and the dielectric material at the distal end of the coaxial feedline; a distal cylindrical dielectric sleeve coupled to the inner conductor (220, 1420) and disposed distal to the proximal cylindrical dielectric sleeve; a first dielectric segment (284; 1564) of a material having a first dielectric constant, coupled to the inner conductor (220, 1420); a second dielectric segment (274; 1562) 1562) of a material having a second dielectric constant, coupled to the inner conductor (220, 1420), wherein the first and second dielectric segments each includes a partial cylindrical surface and at least one flat planar surface for coupling the first to the second segment; wherein the first and second dielectric segments are disposed between the proximal cylindrical dielectric sleeve and the distal cylindrical dielectric sleeve; an elongated shaft (480; 1593) overlying the proximal cylindrical dielectric sleeve, the first dielectric segment (284; 1564), the second dielectric segment (274; 1562) and the distal cylindrical dielectric sleeve, the elongated shaft having an opening defined therethrough, the opening at least partially aligned with the first dielectric segment (284; 1564); and a balun structure disposed on the elongated shaft, the balun structure at least partially overlying the opening in the elongated shaft. a coaxial feedline having an inner conductor (220; 1420), an outer conductor coaxially disposed around the inner conductor (220; 1420), and a dielectric material disposed therebetween; and an antenna assembly, wherein a portion of the inner conductor (220; 1420) extends distally beyond the outer conductor and the dielectric material at a distal end of the coaxial feedline into the antenna assembly; a proximal cylindrical dielectric sleeve coupled to the inner conductor (220, 1420) and disposed at the distal end of the coaxial feedline, surrounding a proximal portion of the portion of the inner conductor (220; 1420) that extends distally beyond the outer conductor and the dielectric material at the distal end of the coaxial feedline; a distal cylindrical dielectric sleeve coupled to the inner conductor (220, 1420) and disposed distal to the proximal cylindrical dielectric sleeve; a first dielectric segment (284; 1564) of a material having a first dielectric constant, coupled to the inner conductor (220, 1420); a second dielectric segment (274; 1562) 1562) of a material having a second dielectric constant, coupled to the inner conductor (220, 1420), wherein the first and second dielectric segments each includes a partial cylindrical surface and at least one flat planar surface for coupling the first to the second segment; wherein the first and second dielectric segments are disposed between the proximal cylindrical dielectric sleeve and the distal cylindrical dielectric sleeve; an elongated shaft (480; 1593) overlying the proximal cylindrical dielectric sleeve, the first dielectric segment (284; 1564), the second dielectric segment (274; 1562) and the distal cylindrical dielectric sleeve, the elongated shaft having an opening defined therethrough, the opening at least partially aligned with the first dielectric segment (284; 1564); and a balun structure disposed on the elongated shaft, the balun structure at least partially overlying the opening in the elongated shaft.