An implantable lead for use with an implantable medical device includes a lead body with first and second electrical conductors extending between its proximal and distal ends. An electrical connector at the proximal end of the lead body includes terminals electrically connected to the first and second conductors. First and second coaxial active fixation helices are coupled to the lead body's distal end, one being an anode, the other an electrically isolated cathode. Each helix has an outer peripheral surface with alternating insulated and un-insulated portions along its length with about a half of the surface area being insulated. The un-insulated portions of the helices may be formed as a plurality of islands in the insulated portions, or as rings spaced by insulative rings, or as longitudinally extending strips spaced by longitudinally extending insulative strips.