An ablation device (101,301,701) includes a feedline (110,426) including an inner conductor (210,420) having a distal end, an outer conductor (260,424) coaxially disposed around the inner conductor (210,420), and a dielectric material (280,422) disposed therebetween, an elongated electrically-conductive member (270) longitudinally disposed at the distal end of the inner conductor (210,420) and having a proximal end, a first balun structure (B1) disposed over a first portion of the outer conductor (260,424) and positioned so that a distal end of the first balun structure (B1) is located at a first distance from the proximal end of the electrically-conductive member (270) and a second balun structure (B2) disposed over a second portion of the outer conductor (260,424) and positioned so that a distal end of the second balun structure (B2) is located at a second distance from the proximal end of the electrically-conductive member (270).
