A Balun-Free Helical Antenna for Minimally Invasive Microwave Ablation

We present a balun-free coax-fed helical antenna for microwave ablation. The proposed antenna produces a localized specific absorption rate pattern at the desired frequency of operation without using a coaxial balun. This reduces the outer diameter of the antenna and, thus, its invasiveness. Balun-free operation of the proposed antenna is achieved by operating the helical antenna at the second resonant frequency of the helix where a current minimum occurs at the feed point, resulting in an intrinsically high feed-point impedance. This efficiently chokes the currents excited on the outer surface of the feeding cable. Using a compact quarter-wavelength impedance transformer or a coaxially implemented pi network of reactive elements at the antenna feed point, we achieve an excellent impedance match between the antenna and the main feeding line. We fabricated a prototype of the proposed helical antenna with an outer diameter of 2.2 mm using the pi matching network. We used this prototype to conduct ablation experiments in ex vivo bovine liver. The dimensions of the resulting ablation zones are similar to those produced by coaxially fed antennas that use coaxial baluns. Our balun-free antenna design offers a promising solution for reducing the invasiveness of interstitial antennas used in microwave ablation.