Toward Flexible Microwave Ablation Antennas With a Balun-Free Helical Dipole Design

We present a balun-free dipole antenna for generating localized heating patterns in microwave ablation. The dipole antenna is created by extending the outer and inner conductors of the coaxial cable. One dipole arm is a helical outer conductor encompassing the other arm that is the extended inner conductor. The overall lengths of the helix and the extended inner conductor are three quarters and one quarter of a wavelength, respectively. The helix's current direction reverses a quarter of a wavelength away from the feed point, becoming aligned with the extended inner conductor's current. This creates a relatively low-input impedance, choking the current on the outer surface of the outer conductor. The fields produced by the currents flowing on the dipole arms destructively interfere closer to the feed point, while constructively interfering further from it. This helps the antenna produce nearly spherical ablation zones. A 1.9 GHz prototype of the antenna was fabricated and its ablation performance was experimentally verified in egg white. Measurement results showed good agreement with the simulations and confirmed the antenna's effectiveness in generating localized ablation zones. Our proposed design can serve as a promising candidate for performing minimally invasive ablation therapy within flexible and maneuverable embodiments.