Embodiments of endfire aperture-based traveling-wave antennas are described. For example, an embodiment, including a Vivaldi antenna, may have a director incorporated into the aperture region of the antenna to provide enhanced radiation directivity. The director may be a shaped dielectric that interacts with an electromagnetic field to reduce the divergence of the resultant beam as it exits the antenna. Additional dielectric substrate layers may be stacked on both sides of the antenna in order to balance the dielectric loading between the different conductors. The dielectric substrates may also eliminate contact between the antenna metallization and the lossy environment. Certain disclosed Vivaldi antennas may be used in tissue screening applications.