This paper introduces a quasi-planar antenna that provides, over a broad frequency bandwidth, highly directive radiation in one plane, phase center stability as a function of the frequency and high efficiency. The structure is composed by a dielectrically filled parallel plate waveguide that supports the propagation of a transverse electromagnetic (TEM) wave. The waveguide is truncated and shaped as a planar lens to achieve directive beams in the H-plane, while a short flaring is added at the waveguide open end to facilitate radiation into free space. High efficiency is achieved thanks to the peculiar feed structure that supports the propagation of directive lateral waves. An antenna prototype has been manufactured and measured. The results highlight the highest directivity for a quasi-planar broadband antenna (3:1 bandwidth) with very low dispersion and frequency-stable phase center.