A plane wave antenna including: a horn antenna; a waveguide at least partially inside the horn antenna, wherein the waveguide includes: a central dielectric slab increasing in width toward the horn antenna and with a first dielectric constant, an upper slab above the central dielectric slab with a second dielectric constant, and a lower slab below the central dielectric slab with the second dielectric constant; wherein the central dielectric slab has a substantially constant thickness less than a quarter of a wavelength at a highest frequency of operation of the plane wave antenna.