Abstract: A high-gain, dielectric loaded, slotted waveguide antenna having a photonic bandgap, a high-impedance electromagnetic structure, in contact with the waveguide surface containing longitudinal slots, and a tailored dielectric material structure in contact with the outer surface of the photonic bandgap structure. The tailored dielectric structure at the inner most surface has the same effective dielectric constant of the waveguide material and the photonic bandgap structure. The effective dielectric constant is then incrementally or continuously reduced to have a dielectric constant close to that of the free-space value at the outer surface further distance from the waveguide array. The tailoring of the effective dielectric constant is achieved by layering a given number of slabs of different dielectric constants with sequentially reduced values, or by varying the chemical composition of the material, or by varying the density of the material imbedded with high dielectric constant particles.