High gain millimeter-wave lens antennas with improved aperture efficiency

The paper presents an effective approach for directivity maximization in high gain integrated lens antennas (ILAs) dedicated for 71-76/81-86 GHz wireless backhaul systems of mobile cellular networks. The proposed approach is based on improvement of ILA´s aperture efficiency by optimization of the dimensions of a primary rectangular waveguide feed. The optimization was performed with 3-D electromagnetic simulation for hemielliptic lenses made of different dielectric materials. It was found that optimal feed dimensions depend only on the lens dielectric permittivity and are valid for lenses of different sizes. In particular, the optimum rectangular feed dimensions for a lens made of a material with the dielectric permittivity of 2.3 are 3.1×2.8 mm. Additionally wide impedance bandwidth is provided by insertion of a dielectric pin with proper size and depth into the waveguide feed structure. The pin is fabricated together with the lens (from a single piece of a raw material). The proposed approach improves the S₁₁ level from -9.7 dB to -16 dB in the whole 71-86 GHz frequency band. The optimized waveguide feed element and a hemielliptic lens with the diameter D = 120 mm were fabricated and measured. Measurement results of the fabricated lens demonstrated a large transmission bandwidth exceeding 60-90 GHz for the 10 dB level of the return loss. The measured gain in the 71-86 GHz frequency band is 37.4 - 38.6 dBi with the half power beam width less than 2°. The measured gain values are only 1.8 dB lower than the diffraction limit calculated for a circular aperture of the same diameter with a uniform field distribution.