Antenna Gain Enhancement and Beamshaping using a Diffractive Optical Element (DOE) Lens (2014)

Dielectric and metamaterial lenses have been designed for gain enhancement and beam shaping. The motivation for this work came from a commercially available slotted waveguide antenna with a dielectric lens that shapes the beam and enhances the gain only in the azimuth plane. When two of these antennas, each with a dielectric lens, are stacked as an array to form the sum and difference patterns the elevation plane gain is low and the beam width too wide to be acceptable for radar applications.\nThe objective of the present work is to design a diffractive optical element (DOE) lens for gain enhancement gain and beam shaping. As compared to other available lenses it is much thinner, lighter and easily machined. The DOE lens is made from rexolite which has a dielectric constant of 2.53. The DOE lens is composed of a series of zones which focus the light at a certain focal length. The phase is the same everywhere on each zone at the focal point. The phase difference between neighboring zones is 2π, resulting in a constructive interference at the focus. These zones are able to focus the radiation from an antenna in order to enhance the gain and shape the beam. The design parameters include the lens diameter, number of zones, the center zone thickness for a particular frequency and refractive index of the dielectric material.\nA comprehensive study has been performed in CST Microwave Studio to illustrate the properties of the DOE lens. The focusing property for image formation is verified by a plane wave excitation. Lenses have been designed and tested at different frequencies and with varying design parameters. Gain enhancement and beam shaping are illustrated by modeling the DOE lens in CST and placing it in front of different antennas. This work presents lenses for 10GHz and 40GHz horn antennas, a 3GHz slotted waveguide antenna array, and a 10GHz microstrip patch arrays. Beam shaping and focusing is clearly illustrated for each type of antenna. It is seen that the size of the lens is directly proportional to gain increase which can be as high 20dB enhancement for a 40-GHz horn antenna. The 3GHz DOE lens illustrates for the slotted waveguide array, a gain enhancement of 7dB in the elevation plane, as well as decrease of the 3dB beamwidth from 20° to 13.5°. It is also proved that the DOE lens allows for the creation of a good difference pattern.\nExperimental validation for the focusing properties and the gain enhancement has been done using the 10GHz DOE, made from rexolite, and fabricated using CNC milling in the RIT Brinkman Lab. The image formation has been verified using an electric field probing station in the Nanoplasmonic lab at RIT. Two types of excitation have been done with a dipole and with a horn antenna, where another dipole probes the field in the transmission plane. The electric field intensity shows clearly the beam focusing by the DOE lens. The X-band anechoic chamber in the Electromagnetics Theory and Application (ETA) lab has been used to demonstrate the gain enhancement of a horn antenna with the fabricated DOE lens. The distance of the lens from the receive antenna has been varied to obtain a maximum received power. The results show a substantial gain enhancement of 6.6 dB for the horn antenna and of 5.6 dB for the patch array.