94 GHz Folded Fresnel Reflector Using C-Patch Elements

A 94 GHz folded Fresnel reflector (FFR) for helicopter collision avoidance Radar is presented. The antenna system consists of a primary source illuminating a semi-reflecting grid that reflects the primary source polarization toward the main reflector opposite the grid. The main reflector has two functions. It focuses the field in the desired direction and rotates the incident polarization by 90^deg to enable it to pass through the grid and radiate. Specific patch elements having a C-shape have been designed for this purpose. In order to increase overall efficiency, the reflector combines 8 correcting zones in its center and 4 at the periphery. The reflector is manufactured using standard photolithographic techniques. The primary source consists of a metal waveguide covered with a small frequency selective surface (FSS) for matching purposes. The maximum measured gain is 36.5 dBi at 94 GHz. The maximum side lobe level is -18 dB. The return loss value does not exceed -25 dB. The frequency bandwidth -3 dB in gain and return loss is 10%. In-flight measurements were conducted demonstrating the ability to detect power lines at distances up to 680 m.