A linear retro-nulling antenna array with the second null at an arbitrary direction

In this study, the design and its practical considerations of a retro-nulling antenna array using 90° hybrid circuits to give a second null at an arbitrary direction is presented. It can adaptively yield two nulls with one at the incident direction and another one at a specified angle. For a 90° hybrid circuit, when its direct and coupled ports are terminated with loads of the same reflection coefficient, the phase difference of the output signals at the other two ports are reversed to that of the incident signals. Thus the array circuit can be implemented by properly using 90° hybrid circuits, phase shifters, and terminations. In this paper, a special case with the second null located at the specular direction of the incident angle is presented and measured, and its practical considerations for the circuits implementation are also discussed.