What is "retrodirectivity"

A retrodirective antenna is an antenna that transmits the signal back in the same direction it came from. This is based on the principle of self-phase conjugation of the input signal. Directivity depends on relative phases of signal radiated from different elements. Arrayed antenna consist of multiple, identical radiating elements.

Retrodirectivity is achieved with phase conjugation by geometric placement of antenna elements (also known as heterodyning). Phase conjugation is obtained by mixing with LO (local oscillator) frequencies twice that of the incoming signal.