A retrodirective antenna is an

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

that transmits the

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

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 can be achieved entirely passively with the use of a van Atta array. Retrodirectivity can also be achieved with

phase conjugation Phase conjugation is a physical transformation of a wave field where the resulting field has a reversed propagation direction but keeps its amplitudes and phases. Description It is distinguished from Time Reversal Signal Processing by the fact t ...

by geometric placement of antenna elements (also known as heterodyning). Phase conjugation is obtained by mixing with LO (local

oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

) frequencies twice that of the incoming signal.

Applications

Using retrodirectivity allows for faster acquisition (up to twelve times faster) than phased array antennas, and with up to twenty-five times the

power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...

output. The key advantages of retrodirective antennas over phased array antennas are less

signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing ''signals'', such as audio signal processing, sound, image processing, images, and scientific measurements. Signal processing techniq ...

time and faster response times.

References

* Angelo Umali's "Retrodirective Noise Correlating Radar" Antennas {{telecomm-stub