  A phased-array antenna is a device that generates radiation patterns whose
shape and direction can be electronically controlled by tailoring the amplitude
and phase of the signals that feed each element of the array. These devices
provide identical responses in transmission and reception due to the constrains
imposed by time-reversal symmetry. Here, we introduce the concept of
nonreciprocal phased-array antennas and we demonstrate that they can exhibit
drastically different radiation patterns when operated in transmission or in
reception. The building block of the array consists of a time-modulated
resonant antenna element that provides very efficient frequency conversion
between only two frequencies: one associated to waves propagating in free-space
and the other related to guided signals. Controlling the tunable nonreciprocal
phase response of these elements with the phase of low-frequency modulation
signals permits to independently tailor the transmission and reception
radiation patterns of the entire array. Measured results at microwaves confirm
isolation levels over 40 dB at desired directions in space with an overall loss
below 4 dB. We believe that this concept can be extended across the
electromagnetic spectrum provided adequate tuning elements are available, with
important implications in communication, sensing, and radar systems, as well as
in thermal management and energy harvesting.
