Nonreciprocity in antenna radiation induced by space-time varying metamaterial cloaks

Conventionally, antennas are reciprocal devices, exhibiting the same properties in transmission and in reception. In order to break antenna reciprocity, in this letter, we exploit the properties of space-time varying metamaterials, whose dielectric properties are modulated in both space and time. We propose to surround the antenna by a space-time varying cloak that imparts a momentum bias to the electromagnetic field propagating through it. The propagating wave interacts with the space-time modulation only in one direction, breaking time-reversal symmetry and, thus, the reciprocity of the system. Here, we show that a properly designed space-time varying metamaterial cloak can be used to significantly change the realized gain exhibited by an antenna in its transmission and reception mode. Since the proposed concept is based on weak coupling between propagating modes, electrically long propagation distance in the cloak is required, increasing the overall dimension of the system. However, the cloak thickness can be controlled by modifying the modulation parameters. The proposed concept may enhance the performance of radio-frequency communication systems, reducing the in-band captured noise or interfering signals.