  Time variation has been recently introduced as an additional degree of
freedom for wave engineering, that enables going beyond the performances that
are expected by linear time-invariant (LTI) systems. In this paper, we
introduce the concept of indirect time-modulation of antennas using an add-on
time-varying scatterer (parasitic element) that gives rise to an inherent
feedback mechanism via the airborne wave system. As opposed to a direct
modulated system where a time-dependent element is in contact with the other
elements, in an indirect time modulation scheme \emph{no} direct physical
contact between the original LTI network and the time-varying add-on scatterer
is needed, thus leading to additional flexibility in the design. Using indirect
time modulation we demonstrate enhanced effective coupling between remote
antenna elements, and the possibility to outperform the gain-bandwidth achieved
for the same antenna structure but without time-modulation.
