  We present a novel, power and hardware efficient, antenna system leveraging
the eigenmodes of the over-the-air propagation matrix from an active
multi-antenna feeder (AMAF) to a large reflective intelligent surface (RIS),
both configured as standard linear arrays and placed in the near field of each
other. We demonstrate the flexibility of the proposed architecture by showing
that it is capable of generating radiation patterns for multiple applications
such as very narrow beams with low side lobes for space-division multiple
access communications, wide-angle beams for short range automotive sensing and
sectorial beaconing, and monopulse patterns for radar angular tracking. A key
parameter in our design is the AMAF-RIS distance which must be optimized and it
is generally much less than the Rayleigh distance. For a given AMAF aperture,
the optimal AMAF-RIS distance increases as a function of the RIS size. The
AMAF-RIS loss is compensated almost exactly by the larger aperture gain of the
RIS leading to almost constant RIS gain with increasing RIS sizes. This allows
to choose different beam angular selectivities with the same center beam gain.
Active RF amplification is done at the AMAF only, thus resulting in a much
higher power efficiency and much lower hardware complexity than conventional
phased arrays with same beamforming performance.
