  A novel metasurface unit cell architecture is proposed to enable independent
control of the reflection magnitude and phase at a desired operation frequency,
while maintaining linear polarization of the incoming fields. The proposed
structure is based on a coupled-resonator configuration where a Dipole Ring
Resonator (DRR) is loaded with a tunable lumped resistive element (e.g. PIN
diode) and Split Ring Resonator (SRR) loaded with a lumped tunable capacitor
(e.g. varactor diode), are interleaved. The surface is next operated around one
of the coupled resonant frequency, where an independent tuning of the lumped
capacitance and resistance elements enable a wide coverage of reflection
amplitude-phase, which is significantly larger than what would have been
achievable using a single resonator configuration. An insightful equivalent
circuit model is further developed for investigating the amplitude-phase
characteristics of a uniform surface as a function of variable resistance and
capacitance, which is next confirmed using full-wave simulations. Finally,
using a variety of full-wave examples, the usefulness of simultaneous and
independent amplitude-phase control is demonstrated, including cases of
variable pattern gain with beam tilting and multi-beam pattern realization,
which otherwise would not be possible using either amplitude or phase control
only.
