  A reconfigurable intelligent surface (RIS) is an essential component in the
architecture of the next generation of wireless communication systems. An RIS
is deployed to provide a controllability to the multi-path environment between
the transmitter and the receiver, which becomes critical when the line-of-sight
signal between them is blocked. In this work, we design an electrically tunable
linearly polarized RIS at 2.5 GHz that yields a controllable reflection phase
and phase-frequency slope; in other words, we add tunability of the
phase-frequency slope to the tunability of the resonance center frequency. The
proposed design consists of two layers of unit cells placed over a ground
plane, with dog-bone-shaped elements in the top layer and patch elements in the
bottom layer. Each patch and dog-bone element is loaded with a varactor, whose
reverse bias voltage is controlled to provide a phase-frequency profile with a
slope value of 9 degrees/MHz or 0.95 degrees/MHz, and a phase shift range of
320 degrees.
