  Reconfigurable intelligent surfaces (RISs) will play a key role to establish
reliable low-latency millimeter wave (mmWave) communication links for indoor
automation and control applications. In case of a blocked line-of-sight between
the base station (BS) and the user equipment (UE), a RIS mounted on a wall or
on a ceiling enables a bypass for the radio communication link. In this work,
we present an active RIS for the mmWave frequency band. Each RIS element uses a
field effect transistor (FET) to amplify the reflected signal and an orthogonal
polarization transformation to increase the isolation between impinging and
reflected radio wave. By switching the bias voltage at gate and drain of the
FET we can establish four states for each RIS element: two reflection states
with different phase shifts, an active amplification and an off state. We
present results of the active RIS with 37 patch antenna elements arranged in a
hexagonal grid for a center frequency of 25.8 GHz. The RIS field patterns
obtained by numerical simulations and by empirical measurements in an anechoic
chamber are compared. They show a good match and the received power is improved
by 12 dB in the active mode of the RIS compared to the reflective mode.
