  Polarization modulation is a powerful technique to increase the stability of
measurements by enabling the distinction of a polarized signal from dominant
slow system drifts and unpolarized foregrounds. Furthermore, when placed as
close to the sky as possible, modulation can reduce systematic errors from
instrument polarization. In this work, we introduce the design and preliminary
drive system laboratory performance of a new 60 cm diameter reflective
half-wave plate (RHWP) polarization modulator. The wave plate consists of a
wire array situated in front of a flat mirror. Using \mbox{50 $\mu$m} diameter
wires with \mbox{175 $\mu$m} spacing, the wave plate will be suitable for
operation in the millimeter wavelength range with flatness of the wires and
parallelism to the mirror held to a small fraction of a wavelength. The
presented design targets the 77--108 GHz range. Modulation is performed by a
rotation of the wave plate with a custom rotary drive utilizing an actively
controlled servo motor.
