  The quasi-optical modulation of linear polarization at millimeter and
sub-millimeter wavelengths can be achieved by using rotating half wave plates
(HWPs) in front of polarization sensitive detectors. Large operational
bandwidths are required when the same device is meant to work simultaneously
across different frequency bands. Previous realizations of half wave plates,
ranging from birefringent multi-plate to mesh-based devices, have achieved
bandwidths of the order of 100%. Here we present the design and the
experimental characterization of a reflective HWP able to work across
bandwidths of the order of 150%. The working principle of the novel device is
completely different from any previous realization and it is based on the
different phase-shift experienced by two orthogonal polarizations respectively
reflecting off an electric conductor and off an artificial magnetic conductor.
