  This work presents a 340-GHz frequency-modulated continuous-wave (FMCW)
pulse-Doppler radar. The radar system is based on a transceiver module with
about one milli-Watt output power and more than 30-GHz bandwidth. The front-end
optics consists of an off-axis parabola fed by a horn antenna from the
transceiver unit, resulting in a collimated radar beam. The digital radar
waveform generation allows for coherent and arbitrary FMCW pulse waveforms. The
performance in terms of sensitivity and resolution (range/cross-range/velocity)
is demonstrated, and the system's ability to detect and map single particles
(0.1-10 mm diameter), as well as clouds of particles, at a 5-m distance, is
presented. A range resolution of 1 cm and a cross-range resolution of a few
centimeters (3-dB beam-width) allow for the characterization of the dynamics of
particle clouds with a measurement voxel size of a few cubic centimeters. The
monitoring of particle dynamics is of interest in several industrial
applications, such as in the manufacturing of pharmaceuticals and the
control/analysis of fluidized bed combustion reactors.
