  We have constructed a Fourier-transform spectrometer (FTS) operating between
50 and 330 GHz with minimum volume (355 x260 x64 mm) and weight (13 lbs) while
maximizing optical throughput (100 $\mathrm{mm}^2$ sr) and optimizing the
spectral resolution (4 GHz). This FTS is designed as a polarizing
Martin-Puplett interferometer with unobstructed input and output in which both
input polarizations undergo interference. The instrument construction is simple
with mirrors milled on the box walls and one motorized stage as the single
moving element. We characterize the performance of the FTS, compare the
measurements to an optical simulation, and discuss features that relate to
details of the FTS design. The simulation is also used to determine the
tolerance of optical alignments for the required specifications. We detail the
FTS mechanical design and provide the control software as well as the analysis
code online.
