  We present a terahertz (THz) frequency-domain spectroscopic (FDS)
ellipsometer design which suppresses formation of standing waves by use of
stealth technology approaches. The strategy to suppress standing waves consists
of three elements \textit{geometry}, \textit{coating} and \textit{modulation}.
The instrument is based on the rotating analyzer ellipsometer principle and can
incorporate various sample compartments, such as a superconducting magnet,
\textit{in-situ} gas cells or resonant sample cavities, for example. A backward
wave oscillator and three detectors are employed, which permit operation in the
spectral range of 0.1--1~THz (3.3--33~cm$^{-1}$ or 0.4--4~meV). The THz
frequency-domain ellipsometer allows for standard and generalized ellipsometry
at variable angles of incidence in both reflection and transmission
configurations. The methods used to suppress standing waves and strategies for
an accurate frequency calibration are presented. Experimental results from
dielectric constant determination in anisotropic materials, and free charge
carrier determination in optical Hall effect, resonant-cavity enhanced optical
Hall effect and \textit{in-situ} optical Hall effect experiments are discussed.
Examples include silicon and sapphire optical constants, free charge carrier
properties of two dimensional electron gas in group-III nitride high electron
mobility transistor structure, and ambient effects on free electron mobility
and density in epitaxial graphene.
