  In this paper we present the outcome of the first international comparison in
the terahertz frequency range among three different kinds of spectrometers. A
Fourier-Transform infrared spectrometer, a Vector Network Analyzer and a
Time-Domain Spectrometer have been employed for measuring the complex
refractive index of three travelling standards made of selected dielectric
materials in order to offer a wide enough range of parameters to be measured.
The three spectrometers have been compared in terms of measurement capability
and uncertainty.
