Ultra-low absorption measurement in dielectrics in millimeter- and submillimeter-wave range

A measurement of ultra-low absorption of microwave radiation in dielectrics is reported. Two Fabry-Perot resonators with ⩾600000 quality factors, fully general-purpose interface bus programmable millimeter-wave frequency synthesizer with 10-15-mW continuous wave (CW) power level, 100-Hz frequency resolution from 78 to 118 GHz, and corresponding hardware and software for signal processing were used. The ±500-Hz accuracy of resonance curve width measurements was reached. This high accuracy allowed loss tangent measurement as small as 10^-6-10^-7 in dielectric samples with a thickness of ~0.5 mm. A convenient method of measurements of almost arbitrary plane parallel samples has been developed and described. Practical applications such as development and control of thin low-loss resonant windows of powerful (~1-MW CW) gyrotrons used in thermonuclear experiments, precise reflection coefficient of metals measurements, as well as other applications are discussed. The existence of such technique up to frequencies exceeding 1 THz makes measurements described at the whole millimeter- and submillimeter-wave bands affordable