A measurement of ultra-low absorption of microwave radiation in dielectrics
is reported. Two Fabry-Perot resonators with greater than or equal to 600
000 quality factors, fully general-purpose interface bus programmable milli
meter-wave frequency synthesizer with 10-15-mW continuous wave (CW) power l
evel, 100-Hz frequency resolution from 78 to 118 GHz, and corresponding har
dware and software for signal processing were used. The +/-500-Hz accuracy
of resonance curve width measurements was reached, This high accuracy allow
ed loss tangent measurement as small as 10(-6) -10(-7) in dielectric sample
s with a thickness of similar to 0.5 mm, A convenient method of measurement
s of almost arbitrary plane parallel samples has been developed and describ
ed, Practical applications such as development and control of thin low-loss
resonant windows of powerful (similar to 1-MW CW) gyrotrons used in thermo
nuclear experiments, precise reflection coefficient of metals measurements,
as well as other applications are discussed. The existence of such techniq
ue up to frequencies exceeding 1 THz makes measurements described at the wh
ole millimeter- and submillimeter-wave bands affordable.