Microwave power measurement using absorption spectrum of ammonia gas

A microwave power measurement technique using power saturation effect of the ammonia-gas spectrum is described. The absolute value of the power irradiated on the gas at resonance frequency is measured by detecting the fractional decrease of resonance absorption. Experiments carried out at 23.870 GHz, (J, K) = (3,3) inversion spectrum of ammonia ¹⁴NH3, are in agreement with the values calibrated by the calorimetrie power standard, within several percent of the error of the apparatus used. By improvements on the resolution of attenuation measurement and the estimation of the relaxation time constant, the error in power measurement could be less than ±1 percent. This technique could also be applied to the power measurement at many different frequency points in microwave and millimeter-wave region, where a huge number of spectral lines of various gases and their isotopie variants are found.