Scanning Microwave Echo Box Spectrometer

A resonant cavity microwave spectrometer for wavelengths between 3 and 30 cm, which uses radar echo boxes as absorption cells, is described. The minimum detectable absorption coefficient at 10 cm wavelength is 10−10 cm−1, and this sensitivity is maintained during frequency scans by automatic feedback tuning of the echo box. The spectrometer is well suited to studies of gaseous chemical reactions, and some preliminary observations are reported, including the negative results of a search for the 10 cm spectrum of CH, and measurements of OH spectra at 4.7 and 6.0 GHz.