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An S-band traveling-wave maser that operates over an 11 per cent tuning range is tuned by a single "frequency-calibrated" dial from 2120 to 2380 Mc. The maser has a relatively constant gain of 30 db and an instantaneous bandwidth of 22 Mc. It uses a comb-type slow-wave structure loaded on both sides with a 6 in length of 0.065 per cent chromium-doped ruby. Ferrimagnetic disks of yttrium iron garnet are periodically distributed in the slow-wave structure and provide sufficient reverse loss for short-circuit stability (100 db). A relatively high gain per unit length of active ruby was realized because of: 1) optimization of the C axis orientation with respect to the dc and RF magnetic fields, 2) utilization of a four spin flip cross-relaxation process and 3) loading both sides of the comb structure with active ruby. The traveling-wave maser and all the necessary auxiliary components have been packaged into an operational unit. The noise temperature of the system is about 10Â°K, and the short-term and long-term stabilities are Â±0.05 db and Â± 0.1 db, respectively. A series of detailed measurements on the important electrical characteristics are described, and the performance data is presented. The system has been designed and successfully tested to meet military field environmental specifications.