Conversion of Airborne HF Receiver-Transmitter from Double Sideband to Single Sideband

In the past, the problem of frequency accuracy and stability has been the major deterrent in the effective utilization of single sideband, suppressed carrier techniques for hf air-ground communications. This paper describes briefly the AN/ ARC-21 airborne hf, double sideband, communication equipment, and the frequency-control system which permits the remote selection and accurate automatic tuning to any selected frequency channel of the 44,000 available at intervals of 0.5 kc in the 2 to 24 mc spectrum. The improvement in the stability of this frequency-control system and the modifications of the several major subassemblies, essential to the conversion of this equipment to the single-sideband mode, are described. The provision for reception and transmission of an equivalent amplitude modulated signal during the transition period is discussed. Block diagrams, performance characteristics, and photographs are presented showing the original DSB equipment and the SSB conversion.