This paper describes the 300A and the auxiliary channel protection switching systems which provide continuity of service on TH‐3 medium‐haul radio systems. Both protection switching systems operate on a one‐by‐one frequency diversity basis. The 300A is a baseband‐to‐baseband system and protects the message circuit load. The auxiliary channel switching system protects the order‐wire and alarm circuits and operates at both IF and baseband frequencies. System design considerations and circuits of special interest, including solid state switches and integrated circuit modules, are discussed in some detail.

This paper describes the transmitter-receiver test set and the FM termina test panel, designed for maintaining the TDS radio equipment, and discusses the instrumentation and measuring techniques used to achieve accuracy consistent with the high performance of the TDS system. Circuits of special interest are covered in some detail.