TH-3 Microwave Radio System: System Considerations
01 September 1971
The T H - 3 microwave radio relay system is a new long-haul facility designed to provide modern solid state equipment with improved performance for use in the 6 GHz band. When work began on the project in 1966, most of the Bell System radio traffic was carried on the 4 GHz T D - 2 system. Also, at that time the first TD-3 1 radio route was being installed. Since T H - 3 routes can be installed most economically as an "overbuild" on existing T D - 2 and T D - 3 (TD radio) routes, it was clear that T H - 3 must be compatible with TD-2, and as much like the new TD-3 as possible. The features of T H - 3 which differentiate it from its predecessor in the 6 GHz band, the T H - 1 2085 2086 T H E BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1971 radio system, 2 and make it more like TD radio are the individual transmitter-receiver bays, and the 70 MHz IF. The latter increases certain difficulties with interchannel interference but allows TH-3 to use the FM terminals, switching systems, and some of the IF circuits developed for TD-3. Also, by adopting the 70 MHz IF, a combined TD and TH-3 facility with crossband protection switching looks feasible at this time. Although TH-3 was originally conceived as a long-haul, multichannel system with a 4000-mile capability, the need to adapt it to shorter routes with a smaller cross section of circuits was quickly recognized. The so-called medium-haul TH-3 then came into being. It is discussed in later articles of this issue.3,4 Many of the design techniques which were assimilated during the TD-3 development were directly applicable to TH-3.