While there was important early work, fixed microwave telecommunications really didn't emerge as a significant component of the telephone system until after Wor

While there was important early work, fixed microwave telecommunications really didn't emerge as a significant component of the telephone system until after World War II. The high cost, short range, and demanding maintenance requirements of microwave systems made them impractical as a toll carrier except in rather narrow circumstances. Through WWII the Bell System, for example, was relying on buried cables (which they used with analog multiplexing, achieving fairly substantial capacity but at a high installed cost). The first major landmark in civilian microwave telecommunications is 1951, when the Bell System completed the first transcontinental microwave route. This too was primitive by the standards of even five years later, and the large transmitters, inefficient waveguides, and basic antenna designs required that most of the equipment be installed in a hut at the top of the tower... and the range was only about 30 miles. The construction cost of these sites was very high but they already showed great promise to be more cost-effective than cable carrier. Basically all of the advancements that enabled this microwave link came out of the military. Improvements in radio transmitters (such as the klystron) were significant, but likely more significant was the rapid evolution of antenna, waveguide, and feedline design. Improved antenna gains allowed for longer ranges (moreso even than increased transmit power) and lower-cost, higher-efficiency waveguide made it possible to locate most equipment at ground level... this type of design was pioneered by radar where steering of the antenna made it impractical to mount the transmitter directly to the antenna. It tremendously reduced construction and maintenance costs. Early microwave telecom systems (through the '70s for the large part) were analog FDM, not digital, and based mostly on established FDM techniques for open-wire and cable but benefiting from the large bandwidth that microwave offered (AT&T would apply similar techniques to both buried coaxial cable and buried microwave waveguide, although the latter is sort of a famous failure). But, the AT&T lineage of digital telecommunications is also a product of WWII, with Bell Labs having developed a practical PCM method for cryptographic use. Innovations from radar design were instrumental in the development of digital radio systems later on, as radar drove a need for increasingly complex modulation techniques and equipment that supported them. This is of course a very US-centric explanation, but the GPO in Great Britain followed a broadly similar timeline to AT&T. Important radio innovations were made there was well during WWII, and imported English equipment (e.g. from General Electric, the London one) was common in US radio telecom equipment through the mid-century.