>>> 2025-08-16 passive microwave repeaters (PDF)
One of the most significant single advancements in telecommunications technology was the development of microwave radio. Essentially an evolution of radar, the middle of the Second World War saw the first practical microwave telephone system. By the time Japan surrendered, AT&T had largely abandoned their plan to build an extensive nationwide network of coaxial telephone cables. Microwave relay offered greater capacity at a lower cost. When Japan and the US signed their peace treaty in 1951, it was broadcast from coast to coast over what AT&T called the "skyway": the first transcontinental telephone lead made up entirely of radio waves. The fact that live television coverage could be sent over the microwave system demonstrated its core advantage. The bandwidth of microwave links, their capacity, was truly enormous. Within the decade, a single microwave antenna could handle over 1,000 simultaneous calls.
Microwave's great capacity, its chief advantage, comes from the high frequencies and large bandwidths involved. The design of microwave-frequency radio electronics was an engineering challenge that was aggressively attacked during the war because microwave frequency's short wavelengths made them especially suitable for radar. The cavity magnetron, one of the first practical microwave transmitters, was an invention of such import that it was the UK's key contribution to a technical partnership that lead to the UK's access to US nuclear weapons research. Unlike the "peaceful atom," though, the "peaceful microwave" spread fast after the war. By the end of the 1950s, most long-distance telephone calls were carried over microwave. While coaxial long-distance carriers such as L-carrier saw continued use in especially congested areas, the supremacy of microwave for telephone communications would not fall until adoption of fiber optics in the 1980s.
The high frequency, and short wavelength, of microwave radio is a limitation as well as an advantage. Historically, "microwave" was often used to refer to radio bands above VHF, including UHF. As RF technology improved, microwave shifted higher, and microwave telephone links operated mostly between 1 and 9 GHz. These frequencies are well beyond the limits of beyond-line-of-sight propagation mechanisms, and penetrate and reflect only poorly. Microwave signals could be received over 40 or 50 miles in ideal conditions, but the two antennas needed to be within direct line of sight. Further complicating planning, microwave signals are especially vulnerable to interference due to obstacles within the "fresnel zone," the region around the direct line of sight through which most of the received RF energy passes.
Today, these problems have become relatively easy to overcome. Microwave relays, stations that receive signals and rebroadcast them further along a route, are located in positions of geographical advantage. We tend to think of mountain peaks and rocky ridges, but 1950s microwave equipment was large and required significant power and cooling, not to mention frequent attendance by a technician for inspection and adjustment. This was a tube-based technology, with analog and electromechanical control. Microwave stations ran over a thousand square feet, often of thick hardened concrete in the post-war climate and for more consistent temperature regulation, critical to keeping analog equipment on calibration. Where commercial power wasn't available they consumed a constant supply of diesel fuel. It simply wasn't practical to put microwave stations in remote locations.
In the flatter regions of the country, locating microwave stations on hills gave them appreciably better range with few downsides. This strategy often stopped at the Rocky Mountains.
In much of the American West, telephone construction had always been exceptionally difficult. Open-wire telephone leads had been installed through incredible terrain by the dedication and sacrifice of crews of men and horses. Wire strung over telephone poles proved able to handle steep inclines and rocky badlands, so long as the poles could be set---although inclement weather on the route could make calls difficult to understand. When the first transcontinental coaxial lead was installed, the route was carefully planned to follow flat valley floors whenever possible. This was an important requirement since it was installed mostly by mechanized equipment, heavy machines, which were incapable of navigating the obstacles that the old pole and wire crews had on foot.
The first installations of microwave adopted largely the same strategy. Despite the commanding views offered by mountains on both sides of the Rio Grande Valley, AT&T's microwave stations are often found on low mesas or even at the center of the valley floor. Later installations, and those in the especially mountainous states where level ground was scarce, became more ambitious. At Mt. Rose, in Nevada, an aerial tramway carried technicians up the slope to the roof of the microwave station---the only access during winter when snowpack reached high up the building's walls. Expansion in the 1960s involved increasing use of helicopters as the main access to stations, although roads still had to be graded for construction and electrical service.
These special arrangements for mountain locations were expensive, within the reach of the Long Lines department's monopoly-backed budget but difficult for anyone else, even Bell Operating Companies, to sustain. And the West---where these difficult conditions were encountered the most---also contained some of the least profitable telephone territory, areas where there was no interconnected phone service at all until government subsidy under the Rural Electrification Act. Independent telephone companies and telephone cooperatives, many of them scrappy operations that had expanded out from the manager's personal home, could scarcely afford a mountaintop fortress and a helilift operation to sustain it.
For the telephone industry's many small players, and even the more rural Bell Operating Companies, another property of microwave became critical: with a little engineering, you can bounce it off of a mirror.
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