Microwave radar brought direct phone calls to millions
spectrum.ieee.org
spectrum.ieee.org
> Although an experimental 40-mile (64 km) microwave telecommunication link across the English Channel was demonstrated in 1931, the development of radar in World War II provided the technology for practical exploitation of microwave communication.
> During the war, the British Army introduced the Wireless Set No. 10, which used microwave relays to multiplex eight telephone channels over long distances. A link across the English Channel allowed General Bernard Montgomery to remain in continual contact with his group headquarters in London.
> The development of radar during World War II provided much of the microwave technology which made practical microwave communication links possible, particularly the klystron oscillator and techniques of designing parabolic antennas.
The fact that pre-war the links were a few dozen miles, vs transcontinental links post-war shows that it really wasn’t a viable technology before radar. And that’s generally true for most technology; war and the military tent to speed up the pace of technology development greatly.
The premise seems overstated to me...there's certainly some crossover, but both radar and microwave telephony were progressing in parallel.
>transcontinental links post-war
That would have to be relays. Even with tropo-scatter, microwave radios are only good for a few hundred miles. The "few dozen miles" limitation would just be the normal earth-curvature limitation that line-of-sight brings.
Looks like it, see what I've been reading:
> Work on klystrons became very intensive during WWII. Radar was independently invented in Germany and Britain before the war, and was initially implemented at UHF frequencies using triodes as sources. The invention of the klystron in the US encouraged the British in a search for a higher power microwave source, one which would make possible both better target discrimination and more compact transmitters. The 1941 invention of the magnetron by Randall and Boot in Britain16 provided such a source for radar transmitters. The same year, (and before Pearl Harbor) a visit by British scientists to America launched an extraordinary collaboration between Britain and the US, which resulted in magnetrons and klystrons being manufactured for the war effort in Western Electric, Sperry, and other American factories. The reflex version of the klystron, perfected at Sperry and in British laboratories, served as the local oscillator in superheterodyne radar receivers. Magnetrons powered the transmitters. Together, the magnetron and the klystron made possible airborne S-band radar, a major factor in securing air superiority for the RAF.
https://web.archive.org/web/20150924102609/http://www.slac.s...
> The klystron was the first significantly powerful source of radio waves in the microwave range; before its invention the only sources were the Barkhausen–Kurz tube and split-anode magnetron, which were limited to very low power. It was invented by the brothers Russell and Sigurd Varian at Stanford University. Their prototype was completed and demonstrated successfully on August 30, 1937.[5] Upon publication in 1939,[3] news of the klystron immediately influenced the work of US and UK researchers working on radar equipment.
- War effort requires a whole lot of capital investment -- factories built, equipment developed, people come up to speed, etc.
- Postwar, radar is still important, but the overall government expenditure falls.
- Much of the capital (manufacturing and expertise) searches for a peacetime use... and what's the low hanging fruit? Telecommunications.
That is, war money was being applied to both.
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.
The earliest widespread sites used square delay lens antennas in the 2 GHz band, later 4 GHz. See black and white photo here: https://long-lines.net/places-routes/Yellow_Springs/index.ht...
Some of the oldest AT&T Long Lines sites with this type of equipment were built in the general corridor from Boston-NYC-WA DC
In the United States there are abandoned Microwave towers left over from this kind of system. People have investigated them out in New Mexico, they run roughly parallel to I-81 between Syracuse and Binghamton and many cities have microwave antennas built into or on top of old central office buildings in cities like Manchester, NH.
American Tower has bought up many of them and could potentially lease the properties for cell phones, broadcast radio and TV, emergency communications, amateur radio, etc. but these are frequently sited in locations not favorable at all for these uses.
One note that much of the same technology was used by the other assorted non-ATT legacy phone system operators like GTE. It all looks the same to an observer on the ground looking at old horn antennas but not everything that looks like an ATT Long Lines site was actually Ma Bell.
However, price does not nearly always follow costs.
International phone call prices are mainly set by monopolies and regulators.
If you want free, make a VoIP call.
Only a few telephone lines on 6MHz!!! How large guard bands do you need on a 3Khz telephone line?
Edit: layout
End of an era, all those squat square masts getting stripped of the huge dishes that shot microwave links all up and down the country.