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I was watching the Peaky Blinders TV Series when something triggered my curiosity: If there were no satellites, nor underseas cables, how come the characters were making overseas phone calls?

After some research, I learnt that those phone calls were made via radio. But due to Earth's curvature, how is it possible that a radio wave from the U.S could reach an antenna in England, and vice versa?

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    \$\begingroup\$ I've got no reference for it, but I thought the radio beams were bounced off the ionosphere. And that is a common method used today by radio hams to get further distances. Famously Marconi showed this could be done in 1901 with a simple Morse code message over radio waves. \$\endgroup\$ – Puffafish Sep 21 '20 at 15:28
  • \$\begingroup\$ This may answer your question: ham.stackexchange.com/questions/16071/… \$\endgroup\$ – Aaron Sep 21 '20 at 15:50
  • \$\begingroup\$ As a sidenote, there might have not been undesea phone cables but there were undersea cables in 1927. Pupin's addition might make an interesting read: hackaday.com/2020/06/19/… (I had read about it from another source, this is the first easy link I've found). "The first transatlantic cable went into service in 1858" \$\endgroup\$ – Sredni Vashtar Sep 21 '20 at 23:56
  • \$\begingroup\$ @Puffafish is correct. Shortwave bounces off the ionosphere. Transatlantic short wave communications are routine. In the US this is not well know since few consumer radios can tune to the shortwave bands. If you happen to have a radio with a shortwave band, even a cheap handheld radio, you can easily pick up broadcast stations from Europe. \$\endgroup\$ – David42 Sep 22 '20 at 15:23
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The system you're referring to was described briefly in the Bell System Technical Journal in 1935. Most of the article is about measuring the advantages of the (then brand-new) "single-sideband suppressed carrier" (SSB) modulation over the conventional double-sideband amplitude modulation (AM), but it mentions some of the history of transatlantic telephone in passing, and says that it used shortwave frequencies of 5 to 20 MHz (5,000 to 20,000 kc in the notation of the time).

Waves at these frequencies are capable of being refracted by the F-layer of the ionosphere (a layer of charged particles 200 - 400 km above the Earth's surface, which was theorized in the 19th century, but first seriously measured between 1925 and 1927). Refraction of signals by the ionosphere makes it possible to communicate between points on the Earth's surface up to 3,000 - 4,000 km apart in a single "hop". This is just about enough to cover the distance between Newfoundland and England, but it's also possible for a signal to take multiple "hops" — conveniently, seawater makes a good reflector of radio signals.

The relatively wide range of frequencies (a 4:1 ratio) was used because ionospheric conditions vary with time of day, time of year, and the solar cycle — generally, lower frequencies perform better at night, in winter, and during solar minimum, while higher frequencies perform better at midday, in summer, and during solar maximum.

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    \$\begingroup\$ Per en.m.wikipedia.org/wiki/Rugby_Radio_Station the call was at 60 khz not 5 to 20 mhz. \$\endgroup\$ – Passerby Sep 21 '20 at 16:29
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    \$\begingroup\$ It would appear that you are describing a slightly later HF system, and that Passerby is right and the original was VLF: rugbyradiostation.co.uk/articles/1927-article \$\endgroup\$ – Chris Stratton Sep 21 '20 at 17:54
  • \$\begingroup\$ In the 80's, my father managed to talk with people in Brazil from Belgium with an amateur CB set. The antena was quite impressive thought, for the kid I was. \$\endgroup\$ – Fredled Sep 21 '20 at 20:03
  • \$\begingroup\$ If the transmissions were at 60kHz, then groundwave propagation is far more likely to have been used than skywaves. \$\endgroup\$ – ocrdu Sep 22 '20 at 8:35
  • \$\begingroup\$ @ocrdu I agree and I would delete this answer for an overhaul but it's already been accepted. \$\endgroup\$ – hobbs Sep 22 '20 at 17:39
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The 1935 paper "A Single Side-Band Short-Wave System for Transatlantic Telephony" , by Polkinghorn and Schlaack is interesting. Stone knives and bear skins indeed.

Here's an excerpt from an earlier paper "The Propagation of Short Radio Waves over the North Atlantic" Published in: Proceedings of the Institute of Radio Engineers ( Volume: 19 , Issue: 9 , Sept. 1931 ). Of course, mc (megacycles) refers to MHz in the below.

Summary-Transmission conditions for each season are shown by "surfaces" giving the received field strength as a function of time of day and frequency. These show that frequencies near 18 mc are best for daytime transmission. In summer the best frequencies for nighttime transmission are those near 9 mc. In winter an additional frequency near 6 mc is required during the middle of the night. A frequency (such as 14 mc) intermediate between the day and night frequency is useful during the transition period between total daylight and total darkness over the path. Day-today variations change the periods of usefulness of these frequencies. In particular the period of usefulness on 14 mc sometimes extends so that it is the best daytime frequency. Transmission conditions on undisturbed days were found to be the same for the same time of year on different years. These undisturbed transmission conditions are presented by "normal" surfaces. Comparison of these surfaces shows that the higher frequencies are less attenuated in winter. Reception on the highest frequency, 27 mc was best in winter; in summer this frequency was never heard. The effect of solar disturbances on short-wave transmission is to reduce reception on all frequencies. Sometimes the higher frequencies are the more adversely affected. Some of the, possible causes of these disturbances are discussed. From the measurements made on "static" at New Southgate, data on the variation of its field strength as a function of frequency, time of day, and season are given.

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  • \$\begingroup\$ Reliable HF radio communication has always been about changing bands to adapt to conditions. \$\endgroup\$ – Chris Stratton Sep 21 '20 at 16:09
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    \$\begingroup\$ @ChrisStratton Early motor-driven PLL from the first paper. \$\endgroup\$ – Spehro Pefhany Sep 21 '20 at 16:35
  • \$\begingroup\$ The PLL would be used to keep the LO or BFO tuned so the other party doesn't sound like Donald Duck without requiring an operator to do so by hand. The actual band switching under different propagation conditions would be a multi-element stacked selector switch touching all RF stages; or else a switch to an entirely different set of gear. Typically switch antennas, too (though there are some possibilities which can work, eg 7 MHz antenna at 21 MHz) \$\endgroup\$ – Chris Stratton Sep 21 '20 at 16:39
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When the first transatlantic telegraph cable was laid in 1858 by Cyrus West Field, it operated for only three weeks; subsequent attempts in 1865 and 1866 were more successful. Although a telephone cable was discussed starting in the 1920s, to be practical, it needed a number of technological advances which did not arrive until the 1940s. Starting in 1927, transatlantic telephone service was radio-based. Source: Wikipedia Transatlantic communications cable.

Timeline of the telephone lists the following:

  • 7 March 1926: First transatlantic telephone call, from London to New York.
  • 7 January 1927: Transatlantic telephone service inaugurated for commercial service (3500 miles).

It doesn't say whether they were full-duplex or half-duplex.

enter image description here

Figure 1. Radio waves can bounce from the ionosphere and the earth's surface. This works on water too. Image source: Bug Out Bag Builder.

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Bouncing the radiowaves like Marconi did when "they said it couldn't be done" is one of the options for over-the-horizon radio communication, but the Wikipedia article on Rugby Radio Station mentioned in the comments says:

In 1927, a second transmitter was installed to initiate the first transatlantic commercial telephone service; linking New York and London on 60 kHz using single-sideband modulation.

At this frequency it is most likely the effect they used is groundwave propagation: at such (lower) frequencies, waves can travel as "ground waves", which can and do follow the Earth's curvature. This works particularly well over conductive surfaces like sea water.

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