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How the Green Hornet provided secure transatlantic telephony during World War II.
Why a telephone call isn't secure
A telephone is just a microphone and a speaker combined. The sound waves from our vocal chords cause air to move backwards and forwards. This is like water in a swimming pool sloshing backwards and forwards as people swim. This vibrates on our ear drums, and they convert that into an electrical signal to our brain.
The telephone extends that distance by converting the moving air, which can only move a few feet, into an electrical signal that varies in voltage in exactly the same way as the sound waves vary.
This means that anybody who taps into that electrical current along its path can pick up an exact reproduction of the sound being transmitted, so it's not secure. These days the sound wave is converted into a binary representation for transmission, which makes it a lot easier to apply digital encryption to it. However, prior to the 1960s the entire end-to-end link was analogue. Moreover, the first transatlantic telephone cable did not enter service until the mid 1950s, so during WW2 telephone calls between Britain and the USA were sent using radio waves, making calls even more vulnerable to interception.
The long wave Atlantic link
A long wave radio telephone link across the Atlantic had first been successfully achieved in 1915, and this historic event marked the beginning of a new era in communication, as voices could cross the vast Atlantic in real time. Long wave radio frequencies were crucial to this, as the lower frequencies were capable of travelling over the very long distances required. In the following decades the long wave radio Atlantic link underwent expansion and improvement. Technological advances and increased understanding of radio propagation allowed for more reliable and efficient communication.
The radio telephone link between Britain and the USA opened to the public in 1927, via radio masts at the GPO radio station at Hillmorton near Rugby and at Wroughton in Wiltshire. Speech was transmitted from Rugby and the return speech was received at Wroughton. On the American side, speech from the UK was received at Houlton in Maine and transmitted from Rocky Point, New York.
The two radio sites interfaced with standard trunk telephone cable circuits, which went to the GPO's International Exchange at Faraday House in London. From there, operators could connect via the national trunk network to any exchange in the country.
The cost of a transatlantic call in the first year was £15 for three minutes — nearly £1,000 in today's prices!
Despite its successes, the long wave radio Atlantic link faced challenges such as signal degradation and atmospheric interference.
The move to short wave
A switch to using short wave radio required less power and shorter aerials, which drastically reduced costs, leading to increased demand.
Engineers and radio enthusiasts began experimenting with short wave frequencies to overcome the limitations of traditional communication methods. The technology progressed rapidly, making it possible to establish reliable and consistent links between Europe and North America.
By the end of the 1930s Rugby was capable of transmitting many simultaneous calls to multiple countries throughout the world. Additional receiving stations were added at Baldock in Hertfordshire and at Cooling, near Tilbury.
