The surplus supply of valves after the First World War led professionals and amateurs to experiment with voice transmission. These first experiments were called ‘voice telephony’, even though they did not imply any aspects of switching and connectivity as expected today from telephony systems. The main drawback for commercial applications of these communications systems was the lack of privacy, since it was easy to eavesdrop

on any conversation. But this drawback was turned into a benefit with the advent of broadcasting. Broadcasting enjoyed spectacular growth in the USA: in less than three years after the opening of the first broadcasting station in 1920 there were 500 stations with 2 million listeners.

In 1924, there were 1100 stations and the unregulated use of radio frequencies led to chaos. In 1927, a first attempt was made to regulate spectrum usage during an international conference in Washington, it was agreed to allocate the frequency band from 550 kHz to 1.5 MHz to broadcasting. Frequencies below this were allocated to maritime communications. Europe
was lagging behind in the evolution of broadcasting, which had been restricted much earlier through the set-up of public companies such as the British Broadcasting Corporation (BBC). There were some 200 broadcasting stations in Europe in 1929 and broadcasting transformed the wireless industry into a high-volume and low-cost industry, especially through
the market for wireless radio receivers. Until the outbreak of the Second World War, the most important technological developments were made in this field.

The first attempts at true mobile date back to the early 1920s. In the USA in 1921, the Detroit Police Department made the first experiments with ‘mobile’ radio (Noble, 1962). At the beginning, the service was limited to a sort of paging, instructing the police car in question to stop and call back to the police station. These one-way systems were widely used in the
USA. Similar experiments were carried out by the Metropolitan Police of London, though with less satisfactory results (Garrard, 1998). In 1932, the Brighton police force was equipped with radio equipment weighing just over 1 kilo, so that they could be carried by patrolling police officers. One-way messages could be sent to all officers within the range of 6 km. A few years later proper two-way communication features were put in place, but for reasons of weight the equipment could be fitted only to vehicles.

The British police, however, were reluctant to introduce these voice communications systems because of lack of privacy. The police preferred to fit its cars with telegraphy systems during the mid-1930s, telegraphy had the advantage of greater reach (up to 100 km) and was less likely to be eavesdropped, as relatively few persons were able to read Morse signals. The US police was less concerned with privacy, and preferred to adopt voice communication systems. Sweden also equipped its police force with two-way voice communication systems in the late 1930s. Most of the police radio systems at that time worked in the 1.5–3.0 MHz band, which at that time was found to give the best compromise between availability, interference and performance.