<P> In 1912, most amateur - radio transmissions were restricted to wavelengths below 200 meters (i.e., frequencies above 1500 kHz) to prevent interference to future commercial broadcasters . The beginning of regular, commercially licensed radio broadcasting in the United States in 1920, along with the concurrent development of sound and color film in that decade, ended the print monopoly of mass media and opened the doors to the immediate (and pervasive) electronic media . By 1928, the United States had three national radio networks: two owned by NBC (the National Broadcasting Company), and one by CBS (the Columbia Broadcasting System). Until 1943, there were four major national radio networks: two owned by NBC, one owned by CBS and one owned by Mutual Broadcasting System . Stations were connected by broadcast - quality, leased telephone lines . NBC's second network became ABC, the American Broadcasting Company . </P> <P> By 1919, after the war, radio pioneers across the country resumed transmissions . The early stations gained new call signs . Some early stations were started by newspapers: the first to do this was 8MK (today WWJ) in Detroit, which was owned by the Detroit News . While many sources cite KDKA in Pittsburgh as the first station, and give its first broadcast as November 2, 1920, the question of who came first is still in dispute among historians . For example, there is evidence 8MK was on the air with Michigan state election returns on August 31, 1920 Many other early stations were owned by small companies, such as AMRAD (the American Radio and Research Company, Medford Hillside MA); by some accounts, it broadcast voice and music as early as March 1916, and was on the air sporadically till it began a daily schedule in May 1921, using call letters 1XE and later, WGI . Perhaps the best - known early station began as experimental station 8XK, which became KDKA in 1920 . AM radio signals traveled long distances, and KDKA's signal covered much of the country . The existence of early experimental radio stations encouraged young men (and a small number of young women) to build crystal sets (with ear phones) to listen to the new technical marvel . Entrepreneurs set up stations primarily with the goal of selling expensive console radio sets that the whole family could listen to, or which restaurants and shops would buy to attract customers . </P> <P> Until early 1922, there were fewer than forty commercial stations broadcasting . But in mid-1922, the so - called "radio craze" began, when several hundred new stations took to the airwaves, and thousands of people wanted to buy their own radio set . Even President Warren G. Harding had a radio installed in the White House . </P> <P> While "direct advertising" was not allowed in radio's formative years, per the order of then - head of the Department of Commerce Herbert Hoover, station owners wanted this policy changed, since it was very expensive to operate a radio station . By 1926, when the National Broadcasting Company went on the air, advertising was gradually allowed . Madison Avenue recognized the importance of radio as a new advertising medium . Advertising provided the major funding for most stations . The United States never had a license fee for set users . </P>

When did the radio became a household item