Defining exactly when broadcasting first began is difficult. Very early radio transmissions only carried the dots and dashes of wireless telegraphy. One of the first signals of significant power that carried voice and music was accomplished in 1906 by Reginald Fessenden when he made a Christmas Eve broadcast to ships at sea from Massachusetts. He played "O Holy Night" on his violin and read passages from the Bible. However, his financial backers lost interest in the project, leaving others to take the next steps. Early on, the concept of broadcasting was new and unusual with telegraphs, communication had been one-to-one, not one-to-many. Sending out one-way messages to multiple receivers didn't seem to have much practical use.

Charles Herrold of San Jose, California sent out broadcasts as early as April 1909 from his Herrold School electronics institute in downtown San Jose, using the identification San Jose Calling, and then a variety of different call signs as the Department of Commerce began to regulate radio. His station was first called FN, then SJN (probably illegally). By 1912, the United States government began requiring radio operators to obtain licenses to send out signals. Herrold received licenses for 6XF and 6XE (a mobile transmitter) in 1916.

He was on the air daily for nearly a decade when World War I interrupted operations. After the war, the Herrold operation in San Jose received the call-sign KQW in 1923. Today, the lineage of that continues as KCBS, a CBS-owned station in San Francisco.

Herrold, the son of a farmer who patented a seed spreader, coined the terms broadcasting and narrow casting, based on the ideas of spreading crop seed far and wide, rather than only in rows. While Herrold never claimed the invention of radio itself, he did claim the invention of broadcasting to a wide audience, through the use of antennas designed to radiate signals in all directions.

By comparison, David Sarnoff has been considered by some, arguably and perhaps mistakenly, as "the prescient prophet of broadcasting who predicted the medium's rise in 1915", referring to his radio music box concept.

A few organizations were allowed to keep working on radio during the war. Westinghouse was the most well-known of these. Frank Conrad, a Westinghouse engineer, had been making transmissions from 8XK since 1916 that included music programming.

However, a team at the University of Wisconsin-Madison headed by Professor Earle M. Terry also had permission to be on the air.  They operated 9XM, originally licensed by Professor Edward Bennett in 1914, and usually sent Morse code weather reports to ships on the Great Lakes, but they also experimented with voice broadcasts starting in 1917. They reportedly had difficulties with audio distortion, so the next couple of years were spent making transmissions distortion-free.

Following the war, Herrold and other radio pioneers across the country resumed transmissions. The early stations gained new call signs. 8XK became KDKA in 1920. Herrold received a license for KQW in 1921 (later to become KCBS). 9XM became WHA in 1922.

The National Broadcasting Company began regular broadcasting in 1926, with telephone links between New York and other Eastern cities. NBC became the dominant radio network, splitting into Red and Blue networks. The Columbia Broadcasting System began in 1927 under the guidance of William S. Paley.

Several independent stations formed the Mutual Broadcasting System to exchange syndicated programming, including The Lone Ranger and Amos 'n' Andy.

A Federal Communications Commission decision in 1939 required NBC to divest itself of its Blue Network. That decision was sustained by the Supreme Court in a 1943 decision, National Broadcasting Co. v. United States, which established the framework that the "scarcity" of radio-frequency meant that broadcasting was subject to greater regulation than other media. This Blue Network network became the American Broadcasting Company (ABC). Around 1946, ABC, NBC, and CBS began regular television broadcasts. Another TV network, the DuMont Television Network, was founded earlier, but was disbanded in 1956.

(Courtesy of Wilkipedia)
Beginning of Broadcasting in the United States
NBC's Blue Network became ABC radio network.