The simple answer is it varies. It is dependant on the range of the transmitter, the popularity of the station or program, the population reached, competition and money available.
Radio -- signaling and audio communication using electromagnetic radiation -- was first employed as a "wireless telegraph", for point-to-point links where regular telegraph lines were unreliable or...
The Federal Communications Commission has announced the following totals for broadcast stations licensed as of March 31, 2004: AM STATIONS - 4781 FM COMMERCIAL - 6224 FM EDUCATIONAL - 2471TOTAL:...