Radio stations in the United States are licensed by the Federal Communications Commission (FCC). When a new radio or television station applies for a license, or when an existing station applies for a change in its facilities, such as a change in location, transmitter power, or antenna height, the FCC, if it approves the application, will grant a "construction permit" to the station.

A construction permit is like a temporary license. It usually lasts for three years. During that time period, the station must get its facilites operating at the new parameters. A new station, for example, must get itself on the air. If an existing station applied for a new transmitter location, then that station must begin operating at the new location by the time the construction permit expires.

If all goes well, and the station complies with the construction permit, then the station is granted a license, which usually lasts for seven years.

For more information on construction permits, check out the FCC's web site.