Before Clear Channel Communications
started gobbling up U.S. radio
stations left and right, the phrase "clear channel" had a specific meaning relating to AM
radio licensing in the United States
If you have ever listened to AM radio late at night, you know that through the mysteries of propagation, you can pick up stations from out of town, some of them as clearly as if they were broadcasting from a mile away. In the early days of radio, the Federal Communications Commission viewed this as a development to be encouraged, and granted several stations exclusive licenses to a particular frequency late at night, granted that they put out a high-power signal worthy of the privilege. Other stations on this frequency in other nearby cities would have to turn off their transmitters at night; stations on nearby frequencies would have to turn their power down to avoid interference, while these licensees got to maintain a steady (usually) 50,000 watts, thus the name "clear channel." Depending on antenna directional orientation, another station might be allowed on that frequency a thousand (or more) miles away. Presumably the FCC coordinated such assignments with the similar Canadian and Mexican communications authorities; radio waves don't tend to respect national borders.
Eventually, the FCC found both that the required power drop was discouraging the development of local AM stations and that they were running out of clear channels to issue. Thus, they ceased to issue clear channel licenses in the late 1960s or early 1970s, but allowed then-current license holders to maintain this status. These licenses have, over time, usually become associated with a city's biggest and best-known AM station, often carrying a news/talk or sports format.
A good list of clear channel assignments for North America can be found at http://www.ac6v.com/clearam.htm.