A 50,000 watt medium-wave (AM) North American radio station that was, long ago (during frequency reallocations which took place from the 1920s to the 1940s), given a "clear channel" by the FCC and CRTC, meaning that it was the sole station on the continent (or on their side of The Mississippi) broadcasting on that frequency. That made what was nominally a local station into a national broadcaster, once the sun started going down, allowing the signal to reach over a greater distance.

Outside of the US and Canada, some enterprising businessmen were able to take advantage of locations in Mexico, broadcasting along the US border at even higher wattage; Wolfman Jack's legend was forged, in part, on such stations, whose signals reached far into the United States and Canada. It is said that Fidel Castro, when he's in one of his more playful moods, turns up the power (no, not personally) on Cuba's AM signals, and you can hear Cuban stations fairly far north of Havana on some nights. There are also a couple of offshore religious broadcasters that like to blast through these frequencies.

Some examples: frequencies owned, at one time or another by NBC, like 660 kHz, formerly their New York flagship WNBC, sold, about 20 years ago to Infinity Broadcasting, which run it as all-sports WFAN. And 750 kHz, for WSB in Atlanta. ABC stations, some of which may have belonged to NBC, before the government-mandated breakup of the network - 890 kHz for Chicago's WLS, 770 kHz for New York's WABC; of course The Mouse owns it all now, and operates 1000 kHz, which was once WCFL (owned by the Chicago Federation of Labor) but is now the flagship for ESPN Radio. CBS still has 880 kHz, for their all-news WCBS, but KMOX (1120 kHz), the station upon which Harry Caray broadcast St. Louis Cardinals games for many years, may have been sold.

There are also frequencies that were probably unaffiliated with those three networks, like WLW (700 kHz, Cincinnati) and WOR (New York, 710 kHz).

After the deregulation of the 80s, some daytime-only broadcasters have been able to use some of those frequencies, so one's ability to hear flamethrowers has been curtailed a bit, depending on where you live.