These calls used a system similar to amateur radio calls, but generally with an "X" as the first letter after the geographic-area number. Experimental calls issued in the very early days of broadcasting, not containing a K/W prefix, are listed separately in this location. Most of these stations were either Apex stations which later became FM stations, FM stations, or television stations.
Frequency-based FM calls
The call-letter plan adopted for FM stations in 1941 was intended to be systematic: first a K or W, then two digits representing the frequency, then one or two letters representing the location. Frequencies at the time were in the range of 43 to 49 MHz, and the two-digit frequency code was obtained by dropping the "4" and the decimal point; thus W81SP was on 48.1 MHz in Springfield, Massachusetts.
FM translator calls
FM stations, rebroadcasting local AM stations in areas where reception is poor, known as FM translator stations, have a new type of call sign, resembling the frequency-based call signs of the 1940s. The call consists of a K or W, a three-digit numeral which starts with 201 for 88.1 MHz and goes up to 300 for 107.9 MHz, and two more letters: