As stated on the page on assignments of FM station call letters, a new call-letter plan was adopted in 1941 that 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. This 1941 systematic call letter scheme lasted only until November 1, 1943, at which time the call letters were normalized to a similar scheme to the AM broadcast calls, except for some experimental-type calls.
The following table shows the changes made at that time (details on the individual stations can be obtained by linking to the individual stations' chronology pages):
|Old call letters||New call letters|