Bill's Radio Historical Pages
US 3-letter call letters.
prior to the 1912/1913 international agreements, the US Navy used N series
1912/1913: KDA to KZZ to ships Atlantic and Gulf; Coast Stns on Pacific Coast. Coast stations received K in front of existing 2 letter call.
1912/1913: WAA to WZZ to ships Pacific and Great Lakes; Coast Stns Grt Lakes, Gulf and Atlantic. Coast stations received W in front of existing 2 letter call or replaced existing M prefix. Exceptions to allocation: WUA to WVZ and WXA to WZZ only for US Army.
1917 ca. first use of 4-letters for ships. Used KE, KI, KO, KU, then went to consonants starting with KD. End of coast differentiation of letters for ships.
1920 temporary policy to give 4 letters to land stations... began with KU and KD series as above. Only KDKA still exists.
1929 KAA to KCZ transferred from Germany to USA
1922 first 4 letter broadcasting call signs assigned on a regular basis. WAAB first (WAAA not allowed due to 3 in a row same) Order was WAAB to WAAZ then WBAB to WBAZ then WCAB to WCAZ, etc. thru to WZAZ, then went in 1923 to WABA. WACA series skipped. WADA series (actually middle of series,such as WHDH) in 1928. WAEA series 1931 and WAFA series in 1934. Somewhat similar for K series. began with KD series. KH series was reserved for aircraft assingments.
1930 last 3 letter broadcasting call sign assigned. Not including suffixed calls.