According to this list on Wiki ( https://en.wikipedia.org/wiki/List_of_airborne_early_warning_aircraft ), the 1st attempt at an AEW helicopter was the Sikorsky HR2S-1W in 1958, but according to various sites (such as this http://www.aviastar.org/helicopters_eng/sik_s-56-aew.php ) it was unsuccessful due to excessive vibrations. The list indicates that the UK was the 1st to field an AEW helicopter in 1982. The Wiki list does not mention any other helicopter attempts in the 24 years in between, were there any? If not, when could one have been developed before 1982? Would guess that turbo-shaft engines reduce vibrations, so sometime in the early to mid 1960s?