Historical clue formulations

• AM radio abbr. — 2020-10-25
• Letters on an AM dial — 2018-01-09
• Radio freq. unit — 2017-10-10
• Radio frequency abbr. — 2016-07-08
• Radio freq. — 2015-10-18
• Radio abbr. — 2011-12-25
• Unit of radio frequency: Abbr. — 1994-11-07