Even in 1977, when I had just built my 'Dyna Micro'
https://en.wikipedia.org/wiki/Single-board_computer microprocessor trainer board, I could tell that its emanations on the AM radio band were were quite distinctive: I could monitor the progress of programs merely by listening to an otherwise-unoccupied AM frequency. While I wasn't particularly interested in the details at that time, it has long been obvious that a program could be written to emit specific data, perhaps by repeating segments of code based on the information to be transmitted.