Do you mean the item itself, or the concept? Do you mean one that worked at "whatever" frequency, or do you mean one that operates on a specific frequency?
Do you mean a modulated signal on a carrier frequency, or do you mean "on and off" mode, like my uncle, SFB Morse did?
I assume you mean wireless, because radio freqs also travel on wire.
Marconi was certainly there, in the beginning.
Noel