I do not know that I have ever seen what I would call a
"sinusoidal" waveform or anything close to sinusoidal in a natural signal.
The "waveform" of natural signals, in my limited experience, is random,
looking more like grass than like a sine wave.
The only reason I ask about broad-spectrum, natural signals is to clarify
the answer to my first question of whether we can observe the waveform
of individual RF photons, and are they sinusoidal.