"Telamon" wrote
Yes that is true that the fundamental by itself will not produce
harmonics of lower frequency but the fundamental is being modulated by
data so mixed products of variable data transmitted mixing with the
carrier will produce frequency energy above and below the fundamental.
If you have long strings of ones or zeros the mixed frequencies will be
as low as the inverse of the period of low frequency data rates.
____________

The spectrum occupied by a data pulse is dependent on the rise and fall
times of the pulse, not on the pulse duration. If the rise and fall times
are constant, spectral bandwidth also will be constant. Only the
distribution of energy within that spectrum will vary for pulses of
different lengths (half-amplitude durations).

A familiar example of this is the "click" created in radios when a nearby
electric light is switched on or off. When the switch contacts make and
break, they create a current transition across a short time interval --
which generates a wideband RF spectrum. This RF energy is radiated by the
AC wiring, and some of it is received and detected by the radio.

RF

Visit http://rfry.org for FM transmission system papers.