Harmonics may be produced in the RF power amplifier of a transmitter if:

Harmonics are generated when an RF amplifier operates outside its intended linear range. When an excessive drive signal is applied to a power amplifier, the amplifier cannot faithfully reproduce the input signal, resulting in clipping or saturation. This severe distortion modifies the pure sine wave, which mathematically translates to the creation of harmonic energy at integer multiples of the fundamental frequency. Options like an unstable oscillator cause frequency drift rather than harmonics, and tuning the output tank circuit to the fundamental frequency is actually the correct procedure to suppress harmonics, not cause them.