Why do modern HF transmitters have a built-in low-pass filter in their RF output circuits?

A low-pass filter allows frequencies below a specific cutoff point to pass through while significantly attenuating frequencies above that point. In HF transmitters, the primary unwanted signals generated during amplification are harmonics, which are multiples of the fundamental operating frequency and are therefore much higher in frequency. By building a low-pass filter into the RF output circuit, manufacturers ensure that these high-frequency harmonic emissions are suppressed before they ever leave the radio. This prevents interference to other services, such as VHF television or public safety bands, which operate at higher frequencies. Distractors like 'adjacent channel interference' are incorrect because adjacent channels are very close to the operating frequency and are managed by band-pass or DSP filters in the earlier transmitter stages, not by the final output low-pass filter.