Harmonics are simple multiples of a frequency, and they matter twice over: an antenna will often load up on them, and a transmitter that spills energy into them lands where it should not be.
Every multiple of your frequency, and which band each lands in.
Harmonics are whole-number multiples of your operating frequency. They matter twice: an antenna often loads up on them, and your transmitter can radiate them if you don’t filter.
A half-wave dipole works on its odd harmonics (3rd, 5th…), which is why a 40m wire also plays on 15m. Clean transmitters keep harmonics well below the fundamental; this tells you which ones to watch.
