TX Harmonic Suppression

A test team user is measuring 2nd, 3rd, … order harmonic suppression in VHF and UHF.

Even (2nd, 4th) suppression is good, odd (3rd, 5th) is not so good but in the real world antennas and amplifiers will add a lot of harmonic suppression.

Question: are there any parameters the user can adjust which affect the harmonics? I don’t think this is a big problem, but would like to know what i can do to create the cleanest possible signal.