WordNet records no synonym and no antonym for wavelength. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the distance (measured in the direction of propagation) between two points in the same phase in consecutive cycles of a wave
a shared orientation leading to mutual understanding
“they are on the same wavelength”
Synonyms, antonyms and the broader and narrower relations for wavelength come from Princeton University’s WordNet. Full sources and licences.