WordNet records no synonym and no antonym for talk down. 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.
speak in a condescending manner, as if to a child
“He talks down to her”
direct and control (the flight of an airplane during landing) via radio
“the control tower talked down the plane whose pilot fell ill”
Synonyms, antonyms and the broader and narrower relations for talk down come from Princeton University’s WordNet. Full sources and licences.