WordNet records no synonym and no antonym for crossbar. 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.
a horizontal bar that goes across something
game equipment consisting of a horizontal bar to be jumped or vaulted over
long thin horizontal crosspiece between two vertical posts
Synonyms, antonyms and the broader and narrower relations for crossbar come from Princeton University’s WordNet. Full sources and licences.