"Multi-" is an adjective combining form that prefixes a singular noun (or sometimes past participle adjective) X to create an adjective that means "with several Xs". Using it with a plural noun is wrong - the plurality is already implied by the form "multi-".
You can find several examples of adjectives created using "multi-" and a singular noun in the Collins dictionary entry.
"Multiband antennas" (or antennae) is the plural of the compound noun "multiband antenna" (several antennas, each having several bands).