"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 X
s". 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 being able to recievereceive more than one frequency band width).