"Multi-" is an [adjective combining form](https://www.merriam-webster.com/dictionary/combining%20form) that prefixes a singular noun (or sometimes [past participle](https://en.wikipedia.org/wiki/Participle#Types_of_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](https://www.collinsdictionary.com/dictionary/english/multi).

"Multiband antennas" (or [antennae](https://en.wiktionary.org/wiki/antennae#English)) is the plural of the compound noun "multiband antenna" (several antennas, each having several bands).