Why do you say "multi-band antenna", not "multi-bands antenna"?
It seems to me there are multiple bands for an antenna.
And, are "multi-band antenna" and "multi-band antennas" different?
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When we make a compound noun, every noun except the most significant (generally the last, but see this English Club article for examples of exceptions) is always singular.
For example, when we put together the noun caravan and the noun site, we get the compound noun caravan site: caravan is singular even though caravan site will definitely involve multiple caravans.
The last noun will be plural only if we are talking about multiples of the last noun... multiple caravan sites in my example.
"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 being able to receive more than one frequency band).