Radio station sign (2,3)

I think it could be either:

Which length description are you looking for?:

2,3

on air

'radio station sign' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for on air that I've seen before include "In the act of broadcasting" , "In the process of broadcasting on radio or TV" , "Broadcasting studio sign" , "Broadcasting at this moment" , "Broadcasting live" .)

I've seen this clue in the USA Today and the Universal.

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