The use for an antenna begs what use a human would have for knowing where its forehead is. As we don't really walk with our head leading, I would think it would be more important for a farsighted human to have more, longer, or more dexterous fingers, or more arms. A human that evolved from a subterranean, digging primate might have something like an antenna or "feeler" organ related to its mechanism for burrowing. Now that I'm thinking about it, unless you're dead-set on antennae, you can characterize the need to feel what's directly around the human with whiskers, or abundant, long, frizzy head hair.
EDIT: Somehow I read "human" in the question when it wasn't present. So generally speaking, a burrowing, subterranean, or cave-living mammal that moves with its head leading would get some use out of head antennae, and as mentioned elsewhere, if your mammal had no obvious nose. There's maybe zero extant exmaples of this in real mammals, so you could write it as large, pronounced hair follicles. There's no good way to represent smell in a similar fashion.