It radiates in all horizontal directions, or if you are a pessimist you might say it radiates equally badly in all horizontal directions. These antennas do a reasonable job of covering a typical home, because a vertical sleeve dipole is omnidirectional.
It’s a sleeve dipole, think of it as a vertical dipole antenna in which the lower radiator is hollow, and through which the feeder is routed. More recent and more fancy routers have more than one such antenna for clever tricks to extend their range or bandwidth, but even if the manufacturer has encased it in mean-looking plastic the antenna inside is the same.
When we take a new Wi-Fi router from its box, the stock antenna is a short plastic stub with a reverse SMA plug on one end.
