Microsoft implemented a proprietary 2.4GHz RF protocol for use with wireless controllers and the Xbox 360. The $399 Xbox 360 comes with a single wireless controller, while the $299 core system features a wired controller; regardless of what it comes with, all Xbox 360 units feature support for up to four wireless controllers.
Amazingly enough, even loaded with a battery pack, the wireless Xbox 360 controllers are lighter than the smaller Xbox S controllers. The new controllers are extremely well designed and mark a true evolution from the original gargantuan Xbox controllers that debuted 4 years ago.
The RF transmitter and receiver antennae are located at the front of the Xbox 360, behind the power button:
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