The disclosure relates to an authentication device for automobile vehicles that controls door locking and/or engine starting and is able to interchange data over a certain distance with an identifier, intended to be carried by a user, when said identifier lies within the emission/reception coverage zone of said authentication device, to enable “hands-free” vehicle access and/or engine starting when said identifier has been authenticated by said authentication device, wherein said authentication device is connected to a single antenna inside the vehicle passenger compartment and is able to emit via said antenna at least two distinct emission powers, a first emission power enabling said antenna to cover a first zone including at least part of said passenger compartment, and a second emission power greater than said first emission power enabling said antenna to cover a second zone including at least the whole of said passenger compartment and possibly a coverage zone outside said vehicle.