Antenna device for an anti-theft system

An antenna device mounted in a vehicle comprising two individual antennae coils 3 communicates with a portable transponder in order to unlock the vehicle doors when a correct answer code signal is given by the transponder in response to a question code signal sent by the antennae. The coils are driven by sinusoidal signals from a control apparatus 4, the signals to one coil being phase displaced with respect to the other so that the resultant electromagnetic field moves to and fro in space. In this way the transponder can be detected regardless of its orientation with respect to the antennae. An individual antenna 3 may have a small energy coil 14 producing a high field density for charging a transponder energy store such as a capacitor, when it is in close proximity to the coil. The transponder may be in a key which is charged when inserted in a door lock. The antennae may be placed in different parts of the vehicle such as in the doors, bumpers, floor, boot, or external mirrors.