A diagnostic device for an antenna includes a coil (20) or the like connected on the one hand to a reference potential and on the other hand to the output of an amplifier (12), including two condensers (C2, C3) mounted in series, a first condenser (C2) being mounted in parallel with the coil (20) and the second condenser (C3) being inserted between a terminal of the coil (20) and the reference potential. The device is configured for the detection of the presence of an antenna in a hands free automotive vehicle system.