An antenna for mounting on a motor-vehicle part formed with a throughgoing hole of nonround shape has a base plate engageable with the part over the hole and carrying antenna components and a stem projecting through the hole. A bayonet plate has an outer edge of nonround shape capable of passing axially through the hole of the part and not able to pass through the hole of the part in another angularly offset position. This bayonet plate can move angularly on the base plate between an install position and a holding position and is urged by a spring toward the base plate and against the part. Spacers engageable in the install position hold the bayonet plate at a predetermined large axial spacing from the base plate and disengage from each other in the holding position to allow the bayonet plate to be pressed by the spring against the motor-vehicle part.