An antenna assembly for use on a vehicle includes telescoping radiating sections that collapse toward an insulated mounting base, so that the radiating sections can be selectively extended. An impedance matching network with concentric outer and inner conductive tubular members slidably receives the telescopically collapsed radiating sections. Both tubular members are attached to the mounting base and electrically insulated therefrom and from each other. The inner tubular member is electrically connected to the radiating sections at a base end thereof. The outer tubular member has at least one longitudinal slot therein through which the central conductor of a transmission line is connected to the inner member. A ground contact is electrically connected to the outer tubular member. A slidable terminal that is selectively adjustable longitudinally along the outer tubular member is connected to the transmission line to select a signal insertion point.