This invention relates generally to a 5/8 wave mobile whip antenna using a re-entrant resonant cavity as an electrically and physically integral component to obtain a narrow bandwith necessary to attenuate off channel interference. One embodiment is a coaxial type of re-entrant cavity resonator as an electrically and physically integral part of a 5/8 wave whip antenna. A second embodiment is a hybrid type of re-entrant cavity resonator referred to as a helical cavity resonator as an electrically and physically integral part of a 5/8 wave whip antenna.