A slot antenna is disclosed for a horizontal dielectric vehicle door in which the ground plane of the antenna sheet does not connect with or overlap the surrounding electrically conducting vehicle body panels but which nevertheless maintains significant RF currents at FM frequencies through the vertical roof pillars of the vehicle. This is accomplished by the formation of a parasitic slot surrounding the antenna slot in the gap between the sheet and the surrounding sheet metal at the trunk opening. The parasitic slot has a first portion of length substantially the same as that of the antenna slot and a similar width and a second portion of a greater width effective to comprise an open circuit for currents. Thus the total effective length of the parasitic slot is equal to that of the antenna slot for similar resulting current flow from the decklid area through the vertical roof pillars; and the desirable FM performance is preserved. If the second portion of the parasitic slot is placed on a vertical portion of the door, backward horizontal radiation is also improved; and interference due to operation of an electric latching mechanism for the door is reduced if it is mounted in the second portion of the parasitic antenna.