A top loaded antenna unit is provided for concealed mounting within a vehicle below a covering or panel of dielectric material. The antenna unit includes a top load plate, a ground plate disposed in a spaced relation to the top load plate and electromagnetically coupled thereto through a matching element. The antenna unit additionally includes a dielectric member which is disposed in proximity to the top load plate for shifting resonance frequency of the antenna to a predetermined extent. The top load plate and the ground plate are assembled together with the dielectric member into a unitary structure for mounting within the vehicle below the covering or panel of dielectric material, at which condition resonance frequency of the antenna can be free from being affected from the covering or panel of the vehicle but affected solely from the dielectric member of which effect on the frequency shifting is known. Whereby, the antenna can give stable resonance frequency as intended only in consideration of the dielectric member and without being substantially influenced by other outside environments.