Anodic tantalum pentoxide is studied up to 3 GHz by using for the first time the Radio-Frequency Impedance Analysis. Over this broad frequency range we extract a better knowledge on the very physical nature of this dielectric. From impedance analysis we obtain the value of the leakage conductance in parallel to the pure capacitance. At frequencies higher than 1 MHz capacitance vanishes to zero. This behaviour appears for all the samples, independently of thickness, anodization precursor or electrode metals, suggesting that it should be due to some intrinsic property of the dielectric. The electrical response of amorphous tantalum oxide is due to dipole oscillations characterized by a constant time; when frequency is high enough, dipoles do not have time to follow the field oscillations and, as a consequence, there is not any capacitive behaviour.