In this paper, we report on simulation results at 10 GHz of different configurations of planar transmission lines (TL) printed on a ferroelectric thin film deposited on sapphire and lanthanum aluminate substrates. The ferroelectric material is KTa0.5Nb0.5O3 (KTN) and its film permittivity is supposed to vary from 700 to 500. First, we present the analyses results of KTN thin films coated considered substrates. Then the main properties of microstrip (MS), coplanar waveguide (CPW) and coplanar strips (CS) are computed as a function of the physical and electrical parameters. Their tunability and figure of merit are defined and compared. CPW and CS lines present a tunability (?eff/effmax) of about 16 % for a gap value g = 30 ?m. The MS lines show a much less tunability of 2%. The figure of merit of CPW and CS configurations are of 6.3 and 12.2 %/dB for a gap value of 30 ?m, respectively. By increasing the gap, these figures of merit can be improved up to the limiting values of 8.7 and 15.4 %/dB, respectively.