Abstract-In order to enhance the accuracy of the complex permittivity data employed in Ground Penetrating Radar (GPR) techniques, an adaptive cavity setup is presented. The use of moveable walls permits to relax the mechanical constraints on the sample dimensions so that it can be employed also in complicate measurement condition as, for example, in the case of wet samples. Moreover, exploiting the cavity resonance phenomenon, low loss materials, such as some type of marbles, can be accurately evaluated. The numerical characterization, the parametric analyses and the L-band measurement results show the validity and the reliability of this configuration.