Knowing the value of the permittivity of the soil is important when using ground-penetrating radar (GPR) systems in outdoor conditions. Soil permittivity can be estimated by a variety of techniques, from destructive methods that require soil sampling though to non-contact methods using the GPR system. This paper introduces a novel method for estimating the permittivity of soils, based on measuring the travel time of common-mode currents on coaxial transmission lines such as semi-rigid coax cables. The common-mode current concept was validated using finite-difference time-domain simulations of dielectric half-spaces. Experimental measurements were taken of air and de-ionised water as reference materials. The estimated value of the permittivity of these materials was within 3% of the expected value.