Electromagnetic properties of soil in the 500 kHz to 5 MHz range are determined experimentally to predict plane wave propagation. The ratio of the characteristic impedance of a coaxial probe filled with fluid or soil to the same probe filled with air is used to determine the material's effective permittivity and conductivity. Open- and short-circuit impedances are measured, corrected, and used to predict wave parameters. Measurements made on dry desert alluvium demonstrated the method. Wave attenuation increased linearly with frequency between 0.5¿10-2 Np/m at 1 MHz to 2.5¿10-2 Np/m at 5 MHz because of increasing soil conductivity; the ratio of soil wavelength to free-space wavelength remained approximately constant at 0.4 because of the almost constant permittivity in this frequency range. ¿American Geophysical Union 1987