The results of measurements of the electromagnetic (EM) field polarization characteristics are considered at various distances from a very-low-frequency (VLF) radiowave transmitter. They are compared with the results of calculations using programs developed on the basis of the complete solution of the problem on EM fields from a dipole and cable in a plane "earth-ionosphere" waveguide. The ionosphere and displacement currents are shown to considerably affect the parameters of polarization of the primary EM field at large distances (hundreds and thousands of kilometers) from the transmitter. The ellipticity of polarization enhances with the distance from the transmitter.