Wavelength Dependence of Microwave Propagation Far Beyond the Radio Horizon

The wavelength dependence of microwave propagation far beyond the radio horizon is studied experimentally and theoretically. The experiments made over a 259‐km path at 1000 MHz and 3000 MHz with scaled antennas give as a main result a wavelength dependence of actually received power relative to received power in free space ranging from λ−1 to λ−3. The exponent of the wavelength dependence is found to be correlated to the level of received signal power. Calculations are made of received signal power under the assumption that the tropospheric refractive index field of the common volume is of a turbulent structure, where the variations obey the −5/3 spectral law. There are also locally sharp boundaries within the common volume, which act as reflecting facets. A meteorological reasoning leads to the conclusion that there is a connection between the intensity of the turbulence and the refractive index gradient through the reflecting facets. Based on this, we get a relation between the exponent of the wavelength dependence and the level of received signal power. As a main result, reflections in the facets at 1000 MHz play a dominating role while at 3000 MHz, turbulent scatter is the most important propagation mechanism. Finally, theoretical and experimental results are compared and a comparatively good agreement is found.