Ultra-Short-Wave Propagation over Land

From theoretical considerations it is found that for ultra-short-wave propagation over level terrain, the received field should equal 4π times the product of the antenna heights divided by the product of the wavelength and the distance times the field that would be received for transmission in free space. This equation has been checked experimentally for horizontal polarization, antenna heights between 2 and 25 meters and frequencies between 17 and 150 megacycles for the two distances 9.4 and 26.3 kilometers. The results indicate that in the absence of detailed information regarding the transmission path, this formula gives the probable value of the received field. The deviations of an actual path from the ideal should cause corresponding deviations in the received field from that calculated by the above formula. For these two paths, the mean of the deviations was found to be about three or four decibels. At 45 kilometers fading was observed with low antennas, and at greater distances there were rather large variations in the received field at all antenna heights available. At higher frequencies and longer distances the curvature of the earth introduces additional attenuation. At the distance d = 5×10
⁴
k
^2/3
λ
^1/3
meters, theoretical considerations indicate that the curvature of the earth will reduce the field strength by a factor of about two, and beyond this "shadow distance" by the factor 2ka/π/ d3/ X, where ka is the effective radius of the earth with refraction and λ is the wavelength, all measured in meters.