Characteristics of Tropospheric Scattered Fields

Experimental results obtained with transmissions at wavelengths of 3.2, 9.3, and 24 cms over a 46.3-mile path are presented. With low terminal heights the scattered field was dominant on this path. Tests with a narrow beam antenna indicate that the scattered field arrives at the receiver spread over an appreciable angle. This angle is some five to seven times as large as the Booker-Gordon theory predicts on the assumption that the scale of turbulence is large compared to the wavelength. Loss in ability to receive power in proportion to antenna gain was encountered for antennas with aperture diameters greater than about 20 wavelengths. This loss occurs for aperture sizes considerably smaller than the Booker-Gordon theory predicts. The speed of fading of the scattered field signal increases aimost linearly with frequency. This agrees fairly well with the concept (due to Ratcliffe and applied to tropospheric scattering by Booker and Gordon) that fading is due to beating between various scattered field components whose frequencies differ by a fractional Doppler shift due to motion of the scatterers. The speed of fading always increases, during the day, with time of day and does not correlate with mean upper wind speed. This increase with time of day is probably connected with the repetitive diurnal meteorological cycle prevalent in Arizona.