The influence of antenna aperture and atmospheric absorption on amplitude scintillations on millimetric wave communications links

Amplitude scintillations observed on a communication link result from multipath scattering from refractive index fluctuations produced by atmospheric turbulence along the propagation path. At certain radio frequencies the atmospheric refractive index is complex (lossy) and consequently both real and imaginary components can contribute to scintillation. Theory to data relating refractivity fluctuation to the variance and spectrum of the scintillation in these absorbing mediums appears to have dealt with hypothetical point antennas or apertures. In practice receivers with finite antenna apertures will see scintillations of reduced magnitude, that is finite apertures perform a spatial averaging of the incident wave front or smoothing. This paper gives smoothing functions for the variance and spectrum of amplitude scintillations as a function of antenna diameter. Also discussed is the relevance of the results to both remote sensing of atmospheric parameters and the gathering of fade statistics used to characterize link availability.