TABLE 3-3 SCINTILLATION CHARACTERISTICS AT 1 GHz DUE TO HIGH POWER INTERACTIONS The correlation scale size D is the distance at which the correlation drops to 70 percent. Nonlinear induced density irregularities will be confined to some fraction of beamwidth; namely, those regions where the power density exceeds the instability threshold (see Appendix B). Because of the finite lateral extent of the unstable region only phase fluctuations with wavelengths greater than contribute to the field of the reference beam at the transmitting antenna face (where is the distance to irregularity region or equivalent phase screen and is the transverse dimension of the unstable region). For example, if , and km then These results can be summarized stating that ionospheric irregularities, self-induced or ambient, will cause phase fluctuations of less than 10 degrees across the wavefront of a reference beam propagated from the center of the ground antenna to the transmitting antenna face. The random phase variations (and amplitude fades) will decorrelate over lengths of a few hundred meters and in tens of seconds. Propagation through the density fluctuations will cause some power beam spreading by diffraction. The increase in beamwidth is estimated by ’’paired- echo" theory applied to Fresnel diffraction of a wave-field modulated by a thin phase screen. At microwave frequencies the refractive index of the ionosphere is given by:
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