Summer 2006 Issue 

Using Radar Data to Predict 

We say that an electromagnetic wave is
“scattered” when it encounters some substance in its path that deflects
some of its energy in a new direction. When one stops to think about it,
most routine propagation at VHF and higher frequencies is a result of some
sort of scattering. At VHF we often observe scattering effects from large
objects close to Earth, such as buildings and aircraft. We also know that
we can make use of small changes in air density in the lower atmosphere
that allow for routine communication of a few hundred miles with amateur
power levels. As we go higher in frequency, we find that smaller and
smaller objects have a significant effect on propagation. Raindrops become
an effective scattering medium in the microwave range. This article will
focus on the mechanics of rainscatter propagation and how freely
available radar data can be used to predict possible propagation paths. There are two sets of scattering equations that are used to calculate the amount of scattering from a medium: Rayleigh and Mie scattering. The type of scattering is a function of the size of the scattering particle relative to the wavelength of the radiation. Rayleigh scattering is simpler, so we will consider it first. Rayleigh scattering applies when the diameter of the scattering particle (d) is much smaller than the wavelength of the radiation (l). Rayleigh scattering is the dominant scattering mode when d < l/10. Figure 1 shows the incoming electric field from an electromagnetic wave as it passes through a particle. When this happens, an electric dipole (p) is induced in the particle. The magnitude of p is given by equation 1: K is known as Beer’s Law absorption coefficient and is a complex number representing the scattering and absorption properties of the dielectric. It is both wavelength and temperature dependent. Typical values of K2 at 10 GHz/0°C are ~0.92 for liquid water and ~0.19 for ice. Therefore, this confirms that ice and snow are poorer scattering media than liquid water droplets of the same size and shape. 
Figure 1. Induction of electric dipole in Rayleigh scattering. Arrows indicate Efield of the incoming EM wave. The two circles show the (re)radiation pattern (Hplane) of the dipole. 
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