Summer 2006 Issue

Using Radar Data to Predict
Rain-Scatter Paths

Rain-scatter propagation has been around for a long time. Radar data is a way
of predicting where rain-scatter propagation can happen and/or is happening in
real time. Here KØSM discusses how to use radar data to predict rain-scatter
propagation. He also discussess his software program, which is designed to be used
for making such predictions.

By Andy Flowers, KØSM

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 rain-scatter propagation and how freely available radar data can be used to predict possible propagation paths.

Scattering Principles I: Rayleigh Scattering

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 |K|2 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 E-field of the incoming EM wave. The two circles show the (re-)radiation pattern (H-plane) of the dipole.

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