What the fog?

What the fog?

What is the effect of fog on a radio link?

I often see concerns that fog and/or clouds will adversely affect your RF link reducing your range and possibly cut you from your aircraft. After all, fog is water and water block microwaves right?

The short answer is no, fog/clouds are not a problem, even for 5.8GHz. Water from the fog just don’t appear from nowhere. The water was there all along, because as temperatures cool, the water is forced out of solution and condense into droplets that we can now see as fog.

The long answer has to do with scattering. Particles in the atmosphere scatter electromagnetic radiation depending on their size. The bigger they are, the longer the wavelength they can affect. The sky is blue because the water molecules in the air are so small that they only scatter the shortest visible wavelength, blue. Clouds and fog are white because the water droplets they are made of are much bigger and scatter the whole visible spectrum. The visible spectra span from 380 to 750 nanometers (that’s 0.75 hundred thousandths of a millimeter) In comparison, the 5.8GHz wavelength is around 51mm, quite a margin. To put it in number, a dense fog at 3g/m3 (30M visibility) will attenuate 5.8GHz by around 0.03dB per km, an insignificant amount for our use.

 

Ref: Attenuation of electromagnetic waves by haze, fog, cloud and rain by C.C. Chen

https://www.rand.org/content/dam/rand/pubs/reports/2006/R1694.pdf

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