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Water vapor attenuation

Attenuation Caused by Water Vapor and Oxygen

By Green Bay Proffessional Packet Radio


Table 1        
Frequency (GHz) Oxygen (Ko in dB/km) Water Vapor (Kw in dB/km per gram/m3)
10 0.007 0.0004
24 0.018 0.024
47 0.300 0.014
76 0.700 0.036
120 3.000 0.090
142 0.030 0.130

To calculate attenuation caused by oxygen absorption, simply multiply the appropriate factor times the total path length in kilometers.

Lo = Ko * d

  • Lo = Loss caused by oxygen absorption, dB.
  • Ko = Attenuation factor, from Table 1, dB/km.
  • d = Path distance, kilometers.

Water vapor attenuation does vary significantly with the weather.  The actual water vapor content, or absolute humidity measured in grams of water per cubic meter, must be determined before the water vapor attenuation factor can be applied.  Absolute humidity can be determined from the more common measurements of temperature and relative humidity.  The maximum amount of water that a given volume of air can hold in the gasous state is known as the saturation vapor content, equivalent to 100% relative humidity.  The following chart shows the saturation vapor content for air at a wide range of temperatures.

water vapor

Absolute humidity is found by multiplying the saturation water vapor content in grams per cubic meter (g/m3) by the relative humidity expressed as a decimal.  The absolute humidity of air at 22°C and 60% relative humidity is:

20 g/m3 * 0.60 = 12 g/m3

It should be noted that warm air can hold significantly more water vapor than cold air. Air at 22°C and 60% relative humidity holds more water (12 g/m3) than air at 10°C and 100% relative humidity (10 g/m3). The absolute humidity figure must then be multiplied by the water vapor attenuation factor, obtained from Table 1. The modified factor can then be multiplied by the total path length to obtain the attenuation caused by water vapor.

Lw = H * Kw * d
  • Lw = Loss caused by water vapor absorption, dB.
  • H = Absolute humidity, g/m3
  • Kw = Attenuation factor, from Table 1, dB/km per g/m3
  • d = Path distance, kilometers.
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