RF ATMOSPHERIC ABSORPTION / DUCTING

10 15 20 25 30 40 50 60 70 80 90100 150 200 250 300 400 30 20 15 10 8 6 5 4 3 2 1.5 1.0 0.8 0.001 0.002 0.004 0.01 0.02 0.04 0.1 0.2 1 2 10 20 40 100 0.4 4 Wavelength (mm) Frequency (GHz) Average Atmospheric Absorption of Milimeter-Waves (Horizontal Propagation) Sea Level H2O O2 H2OO2 H2O 9150 Meters Altitude 5-1.1 Figure 1. Atmospheric Absorption of Millimeter Waves RF ATMOSPHERIC ABSORPTION / DUCTING Signal losses are associated with each stage of signal processing in both the transmitting and receiving portions of the system. The transmitting losses include power transmission efficiency, waveguide and antenna losses, and duplexer losses. In the receiver, losses include antenna, waveguide, RF amplifier, mixer, and IF amplifier. In addition to these losses, energy traveling through the atmosphere suffers from atmospheric attenuation caused primarily by absorption by the gasses. For lower frequencies (below 10 GHz), the attenuation is reasonably predictable. For high frequencies in the millimeter wave range, the attenuation not only increases, but becomes more dependent upon peculiar absorbing characteristics of H O, O , and the like. 2 2 Figure 1 shows the areas of peak absorption in the millimeter wave spectrum. Figure 2 shows how the intensity of precipitation can affect atmospheric attenuation. 0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10 20 50 3 5 10 30 100 100 50 25 12.5 2.5 1.25 0.25 Tropical Downpour Heavy Rain Medium Rain Light Rain Drizzle ATMOSPHERIC ATTENUATION Frequency (GHz) Rainfall rate (mm/hr) UPPER ATMOSPHERE EARTH 5-1.2 Figure 2. Atmospheric Attenuation Figure 3. Ducting Ducting is an increase in range that an electromagnetic wave will travel due to a temperature inversion of the lower atmosphere (troposphere) as shown in Figure 3. The temperature inversion forms a channel or waveguide (duct) for the waves to travel in, and they can be trapped, not attenuating as would be expected from the radar equation. Ducting may also extend range beyond what might be expected from limitations of the radar horizon (see Section 2-9). The ducting phenomena is frequency sensitive. The thicker the duct, the lower the minimum trapped frequency. A similar occurrence takes place with ionospheric refraction, however the greatest increase in range occurs in the lower frequencies. This is familiar to amateur radio operators who are able to contact counterparts “around the world”. . Altitude 5-1.1 Figure 1. Atmospheric Absorption of Millimeter Waves RF ATMOSPHERIC ABSORPTION / DUCTING Signal losses are associated with each stage of signal. Drizzle ATMOSPHERIC ATTENUATION Frequency (GHz) Rainfall rate (mm/hr) UPPER ATMOSPHERE EARTH 5-1.2 Figure 2. Atmospheric Attenuation Figure 3. Ducting Ducting