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# 电子工程代写|通讯系统代写Communication System代考|ELEN90057 Atmospheric gases

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## 电子工程代写|通讯系统代写Communication System代考|Atmospheric gases

Oxygen and water vapor in the lower atmosphere significantly affect path attenuation at higher frequencies. As an example, Figure $1.11$ and Figure $1.12$ present the specific attenuation for a location at the Earth’s surface (a pressure of $\left.1000 \mathrm{hPa}=10^5 \mathrm{Pascal}(\mathrm{Pa})=1 \mathrm{bar}\right)$, a temperature of $20^{\circ} \mathrm{C}$, and $100 \%$ relative humidity $(\mathrm{RH})$. The oxygen curve gives the specific attenuation for $0 \% \mathrm{RH}$. The frequency bands below $22.3 \mathrm{GHz}$ and between the specific attenuation peaks at $22.3,50$ to 70,118 , and $183 \mathrm{GHz}$ are called atmospheric windows. In the frequency window below the water vapor absorption line at $22.3 \mathrm{GHz}$, the specific attenuation increases with frequency and can be more that 10 times higher at $15 \mathrm{GHz}$ than at $2 \mathrm{GHz}$. Long-distance terrestrial microwave links are possible at the lower frequencies in this window but not at the high-frequency limit. Early Earth-space communication systems were developed in the 2- to 5-GHz frequency range to benefit from the low values of path attenuation, but had to compete for the radio frequency (rf) spectrum with terrestrial radio relay systems and long-range radar applications that required low path attenuation.

## 电子工程代写|通讯系统代写Communication System代考|Clouds and fog

Scattering by the very small liquid water droplets that make up liquid water fogs near the Earth’s surface and liquid water clouds higher in the atmosphere can produce significant attenuation at the higher frequencies. Figure $1.13$ and Figure $1.14$ present the specific attenuation per unit liquid water content as a function of frequency. Typical liquid water contents range from $0.003$ to $3 \mathrm{~g} / \mathrm{m}^3$ depending on location, height in the atmosphere, and meteorological conditions. Clouds in the most active parts of mid-latitude thunderstorms may have liquid water contents in excess of $5 \mathrm{~g} / \mathrm{m}^3$. The liquid water cloud heights in the atmosphere can range from $0 \mathrm{~km}$ above ground (a fog) to $6 \mathrm{~km}$ above ground in the strong updrafts in convective clouds. For a $1-\mathrm{g} / \mathrm{m}^3$ cloud at a water temperature of $10^{\circ} \mathrm{C}$, the specific attenuation increases monotonically with frequency through the UHF, SHF, and EHF frequency bands (see Figure $1.13$ and Figure 1.14). For frequencies lower than $10 \mathrm{GHz}$, cloud (or fog) attenuation can be ignored. At a frequency of $30 \mathrm{GHz}$, cloud attenuations on a $50^{\circ}$ elevation angle path may approach 3 to $4 \mathrm{~dB}$. At a frequency of $120 \mathrm{GHz}$, this result translates to 30 to $40 \mathrm{~dB}$.

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