Attenuation Due to Clouds and Fog

In Earth-space radio systems for frequencies higher than 10 GHz the attenuation due to clouds may be important factor affecting the system performance. The mechanism of attenuation is well understood since clouds and fog consist of small droplets and it is possible to express attenuation in terms of total water content per unit volume (12) as

Y [dB/km] = Kt[dB/km x g/m3] M[g/m3]

where y is specific attenuation within the cloud, K specific attenuation coefficient and M liquid water content of the cloud or the fog. The typical water content is about 0.05 g/m3 for medium fog with visibility of the order of 300 m and 0.5 g/m3 for thick fog with visibility of the order of 50 m. The specific attenuation coefficient for the frequencies up to 1000 GHz is given by

0.189 f [GHz]

The estimate of the path attenuation exceeded for the 0.01% of the time is given by

A0.01 [dB] = YRrL

[1+(f/fp)2] [1 + (f/fs)2]

[dB/km/g/m ]

є//(1 + n2)

e/( f) =

2 + e/ err

where

e1 — e2

and complex dielectric permitivity of water is

f (Є0 — Є1) , f(e1 — Є2)

fp[1 + ( f/fp)2] fs[1 + ( f/fs)2]

e//( f) =

X

studies carried out in the 50 to 800 MHz band indicate that loss increases linearly in log scale from 35 dB/decade at 50 MHz up to 40 dB/decade at 800 MHz. The foliage loss is approximately proportional to the operating frequency raised to fourth power which is a good approximation for horizontal polarization. Vertical polarization experienced larger loss in general by 8 to 25 dB at 50 MHz and 1 to 2 dB at 800 MHz. For a transmitting antenna over the tree tops and receiving antenna located in the trees delay spread of

0. 2/xs has been reported due to foliage (7).

Constructions

Although path loss models tend to capture difference be­tween build up and rural areas, few specific conditions should be emphasized. Street orientation has the channel effect on the received signal strength. For the mobile unit closer to the base station in the range of 1-2 miles the differ­ence between signal strengths in the case when the street is in line with base station and when it is perpendicular to the base station is about 10 dB. This phenomenon dimin­ishes for distances over 5 miles.

Mobile receiver moving into tunnel experiences a signal loss which is dependant on the frequency and the transmit­ter/receiver positioning. Experiments carried out for the transmitter at the entrance of the tunnel and the mobile receiver moving through the tunnel show that at 300 m in­side the tunnel about 4 dB loss is observed at 1 GHz. Atten­uation at lower frequencies was much higher approaching 20 dB at 400 MHz (7).

Results from study in New York have established in­verse law dependence of the path loss versus the distance with a inverse law from 4 at 900 MHz reducing to 2 at 2400 MHz. Above this frequency loss is less than in free space indicating some sort of guiding mechanism.

The underpass effect results in signal drop between 5 to 10 dB when a mobile receiver drives through. The period of the attenuation depends on the vehicle speed. In cellular radio this type of effect usually does not affect the voice channel.

Updated: 12.04.2014 — 03:01