Radio path profile

The next step involves identification of the appropriate transmitting and receiving sites. Terminal sites need to be located close to the equipment they will serve, whereas intermediate repeaters are not constrained. The main site selection criteria will be:

• Availability of the land

• Proximity of mains power

• Proximity of road access and above all

• Feasibility of the radio path.

Preliminary site selection is often done from examination of topographical maps of the area, having a scale of at least 1:50 000. The next step is to draw a straight line along the path between the chosen sites, and read from the topographical map the heights of the two sites, and the respective heights and distances of any potential obstructions along the path. The path profile can now be plotted. This can be done using any suitable computer program, such as 'Path Loss II' or the profile can be prepared manually. In both methods the heights measured from the topographical map, need to be adjusted. Where the obstruction to the radio path is trees then the tree height (often 12 m or 40 ft) needs to be added to the ground level read from the topographical map, together with an allowance for tree growth during the system lifetime (typically 3.0 m or 10 ft).

The topographical map heights make no allowance for the curvature of the earth, so an adjustment is necessary. In addition, the radio path seldom follows a straight line between the transmitter and receiver, but is bent due to refraction in the atmosphere.

Under certain atmospheric conditions the radio path is bent toward the earth whilst under other conditions it is bent away from the earth. This is illustrated in Figure 3.19. This variability in the atmosphere is modeled using the K factor, which is used to modify the earth's radius so as to straighten the radio path. When K < 1 the radio beam bends towards the earth and when K > 1 the radio beam bends away from the earth. The K factor typically varies between 0.4 and 1.33 as shown in Table 3.6.

x Earth's radius

x Earth's radius

Figure 3.19

Effect of K factor on radio paths

Figure 3.19

Effect of K factor on radio paths

K Factor

0.4-0.5

0.5-0.66

0.66-1.0

1.0-1.33

1.33

Coastal tropical, water

Coastal

Flat, temperature

Mountainous dry

Temperate standard atmosphere

Fog moisture over water

Fog surface layers

Light fog

No fog No surface layers

No fog No surface layers

Table 3.6

K factor

Table 3.6

K factor

The heights of the intervening obstructions on the radio path need to be modified to compensate for both the ordinary earth curvature and the atmospheric variations by using the K factor in the following formula:

Where h = earth curvature adjustment, added to obstacle height d1 = distance of point to one end of path in km d2 = distance of point to other end of path in km K = earth curvature factor.

The optimum path clearance above the obstructions is defined to be 0.6 of the Fresnel zone radius (F1). If this path clearance (0.6F1) is maintained the system behaves as though it were in free space, however if there is less clearance than this, the beam is obstructed. This is totally unacceptable for microwave systems. The Fresnel zone radius is given at each point by the following formula:

Where

F1 = Fresnel zone radius in m d1 = distance of obstruction from one end in km d2 = distance of obstruction from other end in km d = total path length (d1 + d2) in km f = operating frequency in GHz.

By using 0.6F clearance on the microwave beam we are able to construct an effective system using the minimum height towers at each end. This is illustrated in Figure 3.20. The objective is to minimize the cost of both towers in order to minimize the total cost of system. However it may be preferable to use a taller tower at one site, say an existing TV mast, with a correspondingly smaller tower at the other site.

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