Site Selection and Preparation of a Path Profile

9.2.3.3.1 Site Selection. In this step we will select operational site where we will install and operate radio equipment. After site selection, we will prepare a path profile of each link to determine the heights of radio towers to achieve "line of sight." Sites are selected using large topographical maps. If we are dealing with a long system crossing a distance of hundreds of miles or kilometers, we should minimize the number of sites involved. There will be two terminal sites, where the system begins and ends. Along the way, repeater sites will be required. At some repeater sites, we may have need to drop and insert traffic. Other sites will just be repeaters. This concept is illustrated in Figure 9.4. The figure shows the drops and inserts (also called add-drops) of traffic at telephone exchanges. These drop and insert points may just as well be buildings or other facilities in a private/corporate network. There must be considerable iteration between site selection and path profile preparation to optimize the route.

In essence, the sites selected for drops and inserts will be points of traffic concentration. There are several tradeoffs to be considered:

1. Bringing traffic in by wire or cable rather than adding additional drop and insert (add-drop) capabilities at relay point. This provides additional traffic concentration.

2Bellcore, Bell Communications Research, Piscataway, NJ. Now called Telcordia.

Microwave Relay Towers
Figure 9.4 Simplified functional block diagram of the LOS microwave system shown in Figure 9.2.

2. Siting based on propagation advantages (or constraints) only versus colocation with exchange (or corporate facility) (saving money for land and buildings).

3. Choosing a method of feeding (feeders3): by light-route radio, fiber-optic cable, and wire-pair cable.

9.2.3.3.2 Calculation of Tower Heights. LOS microwave antennas are mounted on towers. Formulas (9.1) allowed us to calculate a rough estimate of tower height. Towers and their installation are one of the largest cost factors in the provision and installation of an LOS microwave system. We recommend that actual tower heights do not exceed 300 ft (~90 m); otherwise, additional expense will be required so that the tower meets twist and sway requirements. Of course, the objective is to keep the tower height as low as possible and still maintain effective communication. The towers must be just high enough to surmount obstacles in the path. High enough must be carefully defined. What sort of obstacles might we encounter in the path? To name some: terrain such as mountains, ridges, hills, and earth curvature—which is highest at midpath—and buildings, towers, grain elevators, and so on. The path designer should consider using natural terrain such as hill tops for terminal/relay sites. She/he should also consider leasing space on the top of tall buildings or on TV broadcast towers. In the following paragraphs we review a manual method of plotting a path profile.

From a path profile we can derive tower heights. Path profiles may be prepared by a PC with a suitable program and the requisite topological data for the region stored on a disk.

3 Here the word feeders refers to feeding a mainline trunk radio systems. Feeders may also be called spurs.

Our recommendation is to use ordinary rectangular graph paper such as "millimeter" paper or with gradations down to one-sixteenth of an inch or better. "B-size" is suggested. There are seven steps required to prepare a path profile:

1. Obtain good topo(logical) maps of the region, at least 1:62,500 and identify the two sites involved, which arbitrarily we call one a "transmit" site and the other a "receive" site.

2. Draw a straight line with a long straight edge connecting the two sites identified.

3. Follow along down the line identifying obstacles and their height. Put this information on a table, labeling the obstacles "A," "B," and so on.

4. Calculate earth curvature (or earth bulge) (EC). This is maximum at midpath. On the same table in the next column write the earth curvature value for each obstacle.

5. Calculate the Fresnel zone clearance for each obstacle. The actual value here will be 0.6 of the first Fresnel zone.

6. Add a value of additional height for vegetation such as trees; add a growth factor as well (10 feet or 3 meters if actual values are unavailable).

7. Draw a straight line from left to right connecting the two highest obstacle locations on the profile. Do the same from right to left. Where this line intersects the vertical extension of the transmit site and the vertical extension of the receive site defines tower heights.

In step 4, the calculation of EC, remember that the earth is a "sphere." Our path is a tiny arc on that sphere's surface. Also, in this calculation, we must account for the radio ray path bending. To do this we use a tool called K-factor. When the K-factor is greater than 1, the ray beam bends toward the earth, as illustrated in Figure 9.3. When the K-factor is less than 1, the ray beam bends away from the earth.

The EC value (h) is the amount we will add to the obstacle height in feet or meters to account for that curvature or bulge. The following two formulas apply:

where d1 is the distance from the "transmit" site to the obstacle in question and d2 is the distance from that obstacle to the receive site.

Table 9.1 is a guide for selecting the K-factor value. For a more accurate calculation of the K-factor, consult Ref. 3. Remember that the value obtained from Eq. (9.2) is to be added to the obstacle height.

In step 5, calculation of the Fresnel zone clearance, 0.6 of the value calculated is added to the obstacle height in addition to earth curvature. It accounts for the expanding properties of a ray beam as it passes over an obstacle. Use the following formulas to calculate Fresnel zone (radius) clearance where F is the frequency in gigahertz, d1 is the distance from transmit antenna to obstacle (statute miles), d2 is the distance from path obstacle to receive antenna (statute miles), and D = d1 + d2. For metric units:

hft = 0.667d1d2/K (d in miles) hm = 0.078did2/K (d in km)

202 CONCEPTS IN TRANSMISSION TRANSPORT Table 9.1 K-Factor Guide3

Propagation Conditions

202 CONCEPTS IN TRANSMISSION TRANSPORT Table 9.1 K-Factor Guide3

Propagation Conditions

Perfect

Ideal

Average

Difficult

Bad

Weather

Standard

No surface

Substandard,

Surface layers,

Fog moisture

atmosphere

layers or fog

light log

ground fog

over water

Typical

Temperate zone,

Dry,

Flat, temperate,

Coastal

Coastal, water,

no fog, no

mountainous,

some fog

tropical

ducting, good

no fog

atmospheric

mix day and

night

K factor

1.33

1-1.33

0.66-1.0

0.66-0.5

0.5-0.4

aFor 99.9 to 99.99% time availability.

aFor 99.9 to 99.99% time availability.

where F is the frequency (the microwave transmitter operating frequency) in gigahertz, and d^ d2, and D are now in kilometers with R in meters.

The three basic increment factors that must be added to obstacle heights are now available: earth curvature (earth bulge), Fresnel zone clearance, and trees and growth (T&G). These are marked on the path profile chart. On the chart a straight line is drawn from right to left just clearing the obstacle points as corrected for the three factors. Another, similar line is drawn from left to right. A sample profile is shown in Figure 9.5.

Fresnel Zone Factor
Figure 9.5 Practice path profile. (The x-axis is in miles, the y-axis is in feet; assume that K = 0.9; EC is the earth curvature and F is the dimension of the first Fresnel zone.)

The profile now gives us two choices, the first based on the right-to-left line and the second based on the left-to-right line. However, keep in mind that some balance is desirable so that at one end we do not have a very tall tower and at the other a small, stubby tower. Nevertheless, an imbalance may be desirable when a reflection point exists at an inconvenient spot along the path so we can steer the reflection point off the reflecting medium such as smooth desert or body of water.

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