ms. Feeders may also be called spurs. 9.2 RADIO SYSTEMS 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 opo(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 informa- tion 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: 0.667d1d2/K (d in miles) (9.2a) 0.078d1d2/K (d in