 km) (9.2b) 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 RA=72.1/dids (9.3a) 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: (9.3b) CONCEPTS IN TRANSMISSION TRANSPORT Table 9.1 K-Factor Guide Propagation Conditions Perfect Ideal Average Difficult Weather Standard No surface Substandard, Surface layers, Fog moisture atmosphere layers or fog light log ground fog over water Typical Temperate zone, Flat, temperate, Coastal Coastal, water, no fog, no mountainous, some fog tropical ducting, good no fog atmospheric mix day and night K factor 1-1.33 0.66-1.0 0.66-0.5 0.5-0.4 a For 99.9 to 99.99% time availability. where F is the frequency (the microwave transmitter operating frequency) in gigahertz, and d1, 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. 23 ft 230 ft 75 ft Path (mi) Adjusted Basic Total Height (Fresnel) T and G