lated values by the fraction d/5, where d is the distance from the earth station to the obstacle in kilometers.

NOTE 2: The values of site-shielding factor shall be used with caution where terrestrial stations may be located, within coordination distance, at sites which are substantially above the horizontal plane passing through the earth station.

(f) Equivalent Basic

Transmission

Loss at 4 Gc/s (Lo'). The propagation data considered in paragraph (g) of this section relates to the frequency of 4 Gc/s and it is therefore in general necessary to convert the minimum permissible basic transmission loss (L) into an equivalent loss at 4 Gc/s (Lb') before using these data to find the coordination distance. The equivalent loss in decibels at 4 Gc/s is given by:

L'L+13-21.6 log10f

where f is the assigned frequency in Gc/s. This relationship is shown in Figure 1 of paragraph (h) of this section.

(g) World Radio-Climatic Conditions and Propagation Data. (1) The propagation curves of Figure 2 are labelled Zone A, Zone B, and Zone C, and correspond to the various basic radio-climatic regions of the world as follows:

Zone A: Land

Zone B: Sea, at latitudes greater than 23.5° N. and 23.5° S.

Zone C: Sea, at latitudes between 23.5° N. and 23.5° S. inclusive.

(2) In any direction from the earth station the required coordination distance is found as follows:

(i) If the equivalent basic transmission loss Lь' is such that the coordination distance in the given direction lies wholly within one of the zones, the coordination distance may be obtained directly from Figure 2 using the appropriate curve;

(ii) If the coordination distance lies partly in one zone and partly in another, the curves for mixed paths, Figures 3, 4 and 5 should be used. These curves show the loss Lb' as a function of the path length in each of the two zones separately. Thus, if the path length in one zone and the required loss are known, the path length in the other zone can be determined. The path length in the first zone is the known

distance from the earth station to the zone boundary in the direction concerned, hence the further length in the second zone can be found. The total path length, or coordination distance, is the sum of the two path lengths. Figures 3, 4 and 5 cover all cases of mixed paths in two zones as follows:

Figure 3: Zones A and B.
Figure 4: Zones A and C.
Figure 5: Zones B and C.

An example of the coordination distance calculation for a mixed path is worked out in paragraph (h) of this section.

(3) In certain geographical areas where propagation losses are known to be less than the values given by the pertinent zonal propagation curves, coordination distances should be calculated on the basis of the known propagation data.

(h) Example of Coordination Distance Calculation for a Mixed Path. (1) The procedure to be followed in the case of a mixed path is illustrated by the following example, in which it is assumed that a basic transmission loss of 190 db is required to avoid interference from an earth station to terrestrial services in a given direction.

(2) As shown in the diagram in Figure 6, the earth station is situated 50 km. from the coast and there is an oversea path of 150 km. before the coastline of neighboring country is reached. It is required to find the coordination distance from the earth station in the given direction using the mixed paths propagation chart in Figure 6. The procedure is as follows:,

(1) Starting from the origin, the distance of 50 km. from the earth station to the coastline is set off along the A axis of the chart as indicated by the point A1.

(ii) The oversea path length of 150 km. is then set off parallel to the B axis of the chart as indicated by the point B1.

(iii) The further overland distance required is then measured parallel to the A axis from the point B1 to the point of intersection with the 190 db curve, as indicated by X. This distance is found to be 90 km.

(iv) The coordination distance is the sum of the A and B coordinates of the point X and is equal to 50+150+90=290 km.