Communications, Radar & Electronic Warfare (201.. - Index of

VHF to SHF Radio Prediction167Figure 10.8Diffraction over a single main diffractIOn obstacle.direct line of sight because of the terrain. The edge of good coverage is shown as thesolid black line. The long dashed line shows the energy in the direction of the receiver.This is energy diffracted over the main diffraction obstacle (circled).Notice in Figure 10.8 that although the terrain is complex, the diffraction edge isonly a single point along it. The received signal strength can be estimated by adding theloss due to distance to the loss due to diffraction over the obstacle. This can besimplified to the form shown in Figure 10.9. Note that the Earth bulge (due to theEarth's radius) must also be included in the calculation for the height of the intrusionabove the link terminals (it must be subtracted from the actual height for the equationthat follows to be accurate).The distances d) and d2 are the distances from the transmitter and receiverrespectively. All of the units should be self-consistent, thus if the distances d) andd2 are in kilometres, the height value must also be expressed in kilometres. FromDiffraction edge- ---h- --------- d 1 --------- -->