to its very high frequency (and corre- include ducting i.e. steering of the microwave signal away from the sponding short wavelength) tends to behave much like visible light. receiving antenna by atmospheric ducts, Figure 50.4; super refrac- Microwave energy can be focused, reflected, and refracted (or bent) tive effects which direct the signal into the earth, Figure 50.5; by the atmosphere. To describe an atmospheric effect that bends the decoupling effects which cause the signal at the receiving antenna microwave energy away from a straight line, the K factor that to arrive outside the main lobe of the antenna pattern and reflective appears in the earth curvature formula is changed. The K factor used effects from atmospheric 'sheets' causing cancellation of the main to describe straight line microwave propagation is 1.0. microwave signal. It is beyond the scope of this discussion to Because the atmospheric density decreases as the height above the elaborate on these effects which, although relatively rare, should earth increases, the lower part of the wavefront of the microwave interest the serious microwave designer. 50/4 Line of sight radio systems The first Fresnel zone consists of the first area around the path line. A path designed SO that only this area is not blocked by any Trapped microwave path obstructions below the path would be described as having a first Fresnel zone clearance above the earth. Another important consideration is the nature of the earth along the path. A reflective surface such as water may actually result in partial or complete cancellation of the desired signal. For example, if the earth is perfectly reflective (R = -1.0), the desired signal will be cancelled with a path clearance equal to 2F as in Figure 50.7. Even with a less reflective earth ( and a typical reflection coefficient is - 0.3) a partial cancellation may occur if the clearance is equal to True earth radius any even Fresnel zone number and the obstruction is reflective. 'More is better' is not necessarily the case wit