pospheric refractive index effects If the path between transmitter and receiver is well clear of inter- A further influence on the received field strength is the refractive vening terrain and obstacles, the Earth's surface is still liable to index of the troposphere. Irregularities in this region bend, scatter modify the received field due to reflections. It is often difficult to and reflect the wave. This is the main cause of time variability, accurately predict the effect of such reflections because the resultant particularly on a long distance path. field strength is a vector addition between the direct and reflected The refractive index n of the atmosphere differs by only a small waves, SO that a small uncertainty about terrain can result in a amount from unity and it is more convenient to use the, SO called, significant uncertainty about relative phases, especially for the N factor. This factor is dependent upon the pressure P (millibars), higher frequencies. temperature T (K) and relative humidity W of the atmosphere, as The strength and phase of the reflected wave depends upon: given by Equation 6.85. N = (n - 1) 10 6 = 77.6 T (6.85) 1. The surface conductivity. 2. The surface dielectric constant. 3. The general shape of the ground profile, e.g. flat, convex, or concave. Table 6.1 Clutter loss 4. The roughness of the ground. 5. The presence of buildings and trees. If the surface of the ground is rough, the reflected energy is V.h.f. (dB) U.h.f. (dB) scattered. Smoothness may be judged by an application of the, SO Urban called, Rayleigh criterion: for small angles of radians between the reflected ray and the ground plane, this criterion states that the Suburban undulations shall not be greater than 16 a for the surface to be Wooded treated as smooth. Rural Of course, the irregular shape of the terrain can result in many Rx foreground slope: Change above by: reflected waves arriving at a reception point. Facing Tx Despite all these difficulties, it is instructive to consider the formulation for a simple case