y to have a good idea of the location and height of buildings and trees. The attenuation to the rooftop close to the receiver can then be obtained by adding a further term to the Deygout knife edge diffraction method given in the Knife Edge Diffraction section. This edge is chosen to represent the most intru- sive building along the path. Then it is necessary to determine the attenuation due to the diffraction from the roof above the street. Again this can be treated as a knife edge diffrac- tion. The loss might be less than would be expected from the angle of roof to receiver because the predominant signal often comes from the walls of the houses opposite. This would produce a smaller diffraction angle. We turn now to loss caused by trees. The predominant signal in this case might be by passage through the trees. This can be determined by figures published for loss per distance Table 7.1 Clutter Loss VHF (dB) UHF (dB) Urban Suburban Wooded Rural Rx foreground slope: Change preceded by Facing Tx Away from Tx The Space Wave traveled through trees. However, care must be taken not to obtain a figure that is greater than that calculated for diffraction over trees. There are now many cellular telephone sites where the base station is also below build- ing height. In this case even more detail is required to make coverage and interference pre- dictions. This detail is to enable calculations to be made of propagation via multiple reflections and diffraction off walls and around the buildings. Ground Reflections If the path between transmitter and receiver is well clear of intervening terrain and obstacles, the Earth's surface is still liable to modify the received field because of reflections. It is often difficult to accurately predict the effect of such reflections because the resultant field strength is a vector addition between the direct and reflected waves, SO a small uncertainty about terrain can result in a significant uncertainty about relative phases, especially for higher frequencies. The strength and phase o