AN ENGINEERING APPROACH FOR SITE SHIELDING CALCULATIONS

In this paper, two models to approximately calculate the field distrib ution in the vicinity of a perfectly conducting finite width screen ar e discussed. The first model is based on the geometrical theory of dif fraction (GTD), and is simplified such that the magnitude of the indiv idual ray contributions can be easily calculated. The second model is based on a graphical approach. Coefficients that are representative fo r the dimensions of the obstacle and the relative position of the obse rvation point are used together with standardized equations to calcula te the field strength in the shadow region behind the screen. Also a m odification of the current CCIR procedure for knife-edge diffraction i s proposed, such that it can be applied to a finite width screen. Resu lts of all models are compared with more accurate and complex results from a model based on the uniform theory of diffraction (UTD). All mod els proposed are simple to apply and present an essential extension to currently available engineering models. It is found that the GTD mode l has the best performance with respect to the UTD approach. The graph ical method, however, is more user friendly than the GTD model, becaus e fewer intermediate results need to be calculated. Moreover, the use of this method yields a field-strength distribution for the whole shad ow region of the obstacle instead of the field at just a single point. Further, it is found that the modified CCIR approach can also be used with good accuracy. The models discussed answer some questions formul ated by the CCIR concerning site shielding, and extend the currently a vailable engineering models for this interference reduction technique.