ln many practical situations, especially in hilly terrain, the propagation path may consist of more than one obstruction, in which case the total diffraction loss due to all of the obstacles must be computed.

Bullington suggested that the series of obstacles be replaced by a single equivalent obstacle so that the path loss can be obtained using single knife-edge diffraction models.

This method, illustrated in Figure oversimplifies the calculations and often provides very optimistic estimates of the received signal strength.

In a more rigorous treatment, Millington et. al. gave a wave-theory solution for the field behind two knife edges in series.

This solution is very useful and can be applied easily for predicting diffraction losses due to two knife edges. However, extending this to more than two knife edges becomes a formidable mathematical problem.

Many models that are mathematically less complicated have been developed to estimate the diffraction losses due to multiple obstructions.