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Algorithm of ITU-R P.526 and multi-edge peak diffraction analysis
Foreword
When an electromagnetic wave encounters an obstacle that is comparable in size to its wavelength during propagation, it bends around the obstacle. This phenomenon is known as diffraction of electromagnetic waves. In the TV and FM broadcasting frequency bands (30 MHz to 1000 MHz), where wavelengths range from meters to hundreds of meters, radio waves are particularly susceptible to diffraction due to complex terrain features.
Traditional propagation models like ITU-R P.370 and ITU-R P.1546 work well in open or low-lying hilly areas but tend to produce significant errors in mountainous regions. To address this issue, the International Telecommunication Union (ITU) introduced Recommendation ITU-R P.526, "Propagation by Diffraction," in 1978. This recommendation provides methods for calculating field strength due to diffraction, taking into account both terrain roughness and the Earth’s curvature. However, it only covers single-edge, double-edge, and single-rounded peaks, without a general solution for complex terrains. This paper explores the application of ITU-R P.526 in multi-edge peak diffraction using the Bullington and Epstein-Peterson methods.
Algorithm for ITU-R P.526
When dealing with a single obstacle, especially one with a complicated shape, it's challenging to find an exact analytical solution. However, two specific cases allow for complete analytical solutions: a knife-edge with negligible thickness and a smooth cylindrical obstacle. These are commonly used in diffraction calculations.
In the case of a single-edge peak, the loss caused by the obstacle can be calculated using specific formulas. For a single circular peak, a correction factor is applied to account for the diffraction effect. The formula includes parameters such as k and b, which depend on the number of edges and other factors.
Multimodal Diffraction
The simplest form of multi-peak diffraction is double-edge diffraction. While it can be calculated using the Fresnel integral, the computation is intensive. The Bullington and Epstein-Peterson methods offer more efficient approaches by using equivalent modeling techniques.
In the Bullington method, multiple obstacles are replaced by an equivalent single-edge obstacle at a junction point. In the Epstein-Peterson method, each segment is treated as a single-edge peak, and losses are calculated sequentially, with an additional correction factor added for accuracy.
Practical Application
In the broadcasting industry, propagation models like ITU-R P.370 have been widely used for predicting field strength in VHF and UHF bands. However, these models do not consider diffraction effects, leading to inaccuracies in mountainous regions. ITU-R P.1546 improved upon these models, but still lacks consideration for diffraction.
To enhance prediction accuracy, efforts are being made to combine analytical solutions with empirical data. ITU-R P.526 plays a key role in this process. When applied in real-world scenarios, such as in the border area between Guangdong and Hong Kong, it provides results closer to measured values. Using electronic maps and accurately extracting geographic parameters is crucial for maximizing the predictive power of ITU-R P.526.
It's important to note that while ITU-R P.526 focuses on terrain obstacles like mountains and hills, it does not typically account for tall buildings. Further research is needed to apply the recommendation effectively in urban environments.