Foreword When an electric wave encounters an obstacle with a similar size and working wavelength during propagation, it will propagate forward around the obstacle. This phenomenon is called diffraction of electric waves.
In the TV and FM broadcasting frequency bands (30 MHz to 1000 MHz), due to the wavelengths in the meter and 100-meter range, the radio waves are prone to diffraction under the geographical conditions of complex terrain and features.
The traditional radio wave propagation prediction models ITU-R P.370 and ITU-R P.1546 are more suitable for open and low-lying hilly areas, while for mountainous areas, the prediction error is relatively large. To this end, ITU in 1978 developed Recommendation ITU-R P.526, "PropagaTIon by diffracTIon", which mainly describes the diffraction effects in propagation and the prediction methods of its field strength, in addition to considering ruggedness in prediction. In addition to the diffraction caused by the terrain, it also involves the diffraction effect of the spherical surface of the Earth on the propagation. However, this Recommendation only provides diffraction algorithms for single-edge, double-edged, and single-round peaks, and does not provide a general solution for complex terrain. This paper attempts to discuss the calculation and practical application of ITU-R P.526 and multi-edge peak diffraction in combination with the Bullington method and the Epstein-Peterson method.
Algorithm for ITU-R P.526When the shape of a single obstacle is complicated, it is difficult to give an accurate analytical solution. Only a single obstacle of two shapes can give a complete analytical solution: one is a knife-edge with negligible thickness, see Figure 1 and Figure 2; one is a cylindrical obstacle with a smooth surface, see image 3.
In the case of a single-edge peak, only one obstacle on the propagation path causes loss to the propagation. The effect produced by the obstacle can be calculated according to the following two formulas.
The meaning of each parameter in the formula is shown in Figure 1 and Figure 2.
In the case of a single circular peak, only one circular obstacle on the propagation path has an effect on the propagation. In the process of calculating the loss, a correction factor is added, which is provided by:
T(m,n)=kmb 3.
Where k=8.2+12.0n, b=0.73+0.27[1-exp(-1.43n)],
The meaning of each parameter in the formula is shown in Figure 3.
Multimodal diffractionThe simplest multi-peak diffraction problem is double-edged peak diffraction. The double-edge diffraction produced by the double obstacle can be obtained by double integration of the Fresnel formula, but the amount of calculation is huge. The Bullington method and the Epstein-Peterson method solve the problem better by using the equivalent method.
In the Bullington method (see Figure 4), obstacles at points A, B, and C are replaced by an equivalent blade-shaped obstacle at point D of the junction, at which point obstacles at points A and B are negligible.
In the Epstein-Peterson method (see Fig. 5), a, b, h'1 and b, c, h'2 each constitute a single-edged peak. First, use the formula to find the diffraction loss L1 between a, b, and h'1, and then find the diffraction loss L2 between b, c, and h'2. After finding L1 and L2, add one more. The correction factor Lc is calculated as follows:
Then the total diffraction loss is:
L = L1 + L2 + LC 5.
In the prediction calculation of the propagation of mountain areas, there are generally multiple obstacles in the propagation path. In this case, the multiple edge peaks must be equivalent to a single-edge peak or a double-edged peak to obtain diffraction loss. The Bullington method can be used to sequentially equalize multiple blade peaks, and finally become the equivalent single-edge peak, thereby calculating the total propagation loss, and can also be equivalent by the Epstein-Peterson method. However, it should be noted that in the double-edge peak treatment method provided by Epstein-Peterson, when the obstacle distance d is large, the obtained result is more accurate. When the distance d of the obstacle is small, the error obtained is large. Therefore, when constructing the blade peak, it is necessary to limit the number of blade peaks, and try to consider the blade peak that has a large influence on the diffraction.
Practical applicationThe radio propagation model based on broadcast field strength predictions (mainly television and FM radio) in the broadcast and television industry is mainly the model in ITU-R P.370, developed by ITU in 1951 - VHF and UHF PropagaTIon curves for the frequency range from 30 MHz to 1000MHz. In 2001, the ITU study developed a new radio wave propagation field strength prediction standard ITU-R P.1546 - "Method for point-to-area predicTIons for terrestrial services in the frequency range 30MHz to 3000MHz" and replaced it with The long-term use of the ITU-R P.370 standard, but in terms of broadcast field strength prediction, China's national standard still uses the ITU-R P.370 standard.
Although ITU-R P.1546 has improved both in terms of prediction accuracy and range of use compared to ITU-R P.370, neither of these Recommendations considers the diffraction effects of obstacles. Therefore, in the mountainous region, when the diffraction influence becomes the main factor of propagation prediction, the prediction errors of the two propagation models are significantly increased. For example, the border area of ​​Guangdong and Hong Kong is a typical example.
In order to improve the accuracy of radio wave propagation prediction, one tries to find an accurate analytical solution to the diffraction problem on the one hand, and on the other hand, as in ITU-R P.370 and ITU-R P.1546, attempts to pass a large number. The actual observation data summarizes the regular propagation prediction curve of the diffraction problem. The results of these two aspects are basically reflected in Recommendation ITU-R P.526.
In the practical application of ITU-R P.526 to deal with multi-peak diffraction, it is necessary to combine electronic maps to fully exploit its predictive accuracy. In electronic maps, constructing and extracting the geographic information parameters required for various calculations is a very professional problem. The extent to which the first Fresnel zone is blocked and blocked, the construction, discrimination, and equivalence of the blade peaks are at the heart of the problem. The actual proof of the coordination of radio and television frequency planning in the border area of ​​Guangdong and Hong Kong is closer to the measured value when dealing with the problem of radio wave propagation prediction in complex mountainous areas.
It should be noted that when applying Recommendation ITU-R P.526 in the television and FM broadcasting bands (30 MHz to 1000 MHz), the main obstacles considered are terrain obstacles such as mountains or hills, and the impact on tall buildings is generally Not involved. How to apply ITU-R P.526 to calculate the diffraction effects of tall buildings is further explored by industry experts.
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