Field Strength Values from Different Propagation Models
Q: Why do I get different values when I compute the field strength for a path with different propagation models?
A: The different propagation models represent different approaches to computing field strength, based on different assumptions and different methodologies.
Not only will using different models often yield different computed field strength values, changing the parameters for a particular model can also have a significant effect on the results. It is important to understand as much as possible about each of the propagation models in order to select the best model for your particular application, as well as the appropriate parameter settings for that model.
For example, the results computed with the Longley-Rice model may be affected significantly by the variability settings in the model, as a comparison of different settings demonstrates. With the Bullington propagation model, changing the topo data increment value will affect the computed field strength. For example, a path with clear line-of-sight and Fresnel zone will show no losses using the Bullington model. But the same path computed with Longley-Rice may include predicted loss values, especially if the Longley-Rice confidence and reliability values are set to high values.
When comparing different models, understanding the differences between the models is also important. The Bullington model sums up knife-edge diffraction losses along a path, while Longley-Rice considers other factors, such as the terrain roughness and signal variability. Okumura includes the type of environment (urban, suburban, etc.). Since each of these factors affects the calculation for the particular model, some variation in computed signal strength for a path is not unusual.
One of the best ways to become familiar with the various propagation models is to compare the computed results with known field strength values. If measured field strength values are available for several locations from a base station, or for several fixed links, set up those paths in HDPath. Compute the field strength using several models with variations in the parameters for the models. If measured field values are not available, a similar approach can be used on a more subjective basis using your knowledge of the performance of an established area coverage system. Compute an area coverage study for an existing system using different models and different parameter settings, to see which configuration is the best fit for what you know to be the real world performance of the system. Those settings will be a good starting point the next time you begin to design a new system.
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Copyright 2007 by SoftWright LLC