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Terrain Roughness
The following explanations and technical notes are provided to help you make the best use of the network path link budget and antenna calculator.
Value to Enter:
Select the option that most accurately describes the terrain underneath the proposed radio link. The options are:
| Calculator Pull-Down Option | Examples of Terrain |
| Generally flat terrain surface | Open parklands (use Foliage Attenuation if the link must pass through trees), farm land, water, airports (across the runway environment), thick jungle-like vegetation (where the link will pass over the consistent-height tree tops) |
| Suburban area; buildings roughly the same height | Housing developments, smaller town downtown area (generally rural communities), apartment complexes (when the link passes over the top of the complex), industrial parks (with similar-height warehouses and offices) ALSO gently rolling hills |
| Urban area; buildings of different heights | City downtown areas with many different building heights, large university campus environments (where the link passes over the campus) ALSO mountain areas with steep, undulating topography |
| Ignore terrain roughness | This option tells the calculator not to add any attenuation factor for terrain roughness |
Significance of This Value:
The Terrain Roughness selection describes the smoothness of the "top" of the span between the antennas. Some of this smooth (or irregular) "top" will penetrate the first 40% of the Fresnel Zone and, consequently, the degree to which it is smooth or rough will effect the degree to which the signal is reflected, diffracted, and attenuated. Remember that the height of the tallest obstruction will, ultimately, have to be added (by you, the designer; not by the calculator) to the antenna heights calculated by the Connect802 Antenna System Designer.
Background and Technical Perspective:
An RF wave, propagating across a relatively smooth portion of the earth's surface (large lake, expansive meadow, etc.) will not "strike" the surface of the earth. By this it is meant that the curving surface always curves "away" or "down" while the wave continues to propagate in a direction that is essentially tangential to the sphere. This (when considered by itself) is the reason that the RF wavefront encounters less-dense atmosphere as it expands (the RF signal is moving tangentially to the sphere) which makes it refract downwards towards the earth resulting in the 4/3 factor for radio Line-of-Sight. (See discussion of Tower Height).
When the earth's surface is not smooth, however, the RF signal will be absorbed, reflected, and diffracted to varying degrees by the terrain features that create "roughness". Very sophisticated RF modeling formulae exist that take into consideration specific terrain obstruction features. These formulae assess incident angles of RF rays as they refract and reflect from surfaces as well as using Guassian distribution models of scattering losses. The Connect802 Antenna System Designer takes a more pragmatic approach to terrain roughness modeling. In "free space" (smooth surface) the assumption is made that an additional 2 dB of gain will be required to overcome the natural irregularities of the earth's surface. In suburban areas (where most buildings are the same height) a 3 dB gain is required, and in urban areas where there is dramatic height difference between buildings a 5 dB gain is added. Since exact sets of incident angle data are not typically available in most practical, field situations these values are appropriate. It's felt that this is a sufficiently reasonable approximation of what would otherwise be a calculation based on potentially unavailable input data.