Appendix H. Distance vs. Antenna Gain, Terrain, and Other Factors

As mentioned before, free space conditions are the ideal, but seldom actually seen. The higher the antenna height relative to the terrain in the line of sight path, the closer to free space conditions. Antenna height is everything!

Here are some additional propagation effects that increase the path losses:

Diffraction

This is caused by objects close to the line of sight path. Real world examples of this would be hills, buildings, or trees. The object may not be in the direct line-of-sight, but if it is close enough, it will cause the RF to diffract around the object, giving additional path loss. “Close enough” is a function of frequency, path length, and position of the obstacle along the path.

An example at 900 MHz: a 10 mile path length with an obstacle halfway along the path will see diffraction “losses” from an obstacle within ~70 ft. of line-of sight. The amount of loss would be from 6 dB to 20 dB, depending on the obstacle surface. A sharp edge (like a rock cliff) would give the minimum loss (6 dB), while a rounded hill would give the maximum loss (20 dB).

Ground Reflections

These are caused by the RF signal being reflected from the ground (or water), and undergoing a phase shift so that it destructively interferes with the line of sight signal. The conditions that cause this the most are propagation over water, or over a low-lying fogbank. The reflected signal suffers little attenuation, gets out of phase, and interferes with the main signal. If antennas need to be sited near water, they should be positioned away from the water’s edge so that the ground vegetation attenuates the reflected RF.

The result of the reflection and interference (worst case) is that the path loss increases as the 4th power of the distance, instead of the 2nd power. This changes the distance term in the path loss equation to: 40 x log ( d ) dB. Then, with each doubling of distance, the path loss increases by 12 dB, instead of 6 dB.

Vegetation

Losses due to vegetation (trees, bushes, etc) cause the path loss to increase by the 3rd to 4th power of the distance, instead of the 2nd power. This is just like in the severe ground reflection case above.

Rain, Snow, and Fog

Below 10 GHz, these don’t have much effect on path loss (see Ground

Reflections).

Real World Distance Estimates

From the above discussion of departures from the ideal “free space” path loss, it is clear that we should usually use something other than the 2nd power distance table.

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