Appendix H. Distance vs. Antenna Gain, Terrain, and Other Factors
RF Path Examples| Distance | | |
| Achieved | Antennas | Path Between Radios |
(miles) | | |
| 14204 OMNI ½ Wave 0 dBd* Whip | Virtual line-of-sight on valley floor with |
2 | to | wetland foliage. |
| 14204 OMNI ½ Wave 0 dBd Whip | |
| 14204 OMNI ½ Wave 0 dBd Whip | Line-of-sight across a valley (on foothills |
10 | to | approximately 300 feet above the valley |
| 14204 OMNI ½ Wave 0 dBd Whip | floor on each end). |
| 14204 OMNI ½ Wave 0 dBd Whip | Line-of-sight across a valley (on foothills |
35 | to | approximately 300 feet above the valley |
| 14201 9 dBd YAGI | floor on each end). |
*dBd = decibel level compared to a simple dipole antenna
LINE-OF-SIGHT
You should arrange for a line-of-sight signal path between RF400s. At 900 MHz or 2.4 GHz there is little signal bending, however, there is reflection from hills, water, and conductive objects. Sometimes reflections provide a helpful path around an obstacle. There can be some trees and bushes in the signal path (with reduction in signal strength), but a hill will block the signal effectively. Thick trees can limit range to as little as 800 feet. Where possible avoid buildings and other man-made structures in the signal path as they absorb or reflect some of the direct wave, possibly below the level needed for communications.
ANTENNA HEIGHT
In situations where the RF400 antennas are situated virtually line-of-sight, the elevation of antennas (by choice of site or by installing a tower or mast) can substantially increase signal strengths. The amount of increase depends on factors in the propagation path between the radios including terrain, foliage, and man-made structures. Elevating one or both of the antennas essentially raises the signal path allowing the direct wave to better avoid absorption or reflection which can sometimes be more helpful than adding higher gain antennas.
GAIN ANTENNAS
Increasing antenna gains improves signal strength and distance. For example, the substitution of a 9 dBd yagi antenna where a 0 dBd OMNI existed theoretically extends the attainable distance by a factor of 2.8. Adding 9 dBd yagi antennas on both ends in place of 0 dBd OMNIs theoretically extends the distance by a factor of 7.9. The higher the yagi’s gain, the narrower the beam width and the more critical it is that it be aimed right on target.
H-1