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Appendix H. Distance vs. Antenna
Gain, Terrain, and Other Factors
RF Path Examples
Distance
Achieved
(miles)
Antennas Path Between Radios
2
14204 OMNI ½ Wave 0 dBd* Whip
to
14204 OMNI ½ Wave 0 dBd Whip
Virtual line-of-sight on valley floor with
wetland foliage.
10
14204 OMNI ½ Wave 0 dBd Whip
to
14204 OMNI ½ Wave 0 dBd Whip
Line-of-sight across a valley (on foothills
approximately 300 feet above the valley
floor on each end).
35
14204 OMNI ½ Wave 0 dBd Whip
to
14201 9 dBd YAGI
Line-of-sight across a valley (on foothills
approximately 300 feet above the valley
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.