Campbell Scientific RF401A-Series Spread Spectrum Radios - Manual (Page 39)

Appendix B. Distance vs. Antenna
Gain, Terrain, and Other Factors
B.1 Introduction
The communication distance you can expect to obtain using the RF401A series
depends on many factors including line of sight, antenna height, and antenna
gain. Examples of distance achieved using different antennas and line of sight
are shown in TABLE B-1.
TABLE B-1. RF Path Examples
Distance
Achieved
(miles) Antennas Path Between Radios
2
14204 Omnidirectional ½ Wave 0 dBd* Whip
to
14204 Omnidirectional ½ Wave 0 dBd Whip
Virtual line of sight on valley floor with wetland
foliage.
10
14204 Omnidirectional ½ Wave 0 dBd Whip
to
14204 Omnidirectional 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 Omnidirectional 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 radios. At 900
MHz, 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 radio 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.
B-1