Campbell Scientific RF401A-Series Spread Spectrum Radios - Manual (Page 40)

Appendix B. Distance vs. Antenna Gain, Terrain, and Other Factors
GAIN ANTENNAS
Increasing antenna gains improves signal strength and distance. For example,
the substitution of a 9 dBd Yagi antenna where a 0 dBd omnidirectional 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 whip antennas 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.
B.2 How Far Can You Go?
Estimating Distance for Spread Spectrum Radios
B.2.1 Overview
There is a great deal of interest in estimating the distance you can expect to
achieve with the RF401A-series radios. Also of interest are the effects of cable
length, antenna gain, and terrain. Some of these items are easy to quantify
(cable loss, for instance); others are difficult to quantify (such as the effect of
ground reflections). They are all important, though, and affect how well the
RF system performs.
Probably the best approach to take in making range estimates is to do a site
survey that considers the topography, location of antennas and radios, and
cable lengths, make some assumptions about the path losses, and see if there is
still some net gain. If there is, or if it is close, the next course is to actually try
it out.
B.2.2 Link Analysis
In an RF system, there are gains (transmitter power, antenna gains, and
receiver sensitivity “gain”) and losses (cable loss and path loss). If the gains
exceed the losses, you have a connection; any excess is the “link margin”.
EXAMPLE GAINS EXAMPLE LOSSES
Transmitter Power 24 Transmitter Cable 3
Transmitter Antenna 6 Free Space 120
Receiver Antenna 6 Receiver Cable 3
Receiver Sensitivity “gain” 109
TOTAL GAINS = 145 dB TOTAL LOSSES = 126 dB
Link Margin = (Total Gains) – (Total Losses) = 145 – 126 = 19 dB
A minimum of 6 dB of link margin is recommended.
Here is a block diagram of the various components of gain/loss:
Cable
Loss
Antenna
Gain
Free Space
Loss
Antenna
Gain
Cable
Loss
Radio
Receiver
Radio
Transmitter
Pt - Lt + Gt - Lp + Gr - Lr = Pr
B-2