Campbell RF401A Series, Instruction Manual

Page: 42 / 52

Appendix B. Distance vs. Antenna Gain, Terrain, and Other Factors
B-2
B.2 How Far Can You Go?
Estimating Distance for Spread Spectrum Radios
B.2.1 Overview
Because the factors affecting communication distance are different for every
system, a generalized estimate of distance cannot be stated with any practical
degree of accuracy. Reasonable estimates of distance can only be achieved by
means of a case-by-case analysis and accounting of these factors. The link
budget is a simple and convenient tool for this purpose.
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.
B.2.2 Link Budget
In any RF system, there are gains and losses. Gains most commonly derive
from the relative gain of the transmitting and receiving antennas. In some
cases, an in-line RF amplifier may be used to provide additional system gain,
but this is rare and not typical of Campbell Scientific installations. Losses
derive from the attenuation of the RF signal as it propagates through the
transmission lines (coaxial cables, surge suppressor, etc.) connecting the
transmitter and receiver to their respective antennas, and more importantly, as
it traverses the path of propagation between the antennas (path loss).
Here is a block diagram of the various components of gain/loss:
Cable
Loss
Antenna
Gain
Path
Loss
Antenna
Gain
Cable
Loss
Radio
Receiver
Radio
Transmitter
Ptx - Ltx + Gtx - Lpath + Grx - Lrx = Prx
Where:
Ptx = transmitter output power, in dBm (24 dBm in the case of the RF401A
series at maximum transmitter power)
Ltx = cable loss between transmitter and antenna in dB (see Appendix B.2.4 ,
Cable Loss
(p. B-3)
Gtx = transmit antenna gain in dBi (dBi = dBd + 2.15)
Lpath = path loss between antennas in dB (see TABLE B-5 , TABLE B-6 )
Grx = receive antenna gain in dBi
Lrx = cable loss between antenna and receiver in dB
Prx = signal power at the radio receiver in dBm
Most of these values can be readily obtained from data sheets or direct
measurements. The notable exception is path loss. Path loss is very difficult to
derive analytically. Probably the best approach is to do a site survey that
considers the topography, elevation, and location of the antennas, any potential
obstructions to the line of sight, and make some assumptions about the path
losses (see the Campbell Scientific application note Line of Sight Obstruction ).