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© Copyright 2007 Trio DataCom Pty. Ltd.
O Series Data Radio – User Manual
Part D – System Planning and Design
Part E – System Planning and Design
Understanding RF Path
Requirements
A radio modem needs a minimum amount of received RF signal to
operate reliably and provide adequate data throughput.
In most cases, spectrum regulatory authorities will also define
or limit the amount of signal that can be transmitted, and the
transmitted power will decay with distance and other factors, as it
moves away from the transmitting antenna.
It follows, therefore, that for a given transmission level, there will
be a finite distance at which a receiver can operate reliably with
respect to the transmitter.
Apart from signal loss due to distance, other factors that will decay
a signal include obstructions (hills, buildings, foliage), horizon
(effectively the bulge between two points on the earth), and factors
such as fog, heavy rain-bursts, dust storms, etc.
In order to ascertain the available RF coverage from a transmitting
station, it will be necessary to consider these factors. This can be
done in a number of ways, including
(a)
using basic formulas to calculate the theoretically available
signal - allowing only for free space loss due to distance,
(b)
using sophisticated software to build earth terrain models
and apply other correction factors such as earth curvature
and the effects of obstructions, and
(c)
by actual field strength testing.
It is good design practice to consider the results of at least two of
these models to design a radio path.
Examples of Predictive Path
Modelling
Clear line of site
Radio path with good signal levels, attenuated only by free space
loss.
Obstructed Radio Path
This path has an obstruction that will seriously degrade the signal
arriving at the field site.
