User's Manual

41
Document Number: 0100SM1401 Issue: 05-15
Part E – Radio Planning and Design
Part E – Radio Planning and Design
Obstructed Radio Path
This path has an obstruction that will reduce the signal
arriving at the field site.
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 strength reduction 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.
Clear line of sight
Radio path with good signal levels, attenuated only by
free space loss.
Radio Path analysis