User's Manual

Document Number: 0100SM1401 Issue: 10-14

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

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