User's Manual

Page 12

M Series Data Radio – User Manual

Part D – 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 (to a minimal extent at

UHF frequencies) 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.