User's Manual

Page 13

E Series Data Radio – User Manual

Part D – System Planning and Design

Selecting Antennas

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.