Technical data

www.westermo.com Theoretical and general applications 105
Radio
Radio communication
Wireless data communications via a radio modem provide a means of maintaining
communications with:
… remote units.
… measuring stations.
… external buildings and unmanned installations.
… temporary or mobile sites.
The purpose may be that of gathering test readings, controlling or regulating equip-
ment or recording various kinds of alarms.
Radio communications technology and how to plan, dimension and cope with noise
and interference, differ greatly from local communications in a data network.
How it works
Communication equipment is provided using a radio modem that converts the data sig-
nal into radio waves for a specific channel with a specific bandwidth. The data signal may
require some form of signal processing or filtering before it can be transmitted by the
radio channel. In addition, the signal is modulated (by a modem) to a correct carrier fre-
quency and can be transmitted via a radio link to the receiver. Irrespective of whether
the source is analogue or digital, the transmission is nearly always analogue. The receiv-
er equipment decodes and reconstructs the original signal.
The available frequency range for radio communications is limited and regulated by
an international agreement (ITU).
Radio waves are propagated in the atmosphere in the layer between the ionosphere
and the surface of the earth. Communication conditions can vary greatly, depending
on the frequency band, ranging from the longest wavelengths of up to 1 000 metres
(0.63 mi) in the ELF band to shortest ones of 10 mm (0.34 in) in the EHF band. Radio
modems operate in the UHF band at around 440 mhz. The UHF band between 300
and 3 000 mHz also contains radar, radio, TV, NMT mobile telephony, mobile radio,
satellite communications, amateur radio and both GSM and wireless telephones.
Frequency band
ELF 300–3000 Hz
VLF 3–30 kHz
LF 30–300 kHz
MF 300–3000 kHz
HF 3–30 MHz
VHF 30–300 MHz
UHF 300–3000 MHz
SHF 3–30 GHz
EHF 30–300 GHz
B A C K