Technical data

106 Theoretical and general applications www.westermo.com
Attenuation and noise
A propagated radio wave is affected by both the ground and the air layers through
which it passes. In the frequency bands in which radio modems operate, with wave-
lengths of around 1 metre (3.28 ft), there are many objects such as hills and buildings
that can cause a radio shadow (cf. Mobile telephony). This is in addition to intermittent
interference from other equipment. Such interference caused by objects is termed
shadow or interference fading, and causes signal attenuation or distortion.
The signal reaching the receiver is often very weak compared with the transmitted
signal but this in itself does not imply any quality deterioration of communication.
What may cause problems is interference outside our control, noise that is added to
the signal. This not only occurs in the receiving equipment but also exists in the form
of thermal noise (thermal motion of particles), atmospheric noise (electrical phenom-
ena such as lightning), cosmic noise (incipient radio-frequency radiation from the sun
or other so-called galactic noise) and locally generated noise (electrical equipment in
the receiver’s surroundings).
B A C K