2.8 Attenuation and distortion
As a signal travels from one device to another it has two problems to overcome. The first is that it gets weaker the further it travels, because some of its energy is absorbed by the transmission medium. This effect is known as attenuation. The extent of attenuation depends on the distance it has to travel and on the type of medium it is travelling through. An amplifier can be used to boost the signal power at the transmitter and receiver, and if necessary at various points in the transmission link, so that signal power can be maintained at a usable level.
The second problem is that the signal can become distorted by external influences as it travels along the communication path. This can be caused by other signals travelling in the vicinity, or by waves of energy such as solar energy, lightning, and pulses of energy from electrical machinery. You might have come across instances of distortion in your own domestic equipment. For example, I have a small TV in my bedroom and when I use my hairdryer nearby I can see spots and lines on the TV screen that are caused by the electromagnetic energy generated by the motor in the hairdryer. If I place my mobile phone next to my radio I often hear 'beeps' on my radio as the phone sends signals to the phone network.
Unless distortion can be removed from the signal at the receiving end then any amplification to overcome the problems of attenuation will also amplify any distortion in the received signal. Binary signals are quite resistant to distortion because they represent only two states that can usually be distinguished quite easily from any unwanted effects.
Activity 11: self-assessment
To test your understanding of what you have read so far about signals, try to write down answers to these questions.
The frequency of an electromagnetic wave is measured in units of __________
In what portion of the electromagnetic spectrum does an electromagnetic wave with a frequency of 108 Hz lie?
What is meant by the term 'modulation'?
Why might a signal become attenuated as it travels?
The frequency of an electromagnetic wave is measured in units of Hertz.
An electromagnetic wave with a frequency of 108 Hz lies within the band of frequencies known as radio waves.
'Modulation' is the term given to the process of modifying a carrier to represent data.
Energy from a signal is absorbed by the transmission medium. This weakens the signal.