P2.4 How are electromagnetic waves used in communications?
Different electromagnetic eaves have different frequencies. This affects their properties and the effect that other materials have on them. Some frequencies can be used for transmitting information:
|Properties and Uses
|● Radio waves are used for transmitting radiation and television programmes because they are not strongly absorbed by the Earth’s atmosphere.
● They can travel long distances through the atmosphere and through space
● Radio telescopes are used in astronomy to pick up radio waves from stars
|● Microwaves are used to transmit mobile phone signals because they are not strongly absorbed by the atmosphere
● They are reflected well by metals so satellite dishes are made of metal and shaped to reflect the signal onto the receiver
|Light and Infrared
|● Light and infrared radiation will travel huge distances down optical fibres without the signal becoming significantly weaker. This makes them very useful for carrying information, e.g. in computer networks and telephone conversations
For communication purposes, information can be superimposed onto an electromagnetic carrier wave to create a SIGNAL.
Before a piece of information is transmitted, it is encoded in the transmitter in either an analogue or digital way. The receiver must then decode the signal to produce a copy of the original information
An ANALOGUE signal can vary continuously, so its amplitude can take any value:
A DIGITAL signal can only take one of a small number of fixed (discrete) – usually two. For transmitting information digitally, the digital code is made up of just two symbols – ‘1’ and ‘0’. Sounds and images can be transmitted digitally.
This coded information can be carried by switching the electromagnetic carrier wave off and on to create short burst of waves (pulses) where ‘0’ = no pulse and ‘1’ = pulse. When the waves are received, the pulses are decoded to produce a copy of the original sound wave or image.
Both DIGITAL and ANALOGUE signals become weaker (their amplitude becomes smaller) as they travel – the transmitted signals therefore have to be AMPLIFIED at selected intervals to make them stronger.
During transmission the signals can also pick up random variations, called NOISE, which reduces the quality of the signal.
|Analogue signals have many different values so it is hard to distinguish between noise and original signal.
This means that noise cannot be completely removed and when the signal is amplified, any noise that has been picked up is also amplified.
|Digital signals, which have two states, on (1) or off (0), can still, be recognised despite any noise that is picked up. Therefore it is easy to remove the noise and clean up the signal, restoring the on/off pattern
This means when it is amplified, the quality of the digital signal is retained
Another advantage of using a digital signal is that the information can be stored and processed by computers. Generally the more information stored or the more bytes, B (image/sound is measured in) the higher the quality of the sound or image.