Science

Wireless signals

Wireless communication is convenient. It is used for radio programmes, mobile phones and computer networks. Digital signals are less affected by interference than analogue signals.

Uses of wireless technology

Electromagnetic radiation can be used for wireless communications. For example:

  • radio waves are used to transmit television and radio programmes
  • microwaves are used to transmit mobile phone calls

Radio stations with similar transmission frequencies can interfere with each other’s signals.

Microwaves are also used to network computers together, especially laptop computers.

Wireless communications can be available all of the time, almost anywhere. They have several advantages over wired communications. These include:

  • no wires need to be run through buildings, over ground or underground
  • wireless devices can be portable

Reflection and refraction

Reflection and refraction are important in communications signals.

Reflection

Wireless signals can be reflected off buildings and other large objects. This means that signals may be received even if the receiver is not in direct sight of the transmitter. But it can also cause ‘ghosting’ on television pictures, for example.

Reflection is also important in wired communications. An optical fibre is a thin rod of high-quality glass that absorbs very little light. Even when the fibre is bent, light getting in at one end undergoes repeated total internal reflection and emerges at the other end.

the light ray enters the fibre at one end and is reflected out at the other end

Optical fibre

Refraction

Radio waves are refracted by different layers in the Earth’s atmosphere. This leads to a reduction in the signal, making it difficult for them to be received over long distances. Unlike radio waves, microwaves are not refracted, so they are used for satellite communications.

microwaves pass through the atmosphere, radio waves relected through a charged layer of the upper atmosphere, signal received even though transmitter and receiver are not in the line of sight

Microwaves and radio waves in the atmosphere

Signals - higher

Analogue versus digital

Digital radio signals maintain their quality better than analogue signals. They are less prone to interference. All signals become weaker as they travel long distances, and they may also pick up random extra signals. This is called noise, and it is heard as crackles and hiss on radio programmes.

Noise adds extra random information to analogue signals. Each time the signal is amplified, the noise is also amplified. Gradually, the signal becomes less and less like the original signal. Eventually, it may be impossible to make out the music in a radio broadcast against the background noise, for example.

Noise also adds extra random information to digital signals. However, this noise is usually lower in amplitude than the amplitude of the ON states. As a result, the electronics in the amplifiers can ignore the noise, and it does not get passed along.

Check your understanding of this section by having a go at the activity:

Radio waves and microwaves

A radio receiver does not need to be directly in view of the transmitter to receive programme signals. The lowest frequency radio waves are reflected from an electrically charged layer of the upper atmosphere, called the ionosphere. This means they can reach receivers that are not in the line of sight, because of the curvature of the Earth’s surface.

microwaves pass through the atmosphere, radio waves relected through a charged layer of the upper atmosphere, signal received even though transmitter and receiver are not in the line of sight

Microwaves and radio waves in the atmosphere

Signals are diffracted as they leave a transmitter dish. This causes them to spread out. Diffraction can be reduced by using larger dishes.

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