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# Waves in Communication

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Sometimes people say hello to each other by waving. However, we can communicate with each other using many different types of waves, not just the wave of our hands. The first example that probably comes to your mind is sound waves. We can produce sound when air passes through our vocal cords, causing them to vibrate. These vibrations propagate through the air and eventually into our eardrums. There is a limit to how far a sound wave can travel as the wave will dampen, meaning the wave will sound quieter with distance from the speaker.

Until a couple of centuries ago, long-distance communications were slow, unreliable, and inefficient. Nearly all messages had to be carried physically. That's why we developed new technologies to help communicate using waves in the electromagnetic spectrum to overcome this problem. Some of these new communication technologies are even useful in short ranges. This article will help you to understand the applications of waves in communication and the physics involved!

## Electromagnetic Waves in Communications

Below is a diagram of the electromagnetic spectrum; it shows the entire distribution of electromagnetic radiation according to increasing frequency or increasing wavelength. Waves with higher frequencies, such as gamma rays and x-rays have the smallest wavelength and the highest energies of electromagnetic waves. While microwaves and radio waves possess the lowest frequencies and energies, but the largest wavelengths.

The different sections of the electromagnetic spectrum are ordered by frequency and wavelength, Wikimedia Commons CC BY 3.0

Electromagnetic waves have many uses within the realms of communication. Different parts of the electromagnetic (EM) spectrum are suitable for different applications. The different energies, frequencies, and wavelengths associated with each separate part of the EM spectrum give advantages and disadvantages depending on their use. We only utilise the low-frequency parts of the electromagnetic spectrum in communications, EM radiation with higher frequencies than visible light becomes impractical for various reasons.

Scientists are researching possible applications of ultraviolet light in communications. UV radiation is absorbed in our atmosphere, so is nearly useless over long ranges. However, with a powerful enough source, UV radiation can be transmitted over distances of up to several miles at ground level. There would be two main advantages of using ultraviolet light when communicating compared to other parts of the EM spectrum.

1. Ultraviolet light has a high frequency, which means it can transfer more information per second than lower frequency waves such as visible light or infrared.
2. Some particles in the atmosphere can scatter or reflect ultraviolet light as well as absorb it, meaning a signal can scatter out over a wide area. This phenomenon can be used to communicate between two points with no line of sight between them, as the UV light can scatter around any obstacles in the way.

Radio waves have the longest wavelengths and lowest frequencies in the electromagnetic spectrum. Most radio wave frequencies are essentially transparent to the atmosphere, so are very useful when communicating long distances. They also pass straight through the human body, so aren't harmful at all. The radio spectrum itself is very broad. It ranges from wavelengths between 1 mm and 10,000 km, which is larger than the radius of the Earth! So radio waves of different wavelengths and frequencies will exhibit very different behaviours. Television and radio broadcasts are typical examples of how these waves are applied in communications.

We can transmit and receive radio waves using oscillations in electrical circuits. When a conductor absorbs a radio wave it generates an alternating current. The frequency of the alternating current matches the frequency of the radio wave. This is how we encode information electronically before radio transmission and decode it after the wave is received.

1. Short-wavelength radio waves have frequencies greater than approximately. These radio waves are used when transmitting between two antennas within line of sight of each other. This means the propagation of the wave is limited to the visual horizon, which is abouton the surface of the Earth, but the waves are large enough to pass through most buildings or foliage unobstructed. Typically used when we require high frequencies to transmit a lot of information at once, such as with radio, television, mobile phones or radar.
2. At frequencies belowradio waves begin to bend over the horizon. This allows radios to broadcast over hundreds of miles, that follows the curvature of the Earth. The lower the frequency the farther the range of the signal. Unfortunately, lower frequencies mean less information can be transmitted per second.
3. Short-wavelength radio waves betweenandcan also be scattered off the ionosphere (charged particles high in the atmosphere) and reflected back towards the Earth's surface. This is called skywave propagation and allows radio communications across continents! Unfortunately, this communication method is rarely used today as atmospheric conditions are unreliable.

Skywave propagation using radio waves, Pixabay

## Visible Light Waves in Communications

Of course, we already use visible light to communicate naturally with each other, so we will discuss visible light purely as communications technology. In 1792 a Frenchman named Claude Chappe invented the semaphore telegraph system. The semaphore was a primitive way of transmitting information long-distance using visual signals. Tall towers were built within line of sight of each other, at distances betweenandmiles depending on terrain. There would be a human operator at each tower with a spyglass, watching their neighbouring towers for signals. After receiving a message the operator would then relay the message to the next tower along. This was the precursor to the electrical telegraph, which would replace the semaphore less than a century later.

In the modern era, we use fibre optic cables to transmit signals as visible light. Fibre optic cables can be used to deliver high-speed internet. Fibre optic cables are made of flexible glass fibre, which allows electromagnetic waves to be internally reflected and therefore travel through the cable at the speed of light. These cables are safely enclosed by a plastic sheathing to protect the delicate glass fibres inside. Optical fibre cables can be used to transmit signals very long distances, as there is only a small loss of the signal due to scattering or absorption of the wave when it is reflected at the boundaries of the cable. The better quality glass allows for smaller losses as the wave travels through the cable.

A visible light ray travelling through a fibre optic cable via internal reflection, adapted from image by Chris Woodford CC BY 3.0

## Infrared Waves in Communications

Infrared light encompasses all electromagnetic radiation between() and. Infrared transmissions are mostly used in short-range communications. Your TV remote control is a common example of this. The main problem with infrared communications is that the waves will not pass through solid objects and are partially absorbed by the atmosphere, giving it a short-range. In some contexts (like the comfort of your own home) this can be turned into an advantage! An infrared TV remote in the living room will not interfere with similar devices in other rooms in the house because the signals cannot travel through walls. Furthermore, infrared transmitters are relatively cheap to make and have low power requirements, which keeps costs low for consumers.

Infrared wavelengths are also used in fibre optic cables when the user wants to minimise the loss in the signal. Visible light in fibre optic cables can be more easily scattered or absorbed by impurities within the cable than infrared light, leading to more loss as the signal travels through the cable. However, due to the lower frequencies of infrared light, there is a smaller data transfer rate compared to the visible spectrum.

## Satellite communications

One way of communicating on a global scale is to utilise satellites, often employed by satellite television companies and mobile phone networks. For instance, a mobile phone tower will transmit and receive data from many nearby mobile phones on the ground via radio waves. The mobile phone tower will then communicate with a satellite in geostationary orbit using microwaves. A geostationary satellite will remain stationary above a specific point of the Earth's surface at all times, so that communication with ground-based equipment can occur uninterrupted, 24/7. The microwave spectrum is used as it's not blocked by the Earth's atmosphere and can transmit more data per second than radio waves, because of their higher frequencies.

The satellite can then communicate with other mobile phone towers within a large area on the Earth's surface. Commercial satellites very rarely directly communicate with each other, signals must be relayed to receivers on the ground.

Using a satellite to communicate over long distances, Wikimedia Commons CC BY 3.0

A geostationary orbit can be achieved above the Earth by sending a satellite into orbit above the equator at aroundabove the Earth's surface. At this height, a satellite will take exactly one day (23 hours and 56 minutes) to complete one orbit. If the satellite's orbit is in the same direction as the Earth's rotation then it will remain stationary in the sky as seen from Earth.

## Waves in Communication - Key takeaways

• Many different wavelengths of light in the electromagnetic spectrum are used in communications.
• The higher the frequency of a wave, the more data/information you can transmit per second.
• Electromagnetic radiation with frequencies higher than visible light becomes increasingly impractical.
• Radio waves can be transmitted hundreds of miles at the lowest frequencies.
• Conductors can absorb and transmit radio waves, allowing information to be encoded and decoded electronically.
• Fibre optic cables transmit data using either visible or infrared light. They are useful because they can transmit a lot of data per second and have only a small loss of signal.
• Infrared spectrum is mostly useful in short-range communications, such as remote controls. Infrared radiation is partially absorbed by the atmosphere and completely blocked by obstacles.
• Satellites allow communications on a global scale. They use microwave radiation, due to the relatively high rate of data transfer compared to radio waves. Also, microwaves are transparent to the Earth's atmosphere.
• Most communications satellites are geostationary so that transmissions between satellites and ground-based equipment is uninterrupted.

Radio waves, microwaves, infrared waves, visible light waves and (experimentally) ultraviolet waves. Waves with relatively low frequencies and high wavelengths.

Radio waves, microwaves, infrared waves, visible light waves, ultraviolet waves, x rays, gamma rays

A way to transmit and receive data

Electromagnetic waves are essentially created by accelerating charged particles.

Waves allow for communications and act as pathways between different energy stores.

## Final Waves in Communication Quiz

Question

Would your next-door neighbour with an infrared receiver be able to eavesdrop on any infrared communications within your own home?

No

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Question

Sound waves will dampen as they propagate from their source. What does this mean?

The sound will get quieter with distance

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Question

Which part of the electromagnetic spectrum has the largest wavelengths and lowest frequency?

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Question

Do we primarily use high-frequency or low-frequency waves in communications?

Low-frequency

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Question

Which part of the electromagnetic spectrum below is NOT absorbed by the Earth's atmosphere?

Microwaves

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Question

Name one reason why ultraviolet communications could be useful.

1. The high frequency of ultraviolet light would allow for a very high data transfer rate.
2. Ultraviolet light is scattered by the Earth's atmosphere, so a UV signal could spread out as it travels and scatter around obstacles blocking line of sight.

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Question

When a radio wave is absorbed by a conductor, what sort of current does it generate?

Alternating current

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Question

Below a certain frequency, radio waves begin to bend over the horizon, allowing for a greater range of transmission. What frequency is this?

3MHz

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Question

Why isn't skywave propagation used regularly today?

Atmospheric conditions are unreliable.

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Question

What sort of wave in the electromagnetic spectrum is able to pass through obstacles such as buildings and foliage?

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Question

What 2 parts of the electromagnetic spectrum are transmitted through fibre optic cables?

Visible & Infrared light

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Question

Name one reason why fibre optic cables are useful.

1. They can transfer signals a very long distances because signal loss is low.
2. The relatively high frequencies of visible light allow for high rates of data transfer per second.

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Question

What part of the electromagnetic spectrum is typically used in satellite communications?

Microwave

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Question

What does geostationary orbit mean?

The satellite will be stationary above the same point of the Earth at all times.

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Question

Why do fibre optic cables have sheathing?

To protect the delicate glass fibre inside

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