Telecommunication Channels

Lessons

Senior 3 · Physics

Telecommunication Channels

4 lessons

Lesson 4 of 4

Wireless Channels: Radio Waves, AM and FM, Satellites and Mobile Networks

30 min

Signals with no wires at all

A small battery radio picks up music in a field far from any cable. A phone shows bars of signal from a mast on a distant hilltop, and a dish on a roof points at a fixed spot in the sky to receive television from space. All of these are wireless channels: the signal travels as electromagnetic waves. This lesson places radio waves in the electromagnetic spectrum, shows two ways of putting a message onto a wave, and follows the waves to satellites and mobile networks.

Definition

Electromagnetic spectrum: the family of electromagnetic waves, arranged in order of wavelength or frequency, from radio waves to gamma rays. All travel at $c = 3.00 \times 10^{8}\,\mathrm{m\,s^{-1}}$ in a vacuum.

Wireless (unguided) channel: a channel in which the signal travels as electromagnetic waves through air or space, with no cable to guide it.

The electromagnetic spectrum

For every electromagnetic wave, speed, frequency $f$ and wavelength $\lambda$ are linked by the wave equation:

c = f\lambda

Communication mainly uses radio waves and microwaves (and infrared in fibres and remote controls). The radio part of the spectrum is divided into bands:

Band	Frequency	Wavelength	Typical uses
LF (low frequency)	30–300 kHz	10 km to 1 km	Long-wave radio, navigation beacons
MF (medium frequency)	300 kHz to 3 MHz	1 km to 100 m	AM (medium-wave) radio
HF (high frequency)	3–30 MHz	100 m to 10 m	Short-wave radio over very long distances
VHF (very high frequency)	30–300 MHz	10 m to 1 m	FM radio (87.5–108 MHz), some television
UHF (ultra high frequency)	300 MHz to 3 GHz	1 m to 10 cm	Television, mobile phones, Wi-Fi, GPS
SHF (super high frequency)	3–30 GHz	10 cm to 1 cm	Satellite television, radar, microwave links

Worked example 1: wavelengths of radio stations

Find the wavelength of (a) an FM station on 100.0 MHz, (b) an AM station on 1.0 MHz, (c) a phone signal at 900 MHz.

\text{(a)}\ \lambda = \frac{c}{f} = \frac{3.00 \times 10^{8}}{1.000 \times 10^{8}} = 3.00\,\mathrm{m} \qquad \text{(b)}\ \lambda = \frac{3.00 \times 10^{8}}{1.0 \times 10^{6}} = 300\,\mathrm{m} \qquad \text{(c)}\ \lambda = \frac{3.00 \times 10^{8}}{9.00 \times 10^{8}} = 0.333\,\mathrm{m}

An efficient aerial is a sizeable fraction of a wavelength long, which is why a phone's aerial fits inside its case while AM transmitters need tall masts.

Putting the message on the wave: AM and FM

Sound itself has frequencies of only about 20 Hz to 20 kHz, far too low to send as radio waves with a practical aerial, and every station would interfere with every other. So the transmitter makes a high-frequency carrier wave (a different frequency for each station) and varies one of its properties to follow the audio signal. This is modulation. The receiver is tuned to one carrier frequency and demodulates it to recover the audio.

Amplitude modulation (AM): the amplitude (height) of the carrier follows the audio signal, while its frequency stays fixed.
Frequency modulation (FM): the frequency of the carrier rises and falls with the audio signal, while its amplitude stays fixed.

	AM	FM
What varies	Amplitude of the carrier	Frequency of the carrier
Usual bands	LF, MF and HF	VHF
Range	Long: ground wave and sky wave	Shorter: mainly line of sight
Bandwidth per station	Narrow (about 10 kHz)	Wide (about 200 kHz)
Sound quality and noise	Lower quality; crackle from electrical noise, which changes amplitude	Higher quality; little noise, because the receiver ignores amplitude changes

Definition

Carrier wave: a high-frequency wave that is modulated to carry a signal.

Modulation: varying the amplitude or the frequency of a carrier wave in step with the signal to be sent. AM varies the amplitude; FM varies the frequency.

How radio waves travel

Ground waves (LF and MF) follow the curve of the Earth and bend round hills, so AM stations can be heard hundreds of kilometres away and behind hills.
Sky waves (mainly HF) are reflected back to Earth by the ionosphere, a layer of charged particles high in the atmosphere, so short-wave radio can cross continents.
Space waves (VHF and above) pass through the ionosphere and travel in straight lines. Transmitter and receiver must be in line of sight, so FM, TV and mobile phone aerials are put on high masts and hilltops, and a deep valley can be in a "shadow".

Satellites

Microwaves pass straight through the ionosphere, so they can reach satellites. A ground station sends an uplink to the satellite, which amplifies it, changes its frequency and sends a downlink back to a wide area. A geostationary satellite orbits above the equator at a height of about 35 800 km, once every 24 hours, in the same direction as the Earth turns. It therefore stays above the same point, and dishes on the ground can point at it without moving. Satellites in much lower orbits (a few hundred to about 2000 km up) move across the sky, so many of them are needed to give constant coverage; they are used for some satellite internet services and for observing the Earth. The navigation satellites of GPS orbit at about 20 200 km.

Worked example 2: the satellite delay

A signal goes up to a geostationary satellite 35 800 km above the transmitter and back down to a receiver directly below it. Find the time taken.

t = \frac{2 \times 3.58 \times 10^{7}}{3.00 \times 10^{8}} = 0.239\,\mathrm{s}

In a telephone call through such a satellite, a reply takes at least twice as long, about half a second, which is why speakers sometimes talk over each other.

Mobile phone networks

A mobile network divides the land into small areas called cells, each served by a base station: a mast with aerials. Your phone exchanges UHF radio signals with the nearest base station, and the base stations are linked by optical fibres or microwave links to switching centres that connect the call or the data. As you travel, the network hands your call from one cell to the next (handover). Using many small, low-power cells means the same frequencies can be used again in cells that are not neighbours, so many more people can be served at once. Each new generation (2G for digital calls and SMS, 3G for mobile internet, 4G and 5G for fast broadband) has carried more data per second.

Important

Radio waves and health. The World Health Organization states that no obvious adverse effect of exposure to low-level radiofrequency fields, such as those from phones and masts, has been found, and research continues. In 2011 the International Agency for Research on Cancer (IARC) classified radiofrequency fields as "possibly carcinogenic" (Group 2B), a category used when the evidence is limited; a large study reported by IARC in 2024 found that the heaviest users of mobile phones had no higher risk of brain tumours than light users. The danger that is proven is distraction: never use a phone while crossing a road, riding a bicycle or moto, or driving.

Warning

Common mistakes

Saying that AM radio waves travel faster than FM, or that microwaves are faster than radio waves. All electromagnetic waves travel at the same speed in a vacuum.
Confusing the carrier with the signal. The carrier is the high-frequency wave; the signal is the audio that modulates it.
Saying a geostationary satellite does not move. It moves very fast, but it turns with the Earth, so it stays above the same point.

Tip

AM changes the height, FM changes the spacing. On the waveforms, an AM wave looks like a sausage chain; an FM wave looks like a spring being squeezed and stretched.

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Practice

List the regions of the electromagnetic spectrum in order of increasing frequency.
Calculate the wavelength of an FM station broadcasting on 89.7 MHz.
A long-wave transmitter has a wavelength of 1500 m. Find its frequency and name its band.
Give two differences between AM and FM broadcasting.
Explain why a satellite-television dish never needs to be turned once it is set up.
A signal travels up to a satellite 36 000 km away and back down the same distance. How long does it take?
Give two reasons why a mobile network uses many small cells rather than one very powerful mast.
A village lies deep in a valley between high hills. Its people can hear an AM station clearly, but not an FM station from a similar distance. Explain why.

Show answersHide answers

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
$\lambda = \frac{3.00 \times 10^{8}}{8.97 \times 10^{7}} = 3.34\,\mathrm{m}$ .
$f = \frac{3.00 \times 10^{8}}{1500} = 2.0 \times 10^{5}\,\mathrm{Hz} = 200\,\mathrm{kHz}$ : the LF band.
Any two: AM varies the amplitude, FM the frequency; AM uses LF, MF and HF, FM uses VHF; AM has a longer range; FM has better sound and less noise; FM needs a wider bandwidth.
It points at a geostationary satellite, which orbits once in 24 hours in the same direction as the Earth turns, so it stays above the same point on the Earth.
$t = \frac{2 \times 3.6 \times 10^{7}}{3.00 \times 10^{8}} = 0.24\,\mathrm{s}$ .
Frequencies can be reused in cells that are not neighbours, so many more users can be served; phones and masts can use low power; coverage can follow where people are.
AM uses medium waves that travel as ground waves, which bend round hills (and sky waves at night). FM uses VHF space waves, which travel in straight lines, so the hills block them and the valley is in a shadow.

Key points

All electromagnetic waves travel at $3.00 \times 10^{8}\,\mathrm{m\,s^{-1}}$ ; $c = f\lambda$ . Radio waves and microwaves carry most wireless communication.
AM varies the carrier's amplitude (long range, more noise); FM varies its frequency (better quality, line of sight).
Ground waves follow the Earth, sky waves reflect from the ionosphere, space waves need line of sight.
Geostationary satellites stay over one point, about 35 800 km up; mobile networks use cells, base stations and handover.

1Signals with no wires at all

2The electromagnetic spectrum

3Putting the message on the wave: AM and FM

4How radio waves travel

5Satellites