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Equipment and SystemsLesson 2 of 2

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EQPS 002

Radio theory

Radio waves

How a signal gets from a transmitter to a receiver: as radio waves, from one antenna to another.

TransmitterReceiverAntennaAntennaRadio waves →E: electricM: magneticOscillating electrons
Electrons oscillating in the first antenna send out the radio waves. At the second antenna, the waves set the electrons oscillating again.
From the transmitterThe transmitter is wired to an antenna. The electrons in the antenna oscillate, up and down, and the antenna sends out radio waves.
The radio waveAn electric part (E) and a magnetic part (M), at right angles to each other and peaking together.
To the receiverThe radio waves set the electrons in the second antenna oscillating, and the receiver wired to it picks up the signal.

The wave's properties

One wave, drawn twice: against distance, and against time.

AGAINST DISTANCEDistanceAmplitude (A)Wavelength (λ)CrestTroughAGAINST TIMETime (t)1 cycle: 1 second at 1 Hz0 s1 s2 s
Against distance, the wave shows its wavelength. Against time, it shows how long one cycle takes: at 1 Hz, 1 second.
Crest and troughThe top and the bottom of the wave.
Wavelength (λ)The distance from one crest to the next.
Amplitude (A)The height of the wave, from the centre line to a crest. A bigger amplitude means more power.
Frequency (f)How many cycles per second, in hertz (Hz). 1 Hz = 1 cycle per second: one cycle takes 1 second.
Frequency and wavelength
Lower frequencyLonger wavelengthHigher frequencyShorter wavelength
Over the same distance, the higher frequency fits in more cycles.

Higher frequency, shorter wavelength.

Creating and receiving the signal

From a microphone to headphones: the transmitter puts the voice on a carrier wave, and the receiver takes it back off.

CREATING THE SIGNALMicrophonePre-amplifierAMmodulatorRFoscillatorCarrier waveModulatedcarrier wavePoweramplifierAmplifiedcarrier waveAntennaRadio wavesRECEIVING THE SIGNALAntennaTunedreceiverRFoscillatorIntermediatemodulatedcarrier waveDemodulatorAudioamplifierAudio signalHeadphones
Orange is the voice, the information signal. Teal is the carrier wave, alone or carrying the voice.
Creating the signal
  • MicrophoneTurns the sound waves of the voice into an audio signal.
  • Pre-amplifierMakes the weak audio signal stronger.
  • RF oscillatorMakes the carrier wave, at radio frequency (RF).
  • AM modulatorChanges the carrier so it carries the voice (Modulation, below).
  • Power amplifierAmplifies the modulated carrier wave for the antenna.
  • AntennaSends the amplified carrier wave out as radio waves.
Receiving the signal
  • AntennaPicks up the radio waves.
  • Tuned receiverTuned to the transmitter's frequency: it picks out the chosen channel.
  • RF oscillatorThe receiver's own oscillator moves the chosen channel to one fixed in-between frequency, which the rest of the receiver is built to filter and amplify: the intermediate modulated carrier wave.
  • DemodulatorTakes the voice back off the carrier.
  • Audio amplifierMakes the audio signal stronger, for the headphones.
  • HeadphonesTurn the audio signal back into sound.

Modulation

The modulator changes the carrier wave so it carries the information signal, the voice. AM changes the carrier's height; FM changes its frequency.

AM: amplitude modulation
Information signalCarrier waveAM signalTimeAM modulatorOscillator
The AM signal's outline, dashed, follows the information signal.
FM: frequency modulation
Information signalCarrier waveFM signalTimeVCOVoltage-controlledoscillator
The FM signal keeps one height. Its crests bunch together where the information signal is high.
AM, amplitude modulationThe carrier's height, its amplitude, follows the information signal.
FM, frequency modulationThe carrier's frequency follows the information signal. Its height stays the same.
VCOA voltage-controlled oscillator; the course slide calls it a voice-controlled oscillator. The voice, as a changing voltage, drives its frequency, and out comes the FM signal.

Analogue and digital

Two kinds of signal waveform.

Analogue signalTimeDigital signalTimeHighLow011010011
The analogue signal runs on without a break. The digital signal steps between two voltages.
Signal waveforms
AnalogueContinuousCan be modified by changing its amplitude and frequency, as AM and FM do.
vs
DigitalHigh and low voltagesA chain of high and low voltage signals, which stand for 1s and 0s.

Reading material

Equipment and Systems

EQPS 002 Radio Theory PowerPoint / PDF
EPN Group Training Document Equipment and Systems, chapter 2 Reading list, course material
International Civil Aviation Organization, Annex 10, Aeronautical Telecommunications, Volume III, Communication Systems, 2nd edition, 2007 Reference, paid
U.S. Congress, Office of Technology Assessment, Wireless Technologies and the National Information Infrastructure, OTA-ITC-622, July 1995 Reference, free
Electronic Communications Committee (ECC), The European Table of Frequency Allocations and Applications in the Frequency Range 8.3 kHz to 3000 GHz (ECA Table), approved November 2020 Reference, free