Your voice is a slow, weak vibration in the air. A radio wave is a fast, powerful oscillation that can leap across a continent. Modulation is the marriage of the two — the trick that lets a feeble 1 kHz voice ride on a 120 MHz carrier and arrive intact at a control tower. Get it right and you understand why aviation insists on AM for VHF and SSB for HF.
SYLLABUS MAP
Part II (iii) Systems for air-ground communication — carrier, AM/FM/SSB, the Tx/Rx chain
Learning objectives — by the end of this chapter you will be able to…
Explain what a carrier wave is and the two reasons modulation is needed.
Describe AM in depth — sidebands, bandwidth, modulation depth and power distribution.
Describe FM — deviation, bandwidth (Carson's rule), noise immunity and the capture effect.
Compare DSB and SSB and calculate the power/bandwidth saving.
Explain why aviation uses AM on VHF and SSB on HF, and read emission designators.
Outline the transmitter chain and the superheterodyne receiver.
10.1 The carrier & why we modulate
10.2 AM — sidebands & depth
10.3 AM power distribution
10.4 FM
10.5 DSB & SSB
10.6 Why AM on VHF, SSB on HF
10.7 Emission designators
10.8 The Tx & Rx chains
Modulation is the process of loading information—like your voice or digital data—onto a high-frequency carrier wave.
10.1 The carrier & why we modulate
FIRST PRINCIPLES — TWO REASONS WE CANNOT SEND THE VOICE DIRECTLY
You cannot simply connect a microphone to an antenna. First, an efficient antenna must be a fair fraction of a wavelength; a 1 kHz audio tone has a wavelength of 300 km, demanding an absurd aerial. Second, every voice would occupy the same low band, so no two stations could share the air. The solution is to put the voice onto a high-frequency carrier: the carrier needs only a short antenna, and each station can use a different carrier frequency. Modulation is the act of loading the voice onto the carrier.
Definition — carrier & modulation
The carrier wave is a high-frequency wave generated by the transmitter; on its own it carries no information. Modulation varies one property of the carrier — its amplitude or its frequency — in step with the low-frequency audio, so the information rides on the carrier and can be recovered at the receiver.
10.2 AM — sidebands & modulation depth
IN PLAIN TERMS
In Amplitude Modulation, the height (amplitude) of the carrier is varied in step with the voice, while its frequency stays fixed. The outline (envelope) of the wave traces your voice.
Sidebands & bandwidth
Mixing a carrier (fc) with an audio tone (fa) produces two new frequencies either side of the carrier: an upper sideband (fc + fa) and a lower sideband (fc − fa). The information lives in the sidebands; the carrier itself carries none. The total AM bandwidth = 2 × highest audio frequency.
Modulation depth m = Vm / Vc
Vm = peak modulating (audio) voltage · Vc = carrier voltage · usually expressed as a %
Modulation depth (index)
The depth of modulation is how strongly the carrier is varied. Too little (low %) wastes power and gives a weak, quiet signal; over 100% (over-modulation) distorts the audio and "splatters" energy into adjacent channels, causing interference.
10.3 AM power distribution
WHY AM IS POWER-INEFFICIENT
Even at full 100% modulation, two-thirds of the transmitted power sits in the carrier, which carries no information; only one-third is shared between the two sidebands. That waste is the motivation for SSB.
At 100% modulation depth, ≈67% of the transmitter's power is wasted in the carrier, leaving only ≈16.7% for each information-carrying sideband.
Worked example — where the power goes
At 100% modulation, of the total power: ~67% is in the carrier, ~16.7% in each sideband. The information-bearing power is only the two sidebands ≈ 33%.
SSB throws away the carrier and one sideband, putting essentially all the transmitter's power into the single information-bearing sideband — a large efficiency gain, decisive over long HF distances.
10.4 FM — Frequency Modulation
IN PLAIN TERMS
In FM the carrier's frequency is varied in step with the voice while its amplitude stays constant. The amount of frequency swing is the deviation. Because most noise affects amplitude, FM is far more noise-resistant — which is why it is used for music broadcasting.
FM bandwidth — Carson's rule
FM bandwidth ≈ 2 × (deviation + highest audio frequency). FM generally needs more bandwidth than AM, which is part of why narrow aviation channels stayed with AM.
Exam trap — the capture effect
FM has a capture effect: the stronger of two signals captures the receiver and the weaker is silenced. In aviation that is dangerous — a second aircraft transmitting would be silently lost. This is the key reason aviation does NOT use FM for VHF voice.
Figure 10.1 — AM vs FM waveforms, the sidebands, and the concept behind Single Side Band (SSB).
10.5 DSB & SSB
Modulation Modes Compared
Mode
What is transmitted
Trade-off
DSB (full AM)
Carrier + both sidebands
Simple receiver; but wasteful — most power in the carrier
DSB-SC
Both sidebands, carrier suppressed
Saves the carrier power; needs carrier reinsertion
SSB
One sideband only; carrier & other sideband suppressed
Far more power- and bandwidth-efficient — ideal for HF long range
10.6 Why AM on VHF, SSB on HF
THE TWO DESIGN DECISIONS, SIDE BY SIDE
VHF voice = AM: if two aircraft transmit together on the same VHF frequency, AM produces an audible heterodyne squeal — everyone hears that two stations "stepped on" each other and can re-transmit. With FM's capture effect, one would simply be lost. AM's "you'll hear the clash" behaviour is a safety feature.
HF voice = SSB: over oceanic distances, efficiency and bandwidth matter most, so HF uses the power- and bandwidth-saving SSB.
Why SSB for HF
SSB puts all the transmitter's power into the one sideband that carries information and uses half the bandwidth of full AM. Over thousands of miles of HF that efficiency is decisive — so HF aviation voice uses SSB. The receiver must reinsert a local carrier to demodulate it.
Cockpit reality
That brief squeal you sometimes hear after a call is two aircraft transmitting together. Because VHF is AM, you know it happened and can say "say again, two together." That awareness is exactly what AM buys aviation — and what FM would hide.
10.7 Emission designators
The ICAO/ITU three-symbol code
An emission is classified by three symbols: type of modulation · nature of the signal · type of information. The two you must recognise: A3E = amplitude-modulated double-sideband telephony (aviation VHF voice) J3E = single-sideband suppressed-carrier telephony (aviation HF voice).
10.8 The transmitter & receiver chains
Transmitter — voice to airwaves
Microphone → audio amplifier → oscillator (generates the carrier) → modulator (mixes audio onto the carrier) → RF power amplifier → antenna.
Receiver — airwaves to voice (superheterodyne)
Antenna → RF amplifier → mixer + local oscillator (shifts the signal to a fixed intermediate frequency, IF) → IF amplifier (most gain/selectivity) → detector / demodulator (recovers the audio) → audio amplifier → speaker/headset. AGC/AVC holds the output level steady (Chapter 11).
Why "superheterodyne"
Converting every incoming signal to one fixed IF means the gain and selectivity stages are always tuned to the same frequency — far easier to make highly selective than tuning every stage to each station. It is the architecture of virtually every aviation receiver.
The essential block diagrams of a radio transmitter and a superheterodyne receiver.
☆ Numbers to memorise
Essential Facts for Chapter 10
Fact
Value
Carrier
High-frequency wave; carries no info until modulated
AM
Amplitude varied; bandwidth = 2 × highest audio; info in sidebands
Modulation depth
m = Vm/Vc; over 100% → distortion & splatter
AM power (100%)
≈ 67% carrier, ≈ 33% sidebands (the only useful part)
One sideband, carrier suppressed; efficient; HF voice
Aviation choice
VHF voice = AM (A3E) · HF voice = SSB (J3E)
Receiver
Superheterodyne — converts to a fixed IF
Question bank
Part A — MCQs (click an option to check)
1. In amplitude modulation, what is varied?
The amplitude of the carrier
The frequency of the carrier
The speed of light
The antenna length
Answer: The amplitude of the carrier. AM varies the carrier's amplitude in step with the audio; frequency stays fixed.
2. One reason a voice cannot be transmitted directly is that it would require:
Too little power
An impractically long antenna (its wavelength is enormous)
A satellite
Frequency modulation
Answer: An impractically long antenna (its wavelength is enormous). Audio wavelengths are hundreds of km; modulating a high-frequency carrier allows a short antenna and frequency sharing.
3. The information in an AM signal is carried in the:
Carrier
Sidebands
Antenna
Local oscillator
Answer: Sidebands. The carrier carries no information; the sidebands do.
4. If the highest audio frequency is 3 kHz, the AM bandwidth is:
3 kHz
6 kHz
1.5 kHz
9 kHz
Answer: 6 kHz. AM bandwidth = 2 × highest audio frequency = 2 × 3 = 6 kHz.
5. Modulating beyond 100% depth causes:
A clearer signal
More range
Distortion and interference to adjacent channels
Lower frequency
Answer: Distortion and interference to adjacent channels. Over-modulation distorts the audio and splatters into neighbouring channels.
6. At 100% AM modulation, the proportion of power in the carrier is about:
33%
67%
100%
0%
Answer: 67%. About two-thirds of the power is in the carrier, which carries no information.
7. In FM, what is held constant?
Frequency
Amplitude
Wavelength of the audio
Bandwidth
Answer: Amplitude. FM varies frequency while amplitude stays constant — hence its noise immunity.
8. FM is generally NOT used for VHF aviation voice because of its:
Low cost
Capture effect (weaker signal silenced)
Wide range
Use of sidebands
Answer: Capture effect (weaker signal silenced). FM's capture effect would silently suppress a second transmitting aircraft — unsafe.
9. Aviation VHF voice uses AM mainly because:
It is cheaper
Two simultaneous transmissions produce an audible heterodyne, alerting users
It has the longest range
It needs no antenna
Answer: Two simultaneous transmissions produce an audible heterodyne, alerting users. AM reveals when two stations transmit together (a squeal); FM's capture effect would silently lose one.
10. SSB is preferred for HF because it is:
Simpler to receive
More power- and bandwidth-efficient
Immune to all fading
Higher frequency
Answer: More power- and bandwidth-efficient. SSB suppresses the carrier and one sideband, concentrating power and halving bandwidth.
11. SSB uses how much bandwidth compared with full AM?
Twice
Half
The same
Four times
Answer: Half. Transmitting one sideband instead of two halves the bandwidth.
12. The emission designator for aviation VHF voice (AM telephony) is:
13. The aviation receiver architecture that converts every signal to a fixed intermediate frequency is the:
Direct-conversion receiver
Superheterodyne receiver
Crystal set
Regenerative receiver
Answer: Superheterodyne receiver. The superheterodyne mixes the signal down to a fixed IF for easy selectivity and gain.
14. In the transmitter chain, the stage that generates the carrier is the:
Modulator
Oscillator
Power amplifier
Antenna
Answer: Oscillator. The oscillator generates the carrier; the modulator then mixes the audio onto it.
15. Modulation depth m is defined as:
Carrier voltage ÷ audio voltage
Audio (modulating) voltage ÷ carrier voltage
Frequency ÷ wavelength
Power ÷ bandwidth
Answer: Audio (modulating) voltage ÷ carrier voltage. m = Vm/Vc, usually expressed as a percentage.
16. Compared with AM, FM generally requires:
Less bandwidth
More bandwidth
No bandwidth
The same bandwidth
Answer: More bandwidth. By Carson's rule, FM bandwidth ≈ 2(deviation + fmax), usually more than AM.
Part B — Oral / viva (tap to reveal model answers)
What is modulation, and why is it needed?
Model Answer:
Modulation varies a property of a high-frequency carrier — its amplitude or frequency — in step with the low-frequency audio. It is needed because audio wavelengths are too long for a practical antenna and every voice would occupy the same band; a high-frequency carrier allows a short antenna and lets stations share the spectrum.
Describe AM, its sidebands and bandwidth.
Model Answer:
In AM the carrier's amplitude follows the audio. Mixing produces upper and lower sidebands either side of the carrier; the information is in the sidebands. The bandwidth is twice the highest audio frequency.
Why does aviation use AM for VHF voice?
Model Answer:
With AM, two aircraft transmitting simultaneously produce an audible heterodyne squeal, so everyone knows a clash occurred and can re-transmit. FM's capture effect would silently suppress the weaker signal — a safety hazard.
Why is SSB used for HF?
Model Answer:
SSB suppresses the carrier and one sideband, putting all the power into the single information-bearing sideband and using half the bandwidth — far more efficient over long HF distances.
What is over-modulation and its effect?
Model Answer:
Modulating beyond 100% depth, which distorts the transmitted audio and causes splatter — interference into adjacent channels.
Outline the superheterodyne receiver.
Model Answer:
The signal is amplified, then mixed with a local oscillator to a fixed intermediate frequency, amplified at the IF (where most gain and selectivity occur), demodulated to recover the audio, then amplified to the speaker, with AGC holding the level steady.
Part C — Numerical problems (tap for worked solutions)
P1. A carrier of 120.000 MHz is amplitude-modulated by a 2.5 kHz tone. Give the sideband frequencies and bandwidth.
