๐ Table of Contents
1. Introduction
What this section covers: The definition of modulation, its purpose, and a summary of the five forms of modulation used in aviation: Keyed, AM, FM, Phase, and Pulse.
Modulation is the process of adding information to a radio wave, or the formatting of radio waves for other purposes. It is the critical step that makes radio useful for communications and navigation.
Before audio information can be transmitted, sound must first be converted to an electrical signal by a microphone (a device that converts sound waves to electrical current). This electrical signal is the audio frequency (AF); the radio wave it is combined with is the carrier wave at radio frequency (RF).
๐ต Five Forms of Modulation Used in Aviation
- Keyed Modulation โ interrupting the carrier (Morse code; NDB ident)
- Amplitude Modulation (AM) โ AF amplitude varies RF amplitude
- Frequency Modulation (FM) โ AF amplitude varies RF frequency
- Phase Modulation โ AF varies phase of carrier (used in GPS, MLS)
- Pulse Modulation โ intermittent carrier bursts (radar, DME)
Modulation is not only used for voice communications. Navigation systems such as VOR, ILS, DME, and GPS use modulation for bearing determination, distance measurement, and data transmission.
2. Keyed Modulation
What this section covers: The simplest form of modulation โ interrupting the carrier wave to create Morse code symbols.
The simplest way to convey information on a carrier wave is to interrupt it โ creating short and long bursts of RF energy. This is keyed modulation. By varying the burst lengths to match Morse code dot/dash patterns, information can be transmitted.
๐ต Morse Code Basics
- Dot (ยท) โ short burst
- Dash (โ) โ long burst (3ร dot duration)
- Morse K = โ ยท โ (dash, dot, dash)
This form of transmission is called telegraphy. Until the development of AM, it was the only means of passing information by radio.
๐ก Aviation Relevance โ NDB Identification
Keyed modulation is still used today by Non-Directional Beacons (NDBs) for station identification. The NDB transmits its two or three-letter ICAO identifier in Morse code superimposed on the carrier. Pilots tune the ADF receiver, and the ident is heard in the headset, confirming the correct NDB is being received. This is covered in Chapter 7.
3. Amplitude Modulation (AM)
What this section covers: How the AF amplitude modifies RF amplitude, the heterodyning process, how sidebands are produced, bandwidth calculation, and power distribution in an AM signal.
In Amplitude Modulation, the amplitude of the audio frequency (AF) modifies (varies) the amplitude of the radio frequency (RF) carrier wave.
AM โ What Varies
ใฐ
AF amplitude
โ RF amplitude
โ RF amplitude
FM โ What Varies
ใใ
AF amplitude
โ RF frequency
โ RF frequency
PM โ What Varies
ฯ
AF amplitude
โ RF phase
โ RF phase
3.1 Heterodyning & Sidebands
The process of combining RF and AF signals is called heterodyning. The result:
- The RF carrier frequency remains unchanged
- New frequencies are created at the sum (RF + AF) and difference (RF โ AF) of the two frequencies
- These new frequencies form an Upper Sideband (USB) and a Lower Sideband (LSB)
๐ Worked Example โ AM Sideband Calculation
RF = 2182 kHz | AF = 3 kHz
USB extends from: 2182.001 kHz โ 2185 kHz (RF + AF)
LSB extends from: 2181.999 kHz โ 2179 kHz (RF โ AF)
Total spread: 2179 kHz โ 2185 kHz
Bandwidth = 2185 โ 2179 = 6 kHz = 2 ร AF
AM Bandwidth Formula
Bandwidth = 2 ร AF (audio frequency)
USB frequency = RF + AF
LSB frequency = RF โ AF
๐ Exam-style AM Bandwidth Calculation
Q1: RF = 4716 kHz, AF = 6 kHz โ Bandwidth = 2 ร 6 = 12 kHz
USB = 4716 + 6 = 4722 kHz; LSB = 4716 โ 6 = 4710 kHz
Spread: 4710 kHz โ 4722 kHz = 12 kHz โ
3.2 AM Power Distribution
In a standard double-sideband AM transmission:
| Component | Power | % of Total | Contains Info? |
|---|---|---|---|
| RF Carrier | 100 W | 67% | No |
| Upper Sideband (USB) | 25 W | 17% | Yes (full copy) |
| Lower Sideband (LSB) | 25 W | 17% | Yes (full copy) |
| Total | 150 W | 100% | โ |
โ ๏ธ Exam-Critical Efficiency Facts
- Only 1/3 of total power carries information (the two sidebands combined)
- The two sidebands contain identical copies of the audio information
- The carrier has no information content โ it has done its job of shifting AF into RF frequencies
- Each sideband = 1/4 of carrier power (25W vs 100W carrier)
โ Capt. Pankaj Pahil | www.ghostaviator.com โ
4. Single Sideband (SSB)
What this section covers: Why SSB is more efficient, how it works, and its three key advantages in HF/MF aviation communications.
Since both sidebands contain identical information, and the carrier contains no information, there is significant redundancy in standard double-sideband (DSB) AM. SSB solves this by:
- Removing one sideband (either USB or LSB)
- Suppressing the carrier
- Transmitting only the remaining sideband โ which contains all the information
โ
Three Advantages of SSB over DSB (Must Know All Three)
- Double the channels: Each SSB transmission uses only one sideband bandwidth โ twice as many channels available in the same frequency band
- Better signal-to-noise ratio: The ionosphere introduces static interference at MF/HF; narrower bandwidth of SSB significantly reduces interference from this static
- Less power required: No power wasted on carrier or redundant sideband โ lighter, more efficient equipment
Additionally, when using sky wave propagation (HF communications), different frequencies within the AM bandwidth refract by different amounts, causing distortion. The narrower bandwidth of SSB significantly reduces this distortion.
๐ก Why SSB for HF?
HF frequencies (2โ30 MHz) are in very high demand globally. SSB doubles the channel count, reduces power consumption, reduces static interference from the ionosphere, and reduces multipath distortion โ all in one step. This is why all HF radio telephony in aviation uses SSB (emission designator J3E).
5. Frequency Modulation (FM)
What this section covers: How FM differs from AM, the relationship between AF amplitude/frequency and RF frequency deviation/rate of change, FM bandwidth figures, and why NBFM is not used in aviation.
In Frequency Modulation, the amplitude of the audio frequency modifies the frequency of the carrier wave (the amplitude of the carrier remains constant).
๐ต Two FM Relationships โ Both Examinable
- Frequency deviation of carrier โ amplitude of AF (greater audio amplitude โ greater frequency swing)
- Rate of frequency change = frequency of the AF modulating wave (higher audio pitch โ faster frequency switching)
FM vs AM Bandwidth Comparison
| System | Bandwidth | Notes |
|---|---|---|
| AM (DSB) | 9 kHz | International standard allocation for AM broadcasting |
| FM Broadcast | 150 kHz | By international agreement for music/broadcast |
| NBFM (voice) | 8 kHz | Narrow Band FM โ reduced bandwidth for voice only |
| Aeronautical Comms limit | 6 kHz | ICAO standard for aeronautical voice channels |
| HF Comms limit | 3 kHz | SSB occupies only one sideband |
โ ๏ธ Why NBFM is NOT Used in Aviation
Even NBFM (Narrow Band FM) with a reduced bandwidth of 8 kHz is still greater than the 6 kHz permitted for aeronautical voice communications. Therefore NBFM communication systems are not used in aviation. Standard aviation VHF/UHF uses AM (A3E for VHF, J3E for HF).
๐ก FM vs AM Summary
- FM has better noise immunity than AM (noise appears as amplitude variations, which FM ignores)
- FM requires much wider bandwidth โ impractical below VHF for aviation use
- FM is used in aviation navigation systems (e.g. VOR uses FM on the variable phase signal) but not for voice comms
6. Phase Modulation
What this section covers: Analogue phase modulation, digital phase shift keying (PSK), and the specific systems that use phase modulation in aviation.
In Phase Modulation, the phase of the carrier wave is modified by the input signal. There are two cases:
Analogue Phase Modulation
The phase of the carrier is modified in proportion to the amplitude of the analogue input signal โ similar to AM but it is the phase angle that changes rather than the amplitude or frequency.
Digital Phase Shift Keying (PSK)
With a digital (binary) input signal, the phase change reflects a binary 0 or 1:
๐ต Binary PSK (simplest case)
- 0ยฐ phase shift โ represents binary 0
- 180ยฐ phase shift โ represents binary 1
- More complex systems use multiple phase shift values to represent more data per symbol (e.g. QPSK = 4 phase states, 8PSK = 8 phase states)
โ
Aviation Systems Using Phase Modulation
- GPS โ uses Binary Phase Shift Keying (BPSK) to transmit navigation data and ranging codes
- MLS (Microwave Landing System) โ uses Differential Phase Shift Keying (DPSK)
โ www.ghostaviator.com | Capt. Pankaj Pahil โ
7. Pulse Modulation
What this section covers: How pulse modulation works and where it is used in aviation.
Pulse modulation is formed by generating and transmitting a sequence of short-period pulses โ the carrier wave is transmitted in intermittent bursts rather than continuously.
๐ต How Pulse Modulation Works
- Carrier wave is transmitted as a series of brief, high-power pulses
- Between pulses, the transmitter is silent โ it can listen for replies
- Information can be encoded in the timing, amplitude, width, or position of pulses
โ
Aviation Systems Using Pulse Modulation
- Radar systems (primary radar, SSR, AWR) โ pulse transmitted, reflected pulse timed to determine range
- DME (Distance Measuring Equipment) โ pulse pairs transmitted/received to measure slant range (emission designator P0N)
- SSR (Secondary Surveillance Radar) โ pulse trains for interrogation and reply
8. Emission Designators
What this section covers: How to decode three-character emission designators, and the specific designators for key aviation equipment.
Emission designators are a standardised three-character code (letter-digit-letter) that describe the characteristics of an electronic transmission. You are not required to memorise the full table, but you must know the key aviation examples.
Emission Designator Structure
Symbol 1 (Letter): Type of modulation of main carrier
Symbol 2 (Digit): Nature of signals used for modulation
Symbol 3 (Letter): Type of information transmitted
Symbol 1 โ Modulation Type
| Letter | Modulation |
|---|---|
| N | Unmodulated carrier (no modulation) |
| A | Amplitude Modulation โ Double Sideband (DSB) |
| H | AM โ Single Sideband, Full Carrier |
| J | AM โ Single Sideband, Suppressed Carrier (SSB) |
| F | Frequency Modulation |
| G | Phase Modulation |
| P | Sequence of unmodulated pulses |
| K | Sequence of pulses modulated in amplitude |
Symbol 2 โ Nature of Modulating Signal
| Digit | Signal Type |
|---|---|
| 0 | No modulating signal |
| 1 | Single channel โ quantized/digital information, no sub-carrier |
| 2 | Single channel โ quantized/digital information, with sub-carrier |
| 3 | Single channel โ analogue information |
| 7 | Two or more channels โ quantized/digital information |
| 8 | Two or more channels โ analogue information |
| 9 | Composite: digital + analogue channels |
Symbol 3 โ Information Type
| Letter | Information Transmitted |
|---|---|
| N | No information |
| A | Telegraphy for aural reception (Morse) |
| B | Telegraphy for automatic reception |
| D | Data transmission, telemetry, telecommand |
| E | Telephony (including sound broadcasting) |
| W | Combination of above |
| X | Cases not otherwise covered |
Key Aviation Emission Designators โ Worked Examples
๐ A3E โ VHF Radio Telephony
A = Amplitude Modulation, Double Sideband
3 = Single channel, analogue information
E = Telephony
โ AM carrier wave modulated with speech (VHF voice comms)
๐ J3E โ HF Radio Telephony
J = AM, Single Sideband, Suppressed Carrier (SSB)
3 = Single channel, analogue information
E = Telephony
โ SSB voice comms over HF (carrier + one sideband removed)
| Equipment / System | Designator | Meaning |
|---|---|---|
| ADF / NDB | N0NA1A / N0NA2A | Unmod carrier + keyed Morse ident |
| VHF RTF | A3E | AM-DSB, analogue, telephony |
| HF RTF | J3E | SSB suppressed carrier, telephony |
| VOR | A9W | AM, composite analogue+digital, combination |
| ILS | A8W | AM, 2+ analogue channels, combination |
| Marker Beacons | A2A | AM, digital sub-carrier, Morse ident |
| DME | P0N | Unmod pulse sequence, no info (ranging only) |
| MLS | N0XG1D | Phase mod, digital data, guidance |
โ ๏ธ EXAM NOTE
"It is not necessary to know the details of the [full emission] table." However you must know A3E (VHF RTF) and J3E (HF RTF) and be able to decode them. Other designators (ADF, VOR, ILS, DME) are "unlikely to be examined" but may appear as distractors.
โก Quick Revision Summary โ Chapter 3
- Keyed: interrupts carrier; Morse code; used by NDBs for ident
- AM: AF amplitude โ RF amplitude; bandwidth = 2 ร AF; USB = RF+AF, LSB = RFโAF
- AM power: carrier = 2/3 total; each sideband = 1/6 total; only 1/3 carries info
- SSB advantages: 2ร channels, better S/N, less power
- FM: AF amplitude โ RF frequency deviation; AF frequency โ rate of change
- FM bandwidth: Broadcast = 150 kHz; NBFM = 8 kHz > 6 kHz limit โ NOT used in aviation voice
- Phase modulation: analogue (amplitude โ phase angle); digital PSK (0ยฐ = 0, 180ยฐ = 1)
- GPS = BPSK; MLS = DPSK
- Pulse modulation: radar systems (DME, SSR, primary radar)
- A3E = VHF RTF; J3E = HF RTF (SSB suppressed carrier)
- DME = P0N (pulse, unmod, no info โ pure ranging)
โ Capt. Pankaj Pahil | www.ghostaviator.com โ
๐ Practice Questions & Detailed Answers
How to Use This Section
Cover the answer panel and attempt each question independently. Read the full explanation and distractor analysis โ knowing why the wrong answers are wrong is as important as knowing the right answer.
Q1.
The bandwidth produced when a radio frequency (RF) of 4716 kHz is amplitude modulated with an audio frequency (AF) of 6 kHz is:
โ Correct Answer: (c) โ 12 kHz
Explanation: AM bandwidth = 2 ร AF = 2 ร 6 kHz = 12 kHz. The USB extends from 4716 to 4722 kHz; the LSB extends from 4716 to 4710 kHz. Total spread = 4710โ4722 kHz = 12 kHz. The RF carrier frequency (4716 kHz) is irrelevant to the bandwidth calculation. See Section 3.1.
Why the other options are wrong:
- (a) โ 6 kHz = the AF itself. This confuses the modulating frequency with the resulting bandwidth. Bandwidth = 2ร AF, not 1ร.
- (b) โ 3 kHz = half the AF. No basis for this value in AM calculations.
- (d) โ 9 kHz = standard AM broadcasting allocation, but it is not the answer to this specific calculation with AF = 6 kHz.
Instructor's Note: The classic trap is answering 6 kHz (the AF itself). Always: Bandwidth = 2 ร AF. The RF value is only needed if asked for the USB or LSB frequency. This is the most likely calculation to appear in the DGCA exam for this chapter.
Q2.
Which of the following statements concerning AM is correct?
โ Correct Answer: (b) โ The amplitude of the RF is modified by the amplitude of the AF
Explanation: In AM, the amplitude of the AF modifies the amplitude of the RF. When AF amplitude is high โ RF amplitude is high; when AF amplitude is low โ RF amplitude is low. The RF frequency remains unchanged. See Section 3.
Why the other options are wrong:
- (a) โ "frequency of AF modifies amplitude of RF" โ this is nonsense for AM. AF frequency determines the frequency of the sidebands, not the RF amplitude.
- (c) โ "frequency of RF modified by frequency of AF" โ this describes neither AM nor FM.
- (d) โ "frequency of RF modified by amplitude of AF" โ this is the definition of FM, not AM. A very common exam trap.
Instructor's Note: Options (b) and (d) are the key pair here. (b) = AM definition; (d) = FM definition. These two are systematically swapped in wrong-answer options. Memorize: AM โ amplitude changes amplitude; FM โ amplitude changes frequency.
Q3.
Which of the following is an advantage of single sideband (SSB) emissions?
โ Correct Answer: (d) โ All of the above
Explanation: SSB has three advantages: (1) more frequencies/channels (each SSB uses half the bandwidth of DSB); (2) reduced power requirement (no power wasted on carrier or second sideband); (3) better signal/noise ratio (narrower bandwidth, less ionospheric static). All three options (a), (b), and (c) are correct individually, so (d) is the best answer. See Section 4.
Why the other options are not the best:
- (a) โ True but incomplete. More channels is one advantage, but not the only one.
- (b) โ True but incomplete. Reduced power is one advantage.
- (c) โ True but incomplete. Better S/N ratio is one advantage.
Instructor's Note: Any time a question asks "which is an advantage" and lists multiple valid advantages individually plus "all of the above" โ the answer is almost certainly "all of the above." Know all three SSB advantages cold: channels ร 2, power โ, S/N โ.