Modulation is the process of encoding information onto a carrier wave by varying its properties. Understanding modulation types is essential for tuning to and decoding radio signals.
How it works: The amplitude (strength) of the carrier wave varies with the audio signal.
Characteristics:
- Bandwidth: ~10 kHz (±5 kHz from carrier)
- Audio quality: Moderate
- Efficiency: Low (carrier always present)
- Noise sensitivity: Moderate to high
Common uses:
- AM broadcast radio (530-1710 kHz)
- Aviation communication (118-137 MHz)
- Some amateur radio
- CB radio (26.965-27.405 MHz, varies by region)
Demodulation: Simple envelope detection
WebSDR Settings:
- Mode: AM
- Bandwidth: 8-10 kHz
- AGC: ON
Visual signature: Carrier visible even with no modulation, symmetrical sidebands
How it works: Similar to AM but with suppressed carrier.
Characteristics:
- Both sidebands present
- No carrier (or reduced carrier)
- More efficient than AM
Common uses:
- Amateur radio experimentation
- Some military communications
Demodulation: Requires carrier reinsertion (BFO)
How it works: One sideband (upper or lower) with suppressed carrier. Most efficient form of amplitude modulation.
Characteristics:
- Bandwidth: ~2.7 kHz
- Audio quality: Excellent for voice
- Efficiency: Very high
- Frequency accuracy required: ±50 Hz for intelligible speech
Types:
- USB (Upper Sideband): Frequencies above carrier, standard for HF >10 MHz
- LSB (Lower Sideband): Frequencies below carrier, standard for HF <10 MHz
Common uses:
- Amateur radio HF voice (3.5-30 MHz)
- Maritime communication
- Military/utility stations
- Aeronautical HF
Demodulation: Product detector with BFO (Beat Frequency Oscillator)
WebSDR Settings:
- Mode: USB or LSB (convention-dependent)
- Bandwidth: 2.4-3.0 kHz
- Fine tune: Critical (±50 Hz)
Visual signature: Single sideband visible, no carrier spike, asymmetric spectrum
How it works: The frequency of the carrier varies with the audio signal.
Types:
- Bandwidth: 8-16 kHz deviation
- Uses: Two-way radio, amateur repeaters, marine VHF, PMR446
- Deviation: ±2.5 to ±5 kHz
- Bandwidth: 150-200 kHz deviation
- Uses: FM broadcast (87.5-108 MHz), weather satellites (APT)
- Deviation: ±75 kHz
- Features: Stereo (using 38 kHz pilot tone), RDS data
Characteristics:
- Noise immunity: Excellent (capture effect)
- Audio quality: Excellent
- Bandwidth: Wide compared to AM/SSB
- SNR improvement: High
Common uses:
- FM broadcast radio
- Amateur VHF/UHF repeaters
- Marine VHF (156-162 MHz)
- Business/commercial two-way radio
- Walkie-talkies (FRS/GMRS/PMR446)
Demodulation: Frequency discriminator or PLL detector
WebSDR Settings:
- Mode: NFM or WFM (bandwidth-dependent)
- Bandwidth: 8 kHz (NFM) or 200 kHz (WFM)
- Squelch: Useful for NFM
Visual signature: Wide spectrum spread, Carson's rule: BW = 2(Δf + fm)
How it works: Carrier is turned on/off in patterns (dots and dashes).
Characteristics:
- Bandwidth: 100-200 Hz (extremely narrow)
- Efficiency: Very high
- SNR advantage: Best of any mode (~10 dB better than SSB)
- Skill required: Must know Morse code
Common uses:
- Amateur radio (especially weak signal work)
- Beacons (propagation indicators)
- Maritime communication (historical)
- Emergency/backup communication
Demodulation: BFO to create audible tone (typically 400-800 Hz)
WebSDR Settings:
- Mode: CW or USB/LSB with narrow filter
- Bandwidth: 100-500 Hz
- BFO: Adjust for comfortable pitch
Visual signature: Single vertical line (carrier)
How it works: Digital data encoded by shifting between two or more frequencies.
Variants:
- BFSK: Binary (2 tones)
- MFSK: Multiple frequencies
- GFSK: Gaussian filtered (smoother transitions)
Characteristics:
- Simple implementation
- Moderate bandwidth
- Good noise immunity
Common uses:
- RTTY (Radioteletype): 45.45 baud, 170 Hz shift
- Packet radio: AX.25 protocol, 1200 baud (VHF)
- APRS: Position reporting, 1200 baud
- Pagers: POCSAG, FLEX
- AIS: Marine vessel tracking, GMSK 9600 baud
- Weather satellites: LRPT (Meteor-M)
Demodulation: Frequency discrimination and bit detection
WebSDR Settings:
- Mode: USB or dedicated digital mode
- Bandwidth: 3 kHz typical
- Use external decoder software (fldigi, direwolf)
Visual signature: Two or more discrete tones, parallel lines in waterfall
How it works: Digital data encoded by changing the phase of the carrier.
Variants:
- BPSK: Binary (2 phases)
- QPSK: Quadrature (4 phases)
- 8PSK: 8 phases (higher data rate)
Characteristics:
- Very efficient
- Narrow bandwidth
- Requires phase coherence
Common protocols:
- PSK31: 31.25 baud, keyboard-to-keyboard chat, 60 Hz bandwidth
- PSK63: 62.5 baud, faster variant
- PSKR: Reporter modes for propagation
Common uses:
- Amateur radio digital communication
- Low power experimentation
- DX (long distance) contacts
Demodulation: Phase detection and symbol decoding
WebSDR Settings:
- Mode: USB
- Bandwidth: 500 Hz - 3 kHz
- Use fldigi or similar software
Visual signature: Constant amplitude, centered carrier, ~60-250 Hz bandwidth
How it works: 8-tone FSK with sophisticated error correction, time-synchronized.
Characteristics:
- Time slots: 15-second transmit periods
- Bandwidth: ~50 Hz
- SNR advantage: Decodes at -21 dB (excellent weak signal)
- Automated: Computer-controlled QSOs
- Data rate: 6.25 baud
Common uses:
- Amateur radio weak signal work
- DX expeditions
- Propagation studies
- Most popular digital mode currently
Frequencies: 1.840, 3.573, 7.074, 10.136, 14.074, 18.100, 21.074, 24.915, 28.074 MHz
Demodulation: WSJT-X software (time-sync required)
WebSDR Settings:
- Mode: USB
- Bandwidth: 3 kHz (to see multiple stations)
- Record audio for WSJT-X processing
Visual signature: Short 15-second bursts, multiple tones, synchronized timing
How it works: Similar to FT8 but faster (7.5 second cycles) for contests.
Characteristics:
- Time slots: 7.5 seconds
- Higher throughput than FT8
- Optimized for rapid QSOs
How it works: Beacon mode transmitting callsign, location, and power.
Characteristics:
- Time slots: 2-minute transmit periods
- SNR advantage: -31 dB
- Purpose: Propagation studies only (not QSOs)
- GPS sync: Requires accurate time
Common uses: Automated propagation monitoring
How it works: Analog image transmission using FM audio tones.
Common modes:
- Martin M1: 114 seconds per image
- Scottie S1: 110 seconds per image
- Robot 36: 36 seconds per image
Characteristics:
- Color images: RGB scan lines
- Audio tones: 1500 Hz sync, 1200-2300 Hz video
- Resolution: 320x240 typically
Common uses:
- Amateur radio image sharing
- ISS (International Space Station) periodic events
- Special event stations
Demodulation: MMSSTV, QSSTV, or similar software
WebSDR Settings:
- Mode: USB
- Bandwidth: 3 kHz
- Record audio for decoder
Visual signature: Diagonal lines in waterfall, changing tones
How it works: FSK using Baudot or ASCII encoding.
Characteristics:
- Standard: 45.45 baud, 170 Hz shift
- Mark/Space: Two tones
- Character encoding: 5-bit Baudot or 7/8-bit ASCII
Common uses:
- Amateur radio contests
- Maritime communication
- Meteorological data (FAX)
- News services (historical)
Demodulation: fldigi, MMTTY
WebSDR Settings:
- Mode: USB (sometimes RTTY-specific mode)
- Bandwidth: 500 Hz
- External decoder software
How it works: Aircraft broadcast position, altitude, speed using PPM (Pulse Position Modulation).
Characteristics:
- Frequency: 1090 MHz
- Modulation: PPM (pulse)
- Data rate: 1 Mbit/s
- Range: Line of sight (50-250 miles depending on altitude)
Common uses: Aircraft tracking
Demodulation: dump1090, Mode-S decoder
Required hardware: 1090 MHz receiver or upconverter
How it works: Digital encoding for AM broadcast bands.
Characteristics:
- Audio quality: Near-FM quality
- Spectrum efficiency: Better than analog AM
- Robustness: Error correction
Common uses: International shortwave broadcasting
How it works: Digital voice codecs for VHF/UHF communication.
Characteristics:
- Voice quality: Clear or silent (no static)
- Data capability: GPS, text messaging
- Networking: Internet linking
Common uses: Amateur radio repeaters
How it works: AX.25 packet radio with position, weather, telemetry data.
Characteristics:
- Frequency: 144.390 MHz (North America), varies by region
- Modulation: 1200 baud AFSK
- Protocol: AX.25 frames
Common uses: Position reporting, weather stations, mobile tracking
Demodulation: direwolf, Xastir
| Use Case | Mode | Why |
|---|---|---|
| Local two-way radio | NFM | Best audio, noise immunity |
| HF voice DX | SSB | Efficient, good audio quality |
| Weak signal HF | FT8, CW | Best SNR performance |
| AM broadcast listening | AM | Standard for these bands |
| FM broadcast listening | WFM | Standard, stereo capability |
| Digital keyboard chat | PSK31 | Narrow bandwidth, efficient |
| Aircraft tracking | ADS-B | Standard for aviation |
| Image sharing | SSTV | Analog image mode |
| Marine VHF | NFM | Standard for maritime |
| Aviation HF | USB | Standard for aeronautical |
CW: [██] ~200 Hz
PSK31: [████] ~60 Hz
FT8: [████] ~50 Hz
RTTY: [████████] ~450 Hz
SSB: [████████████████] ~2.7 kHz
NFM: [████████████████████████] ~12 kHz
AM: [████████████████████████████] ~10 kHz
WFM: [████████████████████████████████████████████████] ~200 kHz
Simple → Complex:
- AM (envelope detection)
- FM (frequency discriminator)
- SSB (product detector + BFO)
- CW (BFO + tone extraction)
- FSK (frequency discrimination + bit sync)
- PSK (phase coherent detection + symbol sync)
- FT8/FT4 (time sync + multi-tone FSK + error correction)
- sigidwiki.com: Identify unknown signals
- fldigi: Multi-mode digital decoder
- WSJT-X: FT8/FT4/WSPR decoder
- direwolf: Packet radio/APRS decoder