If you tune across the bands on a real receiver, every mode has a shape on the waterfall. CW is a single razor-thin line. FT8 is eight ghostly tones, equally spaced. SSB voice is a 2.4 kHz block of modulated noise. Once you know the shapes, you can identify a mode by glance before you even hear it.
These sketches are stylized: real spectra have noise, fading, and neighboring stations. But the characteristic shape is what operators learn to recognize.
The oldest mode, still ubiquitous on HF. Just a single radio carrier turned on and off in Morse patterns: dot, dash, dot, dash. Spectrum-wise it's essentially one vertical line at the operating frequency. Bandwidth is tiny, typically 50 to 200 Hz depending on keying speed.
Where you find it: every band, but especially 80m, 40m, 20m, 15m during contests. CW is the universal weak-signal mode of legacy amateur radio: you can copy CW through QRM and QSB that would kill voice modes outright.
Voice modulation that suppresses the carrier and one of the two sidebands, leaving just one 2.4 kHz wide block of audio-rate modulation. Above 10 MHz hams use USB (upper sideband); below 10 MHz they use LSB (lower sideband).
Where you find it: 80m, 40m, 20m, 17m, 15m, 12m, 10m phone segments. Most "voice on the air" is SSB.
Old-school: a full carrier plus two symmetric sidebands. 6 kHz wide, twice as much as SSB and uses much more power because the carrier itself doesn't carry information. Still has a die-hard following on certain ham bands and on the AM broadcast band (530 to 1700 kHz).
Where you find it: AM broadcast (commercial), parts of 75m, 40m, 160m for nostalgia operators.
Wide. 10 to 15 kHz depending on deviation. Frequency varies with the audio instead of amplitude. Above the threshold, FM sounds clean even when AM and SSB are dropping out; below the threshold it goes to mush instantly. That's why FM is the standard for FM broadcast, ham VHF/UHF repeaters, and most public-safety land mobile.
Where you find it: 88 to 108 MHz commercial FM broadcast, 2m and 70cm repeaters, GMRS, marine VHF voice channels, NOAA weather.
Frequency-shift keying with two tones, traditionally 170 Hz apart (the "shift"). One tone is "mark" (bit = 1), the other is "space" (bit = 0). Send 5-bit Baudot characters at 45.45 baud and you have the radio teletype protocol that dominated digital ham operation from the 1960s through the 1990s.
Where you find it: RTTY contests on 80/40/20/15m, MARS, some weather services. The two-peak signature is unmistakable.
Phase shift keying at 31.25 baud, BPSK variant. Single carrier with ~31 Hz of occupied bandwidth, the narrowest live-keyboard mode that still keeps up with a typist. Designed by Peter Martinez G3PLX in the late 1990s as a deliberate "QRSS" (slow but ultra-narrow) reply to RTTY's appetite for spectrum.
Where you find it: 14.070, 7.070, 3.580 MHz standard PSK watering holes.
8-tone GFSK at 6.25 Hz spacing, 50 Hz total bandwidth, 15-second transmit slots. The mode that swallowed HF: FT8 single-handedly dominates HF activity now because it can decode 25 dB below SSB noise floor. You see eight thin tones marching together on the waterfall, all 8 starting and stopping in lockstep with the WSJT-X sequencer.
Where you find it: 1.840, 3.573, 7.074, 10.136, 14.074, 18.100, 21.074, 24.915, 28.074, 50.313 MHz. Every spot column on /dx with mode = FT8 lives at one of these.
4-tone GFSK at 23.4 Hz spacing, 83 Hz total bandwidth, 7.5-second slots. Half the time of FT8 per QSO at the cost of a few dB sensitivity. Designed for contesting where rate matters more than weak-signal margin.
Where you find it: 3.575, 7.047, 10.140, 14.080, 18.104, 21.140, 24.919, 28.180 MHz.
Gaussian-filtered Minimum Shift Keying at 9600 baud, 12 kHz wide. Used by the Automatic Identification System (AIS) on the two international maritime channels at 161.975 MHz (Ch 87B) and 162.025 MHz (Ch 88B). Every commercial ship over 300 gross tons and most pleasure craft over 65 feet broadcast position + heading + identity here continuously.
This is how K3DPT's /live page sees ships: a global network of shore-based AIS receivers, coupled with satellite AIS receivers in orbit, listen on these two channels and aggregate the decoded position reports. The K3DPT instance subscribes to that aggregated feed via aisstream.io.
Where you find it: Channel 87B / 88B VHF marine, plus satellite AIS overlays.
Pulse-position modulation at 1090 MHz. Every modern aircraft has a transponder that broadcasts position + altitude + identity in short ~120 microsecond bursts. The signature is a stereotyped 8 us preamble followed by a 56 or 112 us data block of evenly-spaced pulses.
A worldwide network of feeder volunteers run inexpensive 1090 MHz SDR receivers and pool their decodes. OpenSky (which the /live page subscribes to) is one of those networks. ADS-B is the aviation equivalent of AIS for ships, except it's faster (position updates every second instead of every 30 seconds) and the receivers are mostly hobbyists.
Where you find it: 1090 MHz worldwide. In the US there's also a secondary system, UAT, on 978 MHz for general aviation.
A waterfall display on a receiver is a 2D plot: frequency across, time down, signal strength as color. Each mode draws its characteristic shape on that display. If you spend any time operating you start reading waterfalls intuitively:
a stuck PTT
The diagrams above give you the static signature. With a real waterfall + a few hours of listening, you'll start identifying modes by glance before you even decode them.
is a link to its section above.
panel for each aircraft links here.
panel for each ship links here.
above interact with the ionosphere.