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Reading the waterfall

Separate frequency, time and colour, and make useful observations from a spectrum display.

Fundamentals2 min readReviewed 15/10/2026

HB3YAQ · Ajdin HadzicLearning to take awayhttps://hb3yaq.ch/en/knowledge/waterfall

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WHAT YOU WILL LEARN

Recognise signal width and behaviour over time.
Distinguish a display from a measurement.

Try it in the simulated SDR learning lab →

Three dimensions of information

The horizontal axis shows frequency. Each new row represents a later time; older rows move up or down depending on the interface. Colour indicates displayed signal strength within the selected scale. A continuous carrier draws a narrow line, while a short transmission produces a limited patch.

The spectrum above the WaterfallA waterfall shows a spectrum over time: one axis represents frequency, the other time. Colour represents relative signal level.Example & full explanation → is usually an instantaneous or averaged view. The waterfall adds history. A signal that has just disappeared remains visible there, allowing you to notice repeated transmissions, pauses and slowly drifting frequencies.

LOOK · UNDERSTAND · TRY

Three traces, three observations

Schematic waterfall: time runs downward. Colour represents relative display level, not calibrated power.

A: narrow trace — a signal remains on the same frequency.
B: short block — a broader signal is visible only briefly.
C: sloping trace — the displayed frequency changes with time.

Quick check

Does the broad trace B prove that the signal uses FM?

Reveal explanation

No. The shape shows width and timing. The mode still needs to be identified; interference can also be broad.

Try the simulated waterfall →OpenWebRX ↗

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Patterns are clues

An AM signal may show a strong carrier with sidebands. SSB · USB / LSBSingle Sideband transmits one sideband, usually with the carrier suppressed. USB is the upper and LSB the lower sideband relative to the carrier reference.Example & full explanation → speech often varies over time and lacks a similarly strong transmitted carrier. Narrow digital signals may form several lines or repeating patterns. Other modes and interference can create similar images, so a pattern alone is not a reliable identification.

Zoom in far enough to see the occupied width. Place the receive filter over the wanted signal and listen in the appropriate mode. Check several clues together: width, timing, frequency and demodulated audio. Avoid conclusions based on colour alone.

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Why brighter does not automatically mean better

Changing the minimum, maximum or contrast of the colour scale changes the image without improving reception. Averaging and FFT resolution also affect the display. Screenshots from different receivers are therefore not reliable level comparisons without ImpedanceImpedance is complex opposition to alternating current: it includes resistance and a frequency-dependent reactance. Antenna input impedance is therefore more than a DC resistance reading.Example & full explanation → settings and calibration.

For a simple comparison, keep display settings fixed and change one thing, such as the receive filter. Also listen for intelligibility. An attractive waterfall and clear speech are different goals: reducing audio noise need not change colours across the whole spectrum.

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