Learn / Guide 4
How to read the waterfall display
The colored screen of receivers shows frequency, time and strength. Each signal type has a distinct look, and you can recognize it before listening.
beginner 6 min read
The three axes
The waterfall display (in English, waterfall display) is a scrolling chart.
- Frequency is on the horizontal axis. Left is lower, right is higher.
- Time is on the vertical axis. Each new line is a moment. Depending on the program, the newest line enters at the top or bottom, and the older ones get pushed along.
- Color is strength. In the most common palette, dark blue is the background noise, and the signal rises through green, yellow and red up to white.
Above the waterfall display there is usually a line graph, the spectrum. It shows the same instant in height instead of color. The waterfall has the advantage of keeping memory: a signal that lasted two seconds is drawn there for a good while.
Before interpreting anything, adjust the contrast. The background should be dark and grainy. If everything is bright, weak signals disappear in the middle. If everything is black, they do not even show up.
The look of each signal
| Signal | Appearance on the waterfall display | Width | Audio |
|---|---|---|---|
| Broadcast AM | a strong, continuous center line (the carrier) with two symmetrical spots on the sides | about 10 kHz | normal voice and music |
| SSB (voice) | one-sided smear with no center line that appears and disappears with speech | 2.4 to 3 kHz | voice, if tuned correctly |
| CW (telegraphy) | fine dashed line, with zoom you can see dots and dashes | 100 to 200 Hz | short and long beeps |
| FT8 | several narrow dashes, side by side, that light up and fade together every 15 seconds | 50 Hz each, in a stretch of about 3 kHz | a chorus of undulating whistles |
| RTTY | two very close parallel lines, flashing alternately | 170 Hz between them, in amateur radio use | a quick two-tone chirp |
| Facsimile | band with two marked edges and a texture that repeats twice per second | just over 1 kHz | a rhythmic scratch |
| Ionosonde (chirp) | thin diagonal line crossing the entire screen | passes through everything | a fraction-of-a-second chirp |
| Over-the-horizon radar | wide block, with straight edges, that turns on and off abruptly | from 10 to over 40 kHz | hum or buzz |
| DRM (digital radio) | solid and uniform rectangle | 10 kHz | pure hiss |
| Narrow FM | wide smear with a marked center that appears when someone transmits | 10 to 16 kHz | clear voice, strong hiss when the signal fades |
Some details that help
AM and SSB. AM voice spends two copies of the same audio plus the carrier. In SSB only one copy is left. That is why the SSB trace has a straight edge on the side where the carrier would be, and a ragged edge on the other. Straight edge on the left means USB. On the right, it means LSB. Just by looking you already know which button to press.
FT8. The block of dashes on 7,074 or 14,074 kHz is the easiest reference on any waterfall display. Each transmission lasts a little under 13 seconds and starts at seconds 0, 15, 30 and 45 of the clock. If the band is open, the block will be full. The guide FT8 and WSPR explains what goes inside each little dot and dash.
Facsimile. Meteorological services still transmit weather charts to ships on shortwave. The image builds line by line, 120 lines per minute, with one tone for black and another for white. The KiwiSDR has an extension that assembles the chart on the screen.
Ionosonde. Research institutes measure the ionosphere by sweeping the spectrum from end to end with a transmitter. On the waterfall display this becomes a slanted line. It does not bother anyone and is a sign that someone is studying propagation at that moment.
Over-the-horizon radar. These are military radars that use the ionosphere to see long distances. They occupy a wide block, pop up anywhere including the amateur radio band, and disappear as quickly as they arrived. The pulse repetition rate determines whether the sound is a low buzz or a woodpecker noise.
What is local noise
Not everything on the screen came from far away. A good portion is electrical noise from the receiver's neighborhood.
- Vertical lines repeated at equal intervals, which drift slowly in frequency: switching power supply, charger, LED lamp, solar inverter.
- Wide and wavy bands, which cover tens of kHz: modem, network cable, electric fence, motor.
- Horizontal lines from side to side, which last an instant: lightning discharge. In the Brazilian summer, the low bands are full of them.
- A very thin vertical line that never changes: a spurious signal generated inside the receiver itself.
The test is easy. Open another receiver, in another city, on the same frequency. If the spot is not there, it was local.
Fading can also be seen
Signal that travels via the ionosphere does not arrive constant. On the waterfall display, fading (QSB) appears as lighter and darker segments over time. On AM broadcasts, it is common to see a dark notch crossing the signal diagonally: part of the frequencies disappears for an instant, and the audio distorts along with it.
A ten-minute exercise
Open a KiwiSDR in Brazil at night and put 7,100 kHz in the center, with enough zoom to see from 7,000 to 7,300.
- At the bottom end, look for the dashed CW traces.
- On 7,074, find the FT8 block and count the 15 seconds.
- From there up, look for SSB traces and see which side the straight edge is on.
- Above 7,200, look for a carrier with two symmetrical spots. It is a broadcast station from another continent.
- Wait for a diagonal trace to pass.
After a few sessions, the eye begins to recognize the signals before the ear.