What a waterfall display shows
A waterfall is a scrolling picture of radio activity. Frequency runs left to right, time runs top to bottom, and the color or brightness of each point shows signal strength. New data appears at one edge and older data scrolls away, so you can see not only what is on the band right now but also what happened a few moments ago.
Next to the waterfall, most programs show a spectrum display, a line graph of signal strength against frequency. The two views complement each other: the spectrum shows instantaneous levels, and the waterfall shows how signals change over time.
How to read a waterfall
- A steady vertical line is usually a continuous carrier, such as a beacon or interference.
- Short dashes and dots in a column are typically Morse code (CW; see CW abbreviations).
- A wider, shifting smear often corresponds to a voice signal such as SSB.
- Neat, evenly spaced tones suggest a digital mode, and different modes have recognizable looks (see digital mode identification).
- A general rise in background can indicate noise.
Reading these patterns takes practice. Slow waterfall speeds can help you spot weak or intermittent signals and slowly changing noise, and some programs, including HDSDR, advertise a very low-speed waterfall for that purpose.
Where waterfalls appear
You can meet a waterfall in three places:
- SDR receiver programs, which display a wide slice of spectrum at once.
- Digital mode software, where the waterfall helps you pick a clear frequency and click on signals to decode them.
- Some transceivers, which have a built-in spectrum scope.
Popular SDR programs with waterfalls
An SDR (software defined radio) uses a computer to do much of the work a traditional receiver does in hardware. The program draws the spectrum and waterfall from the incoming data.
- HDSDR is freeware for Windows. Its listed features include separate spectrum and waterfall displays, demodulation of AM, FM, SSB and CW, noise reduction and notch filters, recording and playback, and CAT control support through Omni-Rig. It builds on Winrad by I2PHD.
- SDR#, SDR++ and similar programs are widely used with inexpensive receivers. Check each project’s site for the platforms it currently supports.
- Manufacturers of SDR hardware often supply their own software too.
Which one suits you depends on your hardware, operating system, and whether you want to transmit or only listen.
Waterfalls in digital mode software
Programs such as WSJT-X and fldigi show a waterfall of the audio from your receiver. You use it to find open space, see who else is transmitting, and select a signal to decode. Common calling frequencies are listed in digital mode frequencies by band. Because the waterfall displays a narrow audio passband rather than a wide RF slice, it is simpler than a full SDR display, but the reading skills are the same.
Free options
There are several free SDR programs, and HDSDR is described by its author as freeware. Free options differ in features, update frequency and supported hardware, so there is no single answer to “the best free SDR software”. A reasonable approach is to try one or two with your receiver and keep the one whose interface you find easiest.
Is RTL-SDR legal?
An RTL-SDR is a low-cost USB receiver. Owning one and listening is legal in many places, but laws vary by country and some jurisdictions restrict listening to certain services or decoding certain transmissions. These receivers are receive-only, so transmitting is not involved. If you want to transmit, you need an amateur licence and must stay within your privileges. Check the rules where you live if you are unsure, and remember this is general information, not legal advice.
Choosing software and getting started
- Decide whether you want to listen only or also transmit.
- Check which operating systems your hardware and software support.
- Install one program, connect your receiver, and tune a known active frequency or beacon.
- Practice identifying signal types, then try zooming and changing waterfall speed.
If your radio has CAT control, check whether the program supports it so the display and your transceiver stay on the same frequency.
Related tools
Maps pair well with a waterfall when you are chasing DX: see ham radio mapping software. If you run group operations, see ham radio net control software.
FAQ
What is the best free SDR software? There is no single winner. HDSDR, SDR# and SDR++ are commonly used, so try one that supports your hardware.
Is RTL-SDR illegal? Receiving is allowed in many places but not everywhere, so check local rules.
What is the most popular ham radio logging software? Waterfall programs do not log contacts. Logging is a separate category, and several programs are popular depending on your needs and operating system.