Below the Noise Floor — Episode 3: Reading the Waterfall

The waterfall is the defining feature of software-defined radio. This episode covers how to read it: what strong and weak signals look like, how to tell a voice signal from a digital mode from a CW transmission just by its shape, and how to use the waterfall as a real-time window into propagation conditions.

Transcript

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One of the first things people say when they connect a software-defined radio to a real antenna and pull up the waterfall for the first time is that they had no idea any of this was happening. They could hear static on a traditional radio, maybe an occasional voice or a burst of something unrecognizable. But the waterfall shows you the whole picture at once, every signal present across a wide swath of the spectrum, simultaneously, scrolling downward in real time. It is a fundamentally different relationship with the radio spectrum than anything a traditional dial-tuned receiver gives you. This episode is about learning to read that display, not just glancing at it, actually reading it the way you would learn to read a map. Understanding what different shapes mean, what colors tell you, and how to use the waterfall as something more than a pretty visualization. Let me start with the basics of what you are looking at. The waterfall display in Ether SDR and in SDR software generally is a two-dimensional representation of the radio spectrum over time. The horizontal axis is frequency, left is lower frequency, right is higher. The vertical axis is time, scrolling downward, newer data at the top, older data toward the bottom. The color of each point represents signal strength at that frequency at that moment. In the default EtherSDR color scheme, strong signals appear as bright yellows and whites. Moderate signals show as oranges and reds, weak signals appear as blues and greens, barely distinguishable from the background. The background itself, the noise floor, is a relatively uniform dark color, usually a deep blue or near black, depending on how the color map is configured. Above the waterfall is the pan adapter, which is the same data displayed differently. The pan adapter is a line graph, frequency on the horizontal axis, signal strength on the vertical axis, showing you the current snapshot of the spectrum as a curve. When a strong signal is present, the line spikes upward at that frequency. The panadapter and waterfall are showing you the same information in complementary ways. The panadapter is better for reading current signal strength precisely. The waterfall is better for understanding how signals are behaving over time, whether they are steady, drifting, intermittent, or pulsing. Now let me talk about what different types of signals actually look like. A voice signal in SSB, single sideband, appears on the waterfall as a relatively broad smear of energy on one side of a center frequency. If you zoom in close enough, you can see that it has irregular density, stronger and weaker at different moments as the person speaks. When they stop talking, the signal largely disappears, except for any carrier or background noise. When they start speaking again, it comes back. The width of the signal in the waterfall corresponds to the audio bandwidth. A typical SSB voice signal is somewhere between 2 and 3 kHz wide, which at normal waterfall zoom looks like a fuzzy vertical stripe with ragged edges. A CW signal, Morse code, looks completely different. It is a single narrow spike that turns on and off in precise patterns. On the waterfall, it appears as a vertical line that blinks, long segments for dashes, short ones for dots, gaps in between. If you see a thin flickering vertical line and wonder what it is, there is a good chance it is someone sending CW. CW signals are very narrow, often less than 100 Hz wide, which means they appear as a sharp, well-defined line rather than the soft smear of a voice signal. Digital modes are where the waterfall gets genuinely interesting and where reading the display starts to feel like learning a visual language. FT-8, which is currently by far the most common mode on HF, has a very distinctive signature. FT-8 transmissions are exactly 12.6 seconds long, they occur on a strict 15-second timing cycle. Transmissions start either at the top of the minute or 15 seconds in, or 30 seconds in, or 45 seconds in, synchronized to GPS time. On the waterfall, this creates a striking pattern, columns of short vertical dashes that appear and disappear in unison at precise intervals. If you are looking at a busy segment of 20 meters, and you see what looks like a comb, multiple narrow signals side by side, all appearing and disappearing at the same moments, that is FT8. The individual tones within each FT8 signal are roughly 50 Hz wide, and the total signal occupies about 50 Hz of bandwidth, which makes them appear as very thin, precise lines compared to a voice signal. WSPR, pronounced whisper, is another weak signal digital mode, and it has a similar comb appearance to FT8, but operates on a two-minute cycle rather than 15 seconds. WSPR signals are designed to be extremely low power beacons used specifically for propagation testing. They are often right at or below what you can see clearly in the waterfall, faint traces that appear for two minutes and then stop. PSK-31 is an older digital mode that looks quite different. It appears as a very narrow, continuous carrier with phase modulations you cannot see directly, but which produce a distinctive sound when demodulated. You will still encounter PSK-31 on 20 meters particularly, and it looks like a thin, steady vertical line that persists rather than appearing in time bursts. AM Amplitude Modulation has perhaps the most visually distinctive signature of any mode on HF. An AM signal has a center carrier that appears as a strong vertical line, flanked symmetrically on both sides by two sidebands that mirror each other. If you see a strong central line with two fainter lines equidistant on either side, that is AM, international shortwave broadcasting, some aeronautical communications, and parts of the 40 and 160 meter amateur bands use AM. The total bandwidth of an AM signal is roughly twice the audio bandwidth. A typical voice AM broadcast occupies around 9 to 10 kHz, which at normal zoom looks noticeably wider than an SSB signal. FM signals on HF are rare. FM is primarily a VHF and UHF mode, but you will encounter it on the 10-meter band and in certain specific allocations. FM signals appear as a steady carrier of moderate width that does not vary in amplitude the way AM does. Now let me talk about using the waterfall as a propagation tool because this is one of the things that SDR operation does that a traditional radio simply cannot match. When a band is open, when propagation conditions are good and signals are traveling long distances, the waterfall fills up. You will see dozens or hundreds of signals present across the band simultaneously. The FT-8 segments in particular become dense with activity. The noise floor may actually rise slightly as distant stations add their aggregate contribution to the background. On a really good day on 20 meters during high solar activity, the waterfall from roughly 14070 to 14100 MHz, the FT-8 segment looks almost solid with signal traces during the peak hours, when conditions are poor, when the solar flux is low, or a geomagnetic storm has disrupted the ionosphere. Or when you're on a band that is not open at that hour, the waterfall is relatively empty. You might see a few strong regional signals on 40 meters at night, or almost nothing on 15 meters during low solar activity. An empty waterfall is not a broken radio. It is information. It is the band telling you conditions are not favorable right now. Watching the waterfall over the course of a day teaches you something that reading propagation theory never quite does the same way. You develop a feel for band behavior. You start to recognize what 20 meters looks like at noon versus 6 p.m. vs midnight. You notice when 10 meters suddenly erupts with signals during a solar flux enhancement. You see the FT-8 segments on 40 meters come alive after sunset as the skip distance shortens and regional stations start appearing. There are a few specific things to look for as you develop your waterfall reading skills. Drifting signals, carriers or tones that move slowly horizontally over time, usually indicate an oscillator that is not stable, often in an older radio that has not warmed up fully. A signal that starts a few hundred Hz to one side and drifts toward a center frequency over the first few minutes of a contact is a classic sign of warm-up drift. Broadband noise that raises the entire noise floor is usually local interference, a switching power supply, a plasma TV, solar panel inverters, certain LED lighting, or a neighbor's equipment. It appears as a general brightening of the entire waterfall background rather than a specific signal at a specific frequency. If you see this, you are dealing with RF interference, which is one of the persistent challenges of urban and suburban amateur radio operation. Impulse noise, very short sharp spikes that appear as horizontal lines across the entire width of the waterfall, is usually electrical in origin. Lightning, power line arcing, or certain motor-driven devices. A lightning strike somewhere in the hemisphere can produce a very distinctive sharp horizontal flash across the entire waterfall simultaneously. Birdies are spurious signals generated inside the radio or the computer itself. They appear at fixed frequencies that do not change with band or conditions and are usually narrow. Most experienced SDR operators learn to recognize their own radio's birdies, the same phantom signals at the same frequencies, regardless of what antenna is connected or what band they are on. One last thing worth mentioning on waterfall configuration. Ether SDR lets you adjust the color map, the reference level, and the display range. The reference level controls what signal strength maps to what color. If the display looks washed out with everything bright, lower the reference level. If everything looks uniformly dark and you cannot see moderate signals clearly, raise it. The display range controls the contrast between weak and strong signals. A wider range shows more dynamic range but makes weak signals harder to distinguish from noise. Most operators find a setting they like and leave it. Getting comfortable reading the waterfall takes a few sessions of just watching. Put on a band that has activity, 20 meters or 40 meters depending on the time of day, and spend some time just observing before you touch anything else. You will start to recognize the recurring patterns. The FT8 comb, the SSB voice smears, the occasional CW line. After a while, it becomes intuitive in the way that any visual skill does with practice. Next episode, we are going to do a band by band tour of the HF spectrum, what each band is typically used for when it is active, and where to point your attention when you are first getting on the air. This is below the noise floor.