Field method
How to identify a radio signal by ear
Every radio signal leaves clues in its sound. With five questions, most of the strange noises on shortwave, ham and scanner bands narrow to a handful of candidates, and often to one.
1. Is it a person, a machine, or nature?
Start with the broadest cut. A voice, even a garbled one, is a person (or a recording). A regular, repeating pattern of tones is almost always a machine: data, a beacon or a radar. Crackles, hiss and buzzes spread across the whole dial are usually noise or interference, not a transmission. Nature makes some sounds too: lightning crashes, and on long-wave the whistles of the upper atmosphere.
2. How many tones can you hear?
Hum along if you can. The number of distinct notes is one of the strongest clues:
- One tone, switching on and off: Morse code.
- One steady tone that never changes: an unmodulated carrier, or a tone signal.
- Two tones alternating quickly: teletype (RTTY), NAVTEX or other two-tone data.
- A handful of tones stepping around: the multitone modes such as Olivia and MFSK, and the weak-signal family (FT8, FT4, WSPR).
- A dense hiss with no tones at all: modes that use many tones at once, such as DRM, MT63 or STANAG 4285.
3. What is the rhythm?
Machines keep time, and each keeps its own. Listen for the beat:
- Starts and stops together on the quarter-minute: FT8 (15-second turns). Every 7.5 seconds: FT4.
- A steady tick, about twice a second: a picture being sent, line by line: weather fax or SSTV.
- A tick every second with a voice each minute: a time station.
- A fast, harsh tapping, dozens of times a second: over-the-horizon radar.
- Short bursts, then silence: packet data such as APRS, pagers, or aircraft and ship data.
4. How wide is it?
Tune slowly across it. Some signals vanish if you move a tiny amount; others fill the whole speaker. Morse and PSK31 are very narrow. RTTY and most data modes are a few hundred hertz wide, about a voice's width or less. DRM, MT63 and many radars are wide enough to cover several neighbors. If your radio or SDR shows a waterfall display, width is easy to see: count how much of the screen the signal paints.
5. Where and when did you hear it?
Frequency and time narrow things fast. Ham digital modes cluster near the bottom of each ham band (FT8 at 14.074 MHz, for example). NAVTEX is on 518 kHz. Weather satellites and pagers sit on VHF and UHF, not shortwave. Many utility and broadcast signals follow published schedules, so the same sound at the same time every day is a strong clue.
Put it together
Two tones, a fast steady rhythm, a few hundred hertz wide, on a ham band in the evening: very likely RTTY. One warbling tone stepping among eight pitches, starting on the quarter-minute near 14.074 MHz: FT8. The signal guide lists the field marks and the look-alikes for 79 signal types, with a real recording of most of them to compare against.
When you still can't place it
Write down what you measured: the number of tones, the rhythm, the width, the frequency and the time. Those details turn a vague "what is this?" into a question an experienced listener can answer quickly, and they're the same details a good identification tool should show you.