Signal guide

WSPR (propagation beacons)

Very low-power beacons that each send a call sign, location and power level over two minutes, so stations can map where signals reach.

I've heard this My life list

WSPR sounds like almost nothing: a faint, pure, very slow tone, often buried in the noise, that drifts in tiny steps for nearly two minutes, starting on the even minute.

What does it sound like?

Plate 79Weak-signal digital

A faint, very slow, steady tone that shifts pitch slightly, lasting almost two minutes.

0 Hz1,0002,0003,000 Hz

12-second example. WSPR made by Cohera: the symbols come from WSJT-X's own encoder, set weaker than the background noise, as it usually is on the air; WSJT-X's decoder read the message back exactly.

Field marks2-minute transmissions4 tones, 1.46 Hz apartAbout 6 Hz wideBeacons, not conversations

Hear it, step by step

This computer-made WSPR clip is a 12-second slice of one transmission, set weaker than the background hiss, which WSPR is built to handle.

0 Hz1,0002,0003,000 Hz

Tap a step to hear that moment. The box shows where it is on the waterfall.

0.0 sWeaker than the hissAcross a 2,500 Hz listening band, the hiss has about 17 times the signal's power: the signal sits 12 dB below the noise. On the waterfall it is only a thin line at 1,500 Hz.0.0 sFour tones, 1.46 Hz apartUp close, that line is four tones at about 1,497.8, 1,499.3, 1,500.7 and 1,502.2 Hz, each 1.46 Hz from the next. The whole signal fits in about 6 Hz.0.2 sEach tone lasts 0.68 secondsThe signal holds each tone for 0.683 seconds, about 1.46 tones per second. This clip shows only about 17 of the 162 tones in one transmission, which lasts about 110.6 seconds.0.0 sLoud in its own laneMeasured only in its own 6 Hz sliver, the signal is about 14 dB stronger than the noise there. Squeezing its power into so narrow a slot is what lets a receiver pick it out.0.2 sA built-in timing patternEach tone choice mixes one message bit with one bit of a fixed timing pattern every receiver knows. All 17 whole tones here match, one for one, the reference encoder's sequence for this practice message.

Make your own: type a message and hear it as WSPR (propagation beacons) in the Signal Maker.

Who uses it?

Amateur radio operators studying propagation; thousands of automatic stations worldwide.

Where and when can I hear it?

A narrow slot on each ham band, for example just above 14.0956 MHz; transmissions start on even minutes.

The science

A WSPR message holds just a call sign, a four-character grid locator and the transmitter power in dBm, compressed into exactly 50 bits. A convolutional error-correcting code stretches those into 162 bits, which are shuffled (interleaved) so a burst of static damages scattered bits instead of a run, and then each bit is paired with a known synchronizing bit to choose one of four tones. It keys at only about 1.46 symbols per second, so a transmission lasts about 110.6 seconds and occupies only about 6 Hz.

Why use it?

Because it sends so little information so slowly, WSPR can be decoded at signal-to-noise ratios as low as -31 dB in a 2500 Hz bandwidth, so even milliwatt-level transmitters can be heard over long distances. Receiving stations can upload what they hear to the public WSPRnet database, which maps and archives the reports and shows where each band is open. Transmit periods are chosen at random to reduce collisions between stations.

Lore

WSPR, pronounced whisper, stands for Weak Signal Propagation Reporter. The WSPRnet site also refers to it as K1JT's MEPT_JT digital mode.

What do the decoded parts mean?

• Call sign: The beacon station.
• Grid square: Its location.
• Power: Transmitter power in dBm (for example 23 dBm is 0.2 watts).

Easily confused with

• FST4 / FST4W: FST4W is a newer, more sensitive relative, with optional longer cycles
• FT8: FT8 is wider and turns every 15 s

How to recognize it

WSPR (pronounced "whisper") changes among four tones spaced only about 1.5 Hz apart, at about 1.5 steps per second. To the ear, that is a single thin whistle with a slight wobble, if you can hear it at all. Each transmission lasts about 110.6 seconds and starts one second after an even minute (00, 02, 04...).

On a waterfall, WSPR looks like a set of very narrow, faint, slightly wavy lines inside a 200 Hz window. Dozens of stations can share that window because each signal is only about 6 Hz wide.

Easily confused with:

• FT8: much busier, with 15-second turns and signals 50 Hz wide.
• An unmodulated carrier or birdie: a steady tone that never stops on the two-minute cycle.
• Morse beacons: switched on and off, not continuous.

Where and when to hear it

WSPR has a slot on nearly every ham band. Common dial frequencies (upper sideband) include 3.5686, 7.0386, 10.1387, 14.0956, 18.1046, 21.0946, 24.9246 and 28.1246 MHz, with the signals 1400-1600 Hz above the dial. The 20- and 30-meter bands are almost always busy.

You rarely hear WSPR by ear. The point is automatic reporting: receiving stations upload what they decode to the WSPRnet database, which has logged billions of reports and shows, minute by minute, which paths are open. Small high-altitude balloons circling the globe often carry WSPR transmitters for tracking.

If you want to hear WSPR by ear, pick 30 or 20 meters, tune to the dial frequency in upper sideband and listen near 1500 Hz in the audio just after an even minute. On a strong night you may catch a faint, steady whistle; more often, only a waterfall display reveals the thin wavering lines.

A little history

Joe Taylor, K1JT, released WSPR in 2008 as a tool for studying how radio waves travel. Its design trades speed for sensitivity: one short message of call sign, four-character grid square and power level takes almost two minutes to send, but it can be decoded at signal levels around 28 to 31 dB below the noise in a normal receiver bandwidth.

Because stations often send with less than one watt, WSPR has shown how far tiny signals can travel, and its archive has become a large public dataset for studying the ionosphere.

Common questions

Sources

• WSJT-X home page (WSPR mode)
• WSPRnet
• WSJT-X User Guide: protocol specifications (WSPR)
• WSJT-X 2.7 User Guide (K1JT and the WSJT Development Team)
• WSPRnet map page
• WSJT-X source, default working frequencies (models/FrequencyList.cpp, GitHub mirror)