WSPR on 20 metres: 14095.6 kHz, milliwatt beacons below the noise

What you hear

Almost nothing, and that is the point. Tune to 14095.6 kHz in upper sideband and the loudspeaker gives hiss with, at most, a faint steady tone somewhere around 1500 Hz that starts just after an even minute and stops just before the next even minute. On the waterfall you see more: within a window only 200 Hz wide, from 14097.0 to 14097.2 kHz, a few hair-thin lines appear every two minutes, some of them visible, most of them not, each one lasting a minute and fifty seconds. Those are WSPR transmissions, and the decoder reads them where the eye sees only noise.

Each line carries a message of fifty bits: a callsign, a four-character grid locator and the transmitter power in dBm. There is no reply and no exchange; the station sends its card into the void every few minutes and other stations report what they heard. The modulation is four tones spaced about 1.5 Hz apart, which is why a signal occupies only six hertz and why it can be decoded around 28 dB below the noise, several times deeper than FT8.

Who transmits and why

WSPR, the Weak Signal Propagation Reporter, was released by the WSJT-X team in 2008 as a tool for measuring propagation rather than making contacts. Every reception is uploaded to a central database, and the result is a map of which paths are open at this minute, on this band, at this power. The transmitters are often tiny: a box the size of a matchbox with 200 milliwatts, a transceiver turned down to a watt, or a small helium balloon drifting around the world at twelve kilometres with a WSPR transmitter weighing a few grams, reporting its position through the locator in each message. On 20 m those balloons are a regular sight in the decodes.

Amateurs run WSPR to compare antennas, to see when a band opens, to test whether a hundred milliwatts can reach Australia from a flat in Europe (on a good day they can), and simply because a map that fills with coloured lines over the course of a day is satisfying to watch.

When and where to listen

Around the clock, with the rhythm of the band: 20 m carries WSPR from an hour after sunrise until well after dark, and in summer through the night. Decodes come every two minutes, so give it at least ten minutes before judging. A receiver with a low noise floor shows the balloons and the milliwatt stations; any receiver shows the one-watt ones. The dial must be exactly 14095.6 kHz in USB: WSPR lives in a 200 Hz window centred 1500 Hz above the dial, and the timing has to be within a second or so of true time.

In ShackDeck

Station catalogue → Amateur radio → WSPR 20 m. The program picks a public receiver covering 14095.6 kHz, sets the WSPR mode with its narrow 1300 to 1700 Hz passband and runs the WSPR decoder on the audio. Every two minutes the decoded callsigns, locators and powers appear in the On air list and the stations land on the map, balloons included. No receiver, antenna or account of your own is needed.