WSPR on 20 metres, 14.0956 MHz: low-power beacons that map propagation

What you hear

Almost nothing. With the receiver on 14.0956 MHz in USB there is the hiss of the band and, if you listen closely, a few faint, thin whistles between 1400 and 1600 Hz, each lasting a little under two minutes and starting on an even minute. On the waterfall they are hair-thin vertical threads, only 6 Hz wide, that drift by a hair as they go: four tones spaced 1.46 Hz apart carry the message. All the traffic of the mode fits into a band 200 Hz wide, and most of the signals in it are below the noise.

Decoded, every thread becomes one line: a callsign, a four-character grid square and the transmitter power in dBm, where 37 means 5 watts, 30 means 1 watt, 23 means 200 milliwatts and 10 means 10 milliwatts. Nobody answers anybody. The stations are beacons, each transmitting on a timetable of its own, a slot or two in every ten minutes, and listening the rest of the time.

Who transmits and why

Radio amateurs, with the smallest transmitters in the hobby: a few hundred milliwatts into a wire is typical, and purpose-built WSPR beacons the size of a matchbox are common. The mode comes from the WSJT project and the name stands for Weak Signal Propagation Reporter. The purpose is not contacts but measurement: every receiving station reports what it heard to a shared database, and the result is a live map of which paths on which bands are open at this hour, down to a tenth of a watt. A watt from Europe reaching Australia is an ordinary WSPR day near the solar maximum.

The same mode carries a second kind of traveller. Small helium balloons weighing a few grams fly around the world at twelve kilometres with a solar-powered WSPR transmitter, encoding their position and altitude in the callsign and grid fields; their tracks are followed for months by listeners on the ground.

When and where to listen

Day and night. 20 metres is open to some part of the world at nearly every hour, and WSPR shows it even when nobody is making contacts: Europe and Africa around midday, the Americas in the evening, Asia and Oceania in the morning. Decodes arrive in batches at the end of every even-minute slot, so the first ones take up to two minutes to appear, and a quiet batch does not mean a dead band, only a quiet slot. Any receiver with 20 metres works, and a receiver far from home shows paths you cannot see from your own location. The decoder needs an accurate clock, because the two-minute slots are timed by UTC; keep the Mac's clock synchronised.

In ShackDeck

Station catalogue → Fax and digital signals → WSPR 20 m (14.0956). The program picks a public receiver with 20 metres, tunes it to 14.0956 MHz in USB and sets the WSPR mode; no receiver, antenna or account of your own is needed. Every two minutes the beacons heard in the last slot appear in the decode list with the callsign, grid, power and signal-to-noise ratio, and each one lands on the map in its grid square with its flag, marked as a WSPR station, with the distance and bearing from your own station. Everything heard goes into the Heard log. Give it twenty minutes and the map fills with dots from every direction the band is open to.