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WSPR Explained: Using 200-Milliwatt Beacons to Test Antennas and Map Propagation

2026-10-18 · Digital Modes

A mode that is not a contact

WSPR, pronounced “whisper,” stands for Weak Signal Propagation Reporter. The name is an unusually honest description: it reports propagation, and it does nothing else. There is no calling, no answering, no signal report exchange and no conversation. You transmit, and then you wait.

The transmission itself is almost absurdly small. Every two minutes, on even UTC minute boundaries, a WSPR station keys up for 110.6 seconds and sends a single narrow 4-FSK signal carrying exactly three pieces of information: callsign, Maidenhead grid locator, and transmitter power in dBm. Then it goes quiet until the next slot.

Any WSPR receiver anywhere on the planet that decodes your signal automatically uploads the reception to a central database at wsprnet.org — the other station’s callsign, frequency, timestamp, measured SNR, the power you reported, and the great-circle distance between the two stations. Minutes later you can open a map and see every station that heard you, each one pinned with its own SNR figure.

It is not a QSO. It is a measurement of the path between two antennas, and the entire global community of receivers acts as your instrument.

How a signal fits into six hertz

WSPR was designed by Joe Taylor, K1JT, and first released in 2008 as part of the WSJT family of weak-signal programs — the same lineage that later produced FT8. The engineering is worth understanding, because the parameters explain why the mode is so useful for antenna work:

That combination — enormous redundancy, extremely slow keying, and power concentrated into a handful of hertz — is what lets receivers pull your beacon out from under the noise floor. It also makes WSPR a sensitive probe of everything in your signal chain. If a feeder connector is corroded or an antenna is mistuned, the effect shows up in the reported SNRs immediately, because there is almost no transmit power to mask it.

One detail that trips people up: stations with compound callsigns, or six-character grid locators, need a two-transmission sequence to carry all the data. The software handles this automatically, but it means your first slot after starting the program may not appear on the map right away.

The clock requirement, again

Like every mode in the WSJT family, WSPR depends on precise timing. Transmissions begin one second into even UTC minutes, and receivers look for the signal at fixed offsets from those boundaries. If your computer clock is off by more than about a second, you will not decode anything, and nobody will decode you.

Modern operating systems sync over the internet by default, so this is usually a non-event at home. It becomes real in the field: a laptop that has been offline for a week can easily drift by seconds. Check against a time reference before you start, and if you want millisecond-level discipline, a USB GPS receiver makes an excellent cheap reference.

What WSPR is actually for

The mode’s most practical use is antenna evaluation, and the method is beautifully simple. Run WSPR for a defined period with one antenna. Change the antenna. Run the same period again. Compare the SNR reported by the same set of receiving stations.

Because the transmit power is low and fixed, the cycle length is identical, and the reporting population is large, the comparison is far more trustworthy than an on-air “you sound stronger” impression. Run it for 24 hours on one band and a single path can yield around 720 reception reports — thirty slots per hour times twenty-four. With hundreds of data points, short-term propagation luck averages out and what remains is your antenna.

The kinds of questions the data can answer include:

The method has a second use worth remembering: frequency stability and transmitter accuracy checking. Because WSPR decodes signals a few hertz wide, a drifting or off-frequency transmitter will either fail to decode or show a consistent frequency offset in the reports.

Running it backwards

There is a free second station hiding in this. Tune any receiver running WSPR and you become one of the measurement points in other people’s experiments. The map of what you hear tells you which directions your receive system works well in, which is the same information you would otherwise need a calibrated test setup to obtain. Several operators I know keep a WSPR receiver running permanently on a Raspberry Pi with no transmitter attached at all, purely as a propagation monitor.

WSPR and its cousins

WSPR is not the only tool that maps where your signal lands, and each has a different job:

In practice these work as a sequence: an antenna analyser confirms the antenna is electrically sound, WSPR quantifies its performance at low power, and PSK Reporter or the RBN verifies the result under your day-to-day operating conditions.

A first session, step by step

The setup is undramatic. Install WSJT-X — free, and available for Windows, Linux and macOS — or the dedicated WSPR program. Connect the transceiver to the computer through its USB sound card, or an audio interface if the radio predates built-in audio. Verify the clock. Select WSPR mode, enter your callsign and grid, set the radio to a low power level, and check that the antenna’s SWR is sane before transmitting.

Then pick one band, let it run for fifteen to thirty minutes without touching anything, and look yourself up on wsprnet.org. Change one variable — band, antenna, or power — and repeat. The interface rewards patience: the first run tells you almost nothing, and the fourth run starts telling you the truth.

One last caution, the same one that applies to every weak-signal mode. A single two-minute transmission is a snapshot of propagation, not a measurement of your antenna. To compare antennas, compare evenings or full days. Only when the data volume is large enough does the ionosphere stop lying to you.

References

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