WSPR (pronounced "whisper", for Weak Signal Propagation Reporter) is not for conversation at all. A station transmits a tiny beacon with its call sign, location and power, and any station that decodes it uploads a report. The result is a live map of where a given signal is reaching, from almost no power.
How it works
The beacon never talks to anyone. The listeners' reports are the result.
The signal is extremely narrow and very slow, so a receiver can integrate it over nearly two minutes and pull it out from deep in the noise. Every message is the same shape: who you are, where you are, and how much power you used. Because the transmit power is in the message, a listener's report includes both how strong the signal was and how much power produced it.
That makes WSPR a quick answer to "is this band open, and how well does my antenna work?" Leave a low-power beacon running, and see which stations around the world hear it.
Power in dBm
Power is given in dBm, decibels relative to one milliwatt, so a milliwatt-level beacon and a transmitter of a few watts fit the same scale. See Decibels.
Power
dBm
1 mW
0
10 mW
10
100 mW
20
1 W
30
5 W
37
In practice
Both the transmitting and the listening station need accurate computer clocks, since slots start on fixed even minutes.
A station cannot listen while it transmits, so it alternates slots. Many people run receive-only to study conditions.
It is one-way: nobody replies, and the "contact" is the report that arrives on the map.
Use low power and modest audio levels, and treat the transmitter as in constant use during its slot.