PC with sound card
With use of the PC with sound card and a frequency stabile receiver, we can receive nowadays much weaker signals than in the past. All modes that are designed for use with weak signals are based on the same principle. They do work with long tone lengths so that you can use very small reception bandwidths. For QRSS signals (morse code with a very low speed), a dot lasts even 3 seconds. And for the digital mode WSPR, the tone length is 0,68 seconds. A disadvantage is that the frequency stability of the transmitter and receiver has to be very good. But are these new modes so much better than that good old CW? No! The data speed when using normal morse code is much higher than that of these special new low power modes. At 12 words per minute, a dotlength is only 0,1 second. For QRSS this is 30x longer! With normal morse code, you can exchange much information, even long stories like news bulletins. That is not possible with the special low power modes. In fact it are not more than just simple beacon transmissions. The only information that is transmitted is the call, possibly added with the QTH locator and the transmit power, nothing more. The exiting experiences are of course where in the world that weak signal has been received.

The WSPR screen

The abbreviation WSPR (say Whisper) means Weak Signal Propagation Reporter. Just as QRSS beacons, it is a mode that is intended to work with weak signals. The WSPR software is designed for the investigation of possible radio propagation paths with use of low power beacon transmissions. It uses modern techniques. Your transceiver is connected to the sound card of the PC and perhaps also with the RS232 connection. The WSPR software controls everything. During 20% of the time you are transmitting with low power in the WSPR mode of your call and QTH locator. During the other 80% you do receive other WSPR stations. It is the intention that the reception results are uploaded to a database on the WSPRnet site (but if you do not want that, it is not necessary). On the website of WSPRnet ( you can look in the database where in the world your station has been received, including a signal to noise report and some more data like frequency drift and time error of your signal. In the database you can also see which stations you have received. You can make these data visible on a world map and so it it is easy to see how the worldwide propagation is.

Database with reception results, see
and select "Database" right above on the screen.

Not a Fuzzy mode
Murray Greenman (ZL1BPU) explains on his website what Fuzzy modes are. That are modes whereby you have to decide yourself what kind of signs or text is received. You have to decode SSB and CW signals yourself by ear out of the noise and interferences. For QRSS signals (morse code with a very low speed), the PC does the digital signal processing and displays these sighals and the noise on the screen, but you have to decode the morse signals on the screen by eye out of the noise. For example, you have to decide if it is an interference, or a dot or a dash of the morse sign. WSPR is not a fuzzy mode, the PC does the signal processing and also the decoding of the signals and prints them in readable text.

Results can be made visible on a worldmap. Here just local conditions.
But mostly station from all parts of the world can be received.

How does WSPR work?
WSPR does work with tones with a length of 0,68 seconds. There are 4 different tone frequencies. (4 frequency shifts), but the difference is minimal, only 1,46 Hz! You do not hear those differences. A WSPR signal sounds like a continuous tone. A standard WSPR beacon transmission consists a call, a QTH locator and an indication for the transmit power. This message has a length of 50 bits.That it is possible to receive weak WSPR signals, is not only due to the long tone lengths. The message with a length of 50 bits is namely extended with even 112 bits, so even more than 2 times the number of bits of the original message. Totally no 50, but 162 bits are transmitted. And with these extra bits it is possible to detect and correct all kinds of errors in the 50 bits of the standard message. Similar techniques are used for the correction of errors in the data of memories and harddisks of our PC. And the bitorder is also changed to improve the error correction for interferences of the signal due to long fading periods. One long, uncorrectable error is converted in many short, correctable errors of the signal.

WSPR signal.

Synchronisation signal
Of course the PC has to be able to find the start of the signal. For this purpose, a synchronisation signal of 162 bits is transmitted simultaneously with the message. This is a known, defined bit pattern of 162 bits. As the WSPR signal has 4 frequency shifts, it is possible to seperate the synchronisation signal and the message. What the PC has to do is the digital signal processing and then looking for synchronisation signals. Of course there are also many interferences, but when larger parts of the audio signal are equal to the synchronisation signal, you may expect that it is a WSPR signal. Then the message is decoded and errors are (if possible) corrected. When the message does contain not correctable errors, then it is not decodec anymore. Only messages of which the errors could be corrected are displayed.

Hardware and frequency spectrum
For every band, a piece of the spectrum of 200 Hz width is assigned. A WSPR signal is only 6 Hz wide, so 33 WSPR signal do fit in this 200 Hz band. And because a WSPR station does transmit only 20% of the time and receives 80% of the time, the total number of active transmitters can be 5x as much or 165.
For the 10 MHz band the WSPR band lies between 10140.1 kHz and 10140.3 kHz. As WSPR operates with an audio bandwidth from 1400 Hz to 1600 Hz, your receiver (or from a direct conversion receiver your local oscillator) has to be tuned to 10138.7 kHz (20 Hz), or 1400 Hz lower than 10140.1 kHz. Unfortunately, I do not have a transmitter but happily I do have a receiver. I used the improved QRSS receiver for it (see this website) and the crystal oscillator is tuned to 10138.7 kHz by modifying the 10 pF of the serial capacitor of the crystal. By coincidence, this receiver was designed for an audio frequency of 1500 Hz, the center of the audio band of 1400 Hz to 1600 Hz.

The receiver for QRSS signals and WSPR. For WSPR reception
the local oscillator is tuned to 10138.7 kHz 20 Hz.

Time synchronisation
The hardware has been completed, the WSPR software is running on the PC. The software contains a detailed manual with much more information than here on this website and that also describes the WSPR signal in detail. Required is a PC with a clockspeed of 1500 MHz. But here the program is also running fine on a PC with windows 98 and a clock speed of 1200 MHz.
But WSPR signals do operate in blockis of 2 minutes. So before you start with the reception of WSPR signals and the WSPR transmissions, you have to synchronise your PC clock (an error of 2 seconds is acceptable). In the WSPR manual is explained houw you can do that automatically via the internet with use of time servers. But I do use a Radio controlled DCF77 clock. An adjustment of twice per day is sufficient.

Radio controlled DCF77 clock for time synchronisation

WSPR signals are displayed on the screen!
At "dial" fill in the frequency of 10.138700 MHz, at TX 10.140200 MHz, even if you are just receiving WSPR signals. When you are only receiving and not transmitting, set the knob "TX fraction" to 0, otherwise to 20%. And at setup you have to fill in your call and 4 character QTH locator "AB12", even when you are just receiving. After a while the text "Waiting to start" in the textbox left below will change to "Receiving". Adjust the audio level so that the RX Noise (textbox left below) is between -10 dB and +10 dB. Remove the "Idle" sign and after a few minutes the stations will be displayed. It becomes however really interesting when you do activate "Upload spots" and your PC does have an internet connection. The stations that you do receive are uploaded to a central database that you can display via internet. In this database you can select that you only want to see the station you have received and there are many more selection criteria. The station you have received can also be made visible on a worldmap. During the daytime, my receiver is switched on quite often and then I can see everywhere (on my work for example), in the database which stations I have received. And when you also transmit in WSPR, you can look in the database where and who did receive your WSPR signal. Most WSPR signals are 5 watt or less and have been received wordldwide. Nice! In the database, also a reception report is displayed as signal to noise ration. WSPR signals can be decoded by a signal to noise ratio of -28 dB. When your report has a signal to noise ratio of -8 dB, the signal can be reduced by 20 dB or 100x in power. And some more data is displayed in the database: frequency, frequency drift, time error, power, direction and distance.

Dr. Joe Taylor, K1JT
We have to thank Dr. Joe Taylor (K1JT), a Nobelprice winner and professor for this digital mode. He has designed more digital modes like WSJT, that is used for moon bouncing. On internet you can find much more information about Joe, his work and the digital modes.