WSPR - digging deep into the noise

"WSPR (pronounced "whisper") stands for 'Weak Signal Propagation Reporter'.

It is a computer program used for weak-signal radio communication between amateur radio operators.
The program was initially written by Joe Taylor, K1JT, but is now open source and is developed by a small team.
The program is designed for sending and receiving low-power transmissions to test propagation paths on the MF and HF bands.

WSPR implements a protocol designed for probing potential propagation paths with low-power transmissions.
Transmissions carry a station's callsign, Maidenhead grid locator, and transmitter power in dBm.
The program can decode signals with S/N as low as -28 dB in a 2500 Hz bandwidth.

Stations with internet access can automatically upload their reception reports to a central database called WSPRnet, which includes a mapping facility."

This is a very good description, which I have taken over from Wikipedia.

So what is the trick ?

Remember Claude Elwood Shannon:
The more of a signal is known (e.g. frequency and time of start of the transmission), the lower SNR you can afford for correct reception.

So, a critical point to WSPR is time- and frequency synchronization of your rig.
 
- If you use a PC for modulation/demodulation and a standard TRX, you should have your PC periodically synchronized to a time server.
  Dimension 4 is a free program that I can highly recommend for that purpose.

- In case you use a dedicated stand-alone rig, you should consider an additional GPS receiver.


Find an overview of the WSPR TX setups that I have tested in the past 5 years up to now:
rating
 quality of output signal
 synchronization
 power consumption
 antenna outputs
 ease of setup
++
 stand-alone TRX
 GPS
 Raspberry Pi with WsprryPi
or
QRP labs Ultimate
up to 5:
QRP labs Ultimate 3
 PC-based solution
with standalone TRX
o
 QRP labs Ultimate 2
or
QRP labs Ultimate 3
 NTP over permanent internet connection
stand-alone TRX
 Raspberry Pi with WsprryPi
-
 Raspberry Pi with WsprryPi
PC-based solution
with standalone TRX
 1 output only :
QRP labs Ultimate 2
or
Raspberry Pi with WsprryPi		 QRP labs Ultimate 2
or
QRP labs Ultimate 3


See below what can be achieved using a modest setup:

My current hardware consists of a Ultimate 2 kit by Hans, GØUPL in connection with a GPS module from Sure Electronics.
This is a very compact setup for PC-less allband-operation.
The power is fed into a quarterwave whip on a bamboo stick out of the kitchen window:

  


Mileage highscore:
Band used power (equivalent isotropically radiated power) mileage in km per Watts EIRP
28 MHz 0.002 W EIRP 3 889 000 km / W EIRP  (tnx KM4LK !)
28 MHz 0.002 W EIRP 3 580 000 km / W EIRP  (tnx KZ8C !)
28 MHz 0.002 W EIRP 3 373 500 km / W EIRP  (tnx W3CSW !)

ODX:
Band used power (equivalent isotropically radiated power) mileage in km
28 MHz 0.2 W EIRP 18 098 km (tnx ZL1RS !) via short path
28 MHz 0.2 W EIRP 21 976 km (tnx ZL1RS !) via long path

Reception reports, received so far:

10 m - ca. 200 mW into a horizontal quarterwave:
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