This was a quick and dirty experiment to try and turn a Raspbeerry Pi into a shortwave transmitter. The aim was transmit a digital Weak Signal Propagation message (WSPR pronounced Whisper) on the 20 metre (14Mhz) Amateur Radio band using only a Raspberry Pi, a low pass filter and a length of wire as an aerial.
WSPR (Weak Signal Propogation)
WSPR implements a protocol designed for probing potential propagation paths with low-power transmissions. Normal transmissions carry a station’s callsign, Maidenhead grid locator, and transmitter power in dBm. The protocol is such that messages can be decoded 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. A live version of the map can be seen here.
The Raspberry Pi
There are numerous examples on the Internet of transmitting signals by simply attaching an aerial to the clock O/P pin of the Pi. However, for this experiment I used the WsprryPi software.
The WsprryPi software make light work of controlling the Raspberry Pi Clock output. The software controls the radio frequency, in this case 14.097120 MHz, and then modulates this with the WSPR radio data. Full details of how this works is given here.
The Low Pass Filter
The clock output is a square wave, therefore, a low pass filter is required to reduce the harmonic transmissions. This was constructed using this GQRP Club article as a guide.
The low pass filter was built using islands on a copper clad board. There are many examples on the Internet of how to build a suitable tool to create these islands, however, I find that a standard unmodified wood drill (the type shown in the photo) works very well and keeps things very simple. Start by drilling a 3mm pilot hole and then pop the wood drill into the hole and watch the island appear as the drill turns.
The image here shows the completed low pass filter with its input connected to the clock output of the Pi, and its output connected to a 5 meter length of wire laid out on the patio.
The WsprryPi software was used to transmit my call sign (G6AML) and my Maidenhead locator square (IO83XX). Within a very short time I was heard by DK6UG in Germany, EW1LN in Belarus and LA9JO in the north of Norway. Awesome!
Thank you to the many amateurs around the globe who have WSPR receiving stations setup. These stations send what they have heared, to a central database at WsprNet.org. The map is a great way to see how far these signals have traveled.