Back in October 2015 we posted about a piece of software for the Raspberry Pi called PiTX. PiTX allows you to turn your Raspberry Pi into a fully functional RF transmitter. When combined with an RTL-SDR a full transceiver radio can be built using the QTCSDR software.
PiTX works by modulating the GPIO pins on the Pi in such a way that it is able to produce FM modulation. The major problem with using this method of producing radio is that it creates large amounts of harmonics and interference outside of the intended transmit frequency. Interference like this is illegal and could potentially disrupt life critical radio systems such as emergency services, cellphones and air traffic control.
In order to cleanly transmit with PiTX an output RF filter should be used. Recently, the team over at TAPR.org have released a 20M WSPR TX filter shield. WSPR is pronounced “Whisper” and is short for “Weak Signal Propagation Reporter Network“. It is a type of amateur radio signal that can be broadcast and received around the world by using very low transmit power. Radio amateurs use it to see how far their signal can travel when using very low power (QRP) and to investigate signal propagation conditions.
The 20M WSPR shield sells for $20 at www.tapr.org/kits_20M-wspr-pi.html.