I'm a ham radio operator and I run a WSPR beacon using a RPi, a TAPR-QRPi shield, the WsprryPi software and a random wire antenna. The RPi generates an HF signal on GPIO_4 at around 14 MHz (20 meter short wave band), the TAPR-QRPi shield filters out the harmonics and amplifies the carrier to around 200 milliwatts. Using a 12 feet wire on my balcony I get automated reception reports from 300 to 2800 miles away.