This month’s Gadget Freak comes courtesy of investment banker Hans Summers. He built a simple receiver capable of detecting QRSS transmissions on a fixed frequency. The QRSS receiver is powered by a computer’s USB port and the audio output feeds into the PC’s sound card. Hans used a toroidal transformer as matching and input filter, connected directly to a 30m (10MHz) dipole antenna and a useful oscillator/mixer IC as a crystal oscillator and mixer. Link.
Build a QRSS (extreme slow speed continuous wave) transmission detector