The receiver is made from cheap software defined radio modules called RTL-SDR dongles. These are small USB stick shaped receivers that were initially designed for receiving digital TV signals. In 2009, some people at Osmocom figured out that the dongles have a special debug mode that allows them to send raw IQ samples to a computer via USB, making them attractive as general-purpose software defined radios.
In 2013, Juha Vierinen discovered that you can share a clock between two RTL-SDR dongles to get a coherent two-channel receiver.1http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html He used this to do some impressive passive radar measurements and later wrote a Hackaday article about them which was actually the original source of inspiration for this project. The clock-sharing modification involves removing the crystal oscillator from one of the dongles and connecting the oscillator output and ground nodes between the dongles. We found that it is also necessary to remove some capacitors in the oscillator circuit of the second dongle.
Here is a picture of the receiver module. The red and black wires carry the clock signal between the dongles. This is very questionable RF design but it seems to work.
The observation channel antenna is a 5-element Yagi-Uda antenna based on Brian Beezly’s design. The elements are made from 3/8″ solid aluminum rod, and the boom is made from 2″ OD ABS tubing. The elements can be removed and stored inside the boom when the antenna is not in use.
The gain and directivity of the reference channel antenna is less critical for the radar’s performance, so we used a simple half-wave dipole.