The days when software defined radio techniques were exotic are long gone, and we don’t miss them one bit. A case in point: [Laakso Mikko’s] research group has built a multichannel receiver using 21 cheap RTL-SDR dongles to create a phase coherent array. This is useful for everything from direction finding and passive radar or beam forming. The code is also available on GitHub.
The phase coherence does require the dongle’s tuner can turn off dithering. That means the code only works with dongles that use the R820T/2. The project modifies the dongles to use a common clock and a switchable reference noise generator.
A single coupler can accommodate seven dongles for a cost of about $180. The team has used up to 5 of these to create arrays of 35 receivers. The processing is a bit math-intensive and some of the code is in Matlab.
In addition, some of the paper deals with increasing performance of sparse arrays of receivers to approach the performance of a fully-populated array. Obviously, even at this low price, fewer receivers will be desirable for cost, space, and power concerns.
Of course, not everyone needs dozens of channels of phase coherent receiving, but if you do, it is good to know an inexpensive way to get there.