RTL-SDR phase correlative direction finder

[IK0ZCW]

Testing direction finding by measuring phase difference between 3 antennas using 3 coherent rtl-sdr receivers.

The antenna is a uniform circular array of monopoles spaced by 1/3 wavelength. Better picture of the antenna prototype: http://prkele.prk.tky.fi/~peltolt2/DSC_2710.jpg

Each rtl-sdr is fed from the same reference clock to make their phase shift remain constant. They still have unknown phase shifts and sampling time differences relative to each other. This is calibrated by disconnecting them from antennas and connecting every receiver to the same noise source. Cross correlation of the noise gives their time and phase differences so that it can be corrected. Currently the signal is received and processed in short blocks with each block starting with a burst of calibration noise.