[TowerTalk] pattern measurements with a drone

[Jim Lux]

I'm in the middle of testing a scheme at work where we use 3 RTL-SDRs and a RPi with 3 whip antennas at 90 degrees from each other.
If all you want is amplitude, that's all you need. 
If you want phase and direction of arrival, then you need to sync the RTL-SDRs, either by driving with a common clock, or by using a sync calibration signal.  You use the RTL-SDR in "direct" mode which bypasses the front end down converter. A LeoBodnar GPSDO with 2 outputs can make a nice pulsed signal at any frequency as a cal reference. You use the 1pps output on the IF port of a mixer to modulate the CW tone. 
Then you get nice 100 millisecond long pulses at a known frequency, exactly one second apart, so you can calculate the oscillator frequency offset, etc.