Radio Direction Finding

http://robowarner.com/portfolio/homing-robots/
http://robowarner.com/portfolio/radio-homing-robot-diy/
http://theleggios.net/wb2hol/projects/rdf/rdf.htm
http://theleggios.net/wb2hol/projects/rdf/tdoa2.htm
http://theleggios.net/wb2hol/projects/rdf/tdoa1.htm
https://www.qsl.net/k/kd8nxh//Tech%20Articles--%20From%20KB3JQQ.com/Tech%20Articles/Double-Ducky%20Direction%20Finder.pdf
http://web.archive.org/web/20050224191812/http://members.aol.com/homingin/hfinderfix.html
http://hamradio.uz/media/uploads/2018/04/19/arrl_antenna_book_21st_ed.pdf see Chapter 14

https://www.instructables.com/Arduino-Audio-Input/