Radio direction finding (RDF) or T-hunting or fox hunting has been a hobby in the ham radio world for a long time. If you've ever seen a car equipped with a big, crazy looking antenna, a square antenna array or people running around with funny antennas and radios, they are probably T-Hunting. If it's a weekend it's probably hams and a weekday someone looking for a transmitter for another reason.
T-hunting is still a thing with communities of hunters. In San Francisco there is rdf-sf and there may be a club in your neighborhood. The rdf-sf site is a great source of information and there's also http://www.homingin.com.
Do a Google search for Doppler Direction Finding for theory of operation. The May and June 1999 QST issue with WA2EBY's article on a radio direction finder is based on simulated Doppler. For even more information find a copy of "Transmitter Hunting - Radio Direction Finding Simplified" - available in dead-tree form only. The 1999 article is an update to WA4BVY's early 80's design. It's been 20 years so a technology update is overdue.
I'm using four antenna that are electronically switched to impart a Doppler tone FM modulated on the received signal. The Doppler tone is isolated with a very narrow band-pass filter and the phase of the Doppler relative to the switched antenna is measured to determine direction. The relative position of the signal source is derived from the difference.
The WA2EBY DDF is built with nine chips (74HC), a couple of transistors, a voltage regulator and a bunch of passive parts. I am using the same idea for the antenna switching but trying to do the instrumentation with many fewer parts. WA2EBY's used cheap 1n4148 because at the time PIN diodes could be over $10 and the design needs 8. PIN diodes have become commodity parts and BAP64-03 PIN diodes I'm using are $0.09. I am using a Teensy 3.2 ($20) for most all the logic and filtering. I have just gotten started with the Teensy Audio Library. I am using 16 Bi-Quad filters but have not gotten around to characterize performance yet. The code is pretty simple and sloppy. It mostly "rotates" the antenna and measures the zero crossing of the filter output.
The design uses a 16 element Neopiixel display - the Neopixel is a nice update of the '80s ring of 16 plain old blinken' LEDs. I am using a single white pixel to point direction and the rest of the array to display signal quality.
I am using a cheap 0.5 W FM transmitter module for my test signal. I fired everything up and the display was crazy random. I have come to learn that was to be expected. I am testing in a small office with metal filing cabinets so there are lots of reflections. Even my movement in the office makes a difference. I did some testing on the major functions and everything looks right. I'd like to test with a RF signal generator that somehow was synchronized to the antenna switching. I tried putting together a gnuradio flowgraph but I've yet to find a PC fast enough to make it work. I found a simple way to test the basic phase measurement by using another Teensy to delay the antenna switching pulse to a comparator output. Just getting a stable delay out of the tester was a challenge.
I plan on testing outside to operate in an open area without so many reflections. As visible in the photo, the bread board isn't travel worthy. Next up is redoing the the antenna switcher board with edge mount sma connectors and a new design for the antennas. The antenna cookie sheet is deep enough for the PCB to go in underneath. BTW - the antenna shown is for UHF. Most serious T-Hunting goes on in the VHF band...Read more »