Radio Direction Finding - TDOA

This article describes a few projects that I built from plans for Amateur Radio Fox Hunting.

Basically, the hobby/sport is for someone to hide a few low power "Foxes", which are small transmitters that beep or send Morse Code, and then have a bunch of HAM's try to find them.

The first project that I built was a Yagi Antenna (Directional receiver) and a 4MHZ Offset Attenuator.  The attenuator is needed when you get close to the fox and you need to reduce the power of the received signal. 

I struggled with this setup because, on my first 2 fox hunts, there was too much reflection and multi-path interference.  This causes false direction detection and if you are not mentally prepared for this, you want to throw all of your equipment into a garbage can and take up a different hobby.  

There are some truly strange individuals who find this frustration fun and enjoy making other people suffer, but I will leave that discussion for another day. 

Anyway, in one particularly frustrating hunt, when the fox was hidden in the trunk of a car next to an apartment building, one of my peers walked right up to the fox and identified where it was using a TDOA detector.  (Time Difference of Arrival).    I was hooked. 

The Theory of TDOA is that you have a simple electronic project and 2 identical antennas.

The TDOA detector switches between the 2 antennas at a speed that is in the audio band, and you listen to the received signal on an FM radio (2 Meters works perfectly for this).  146MHZ. 

If the 2 antennas are exactly the same distance from the source, you hear a perfectly clean signal.  But, if there is a slight different in the distance to the fox, as in when the detector is point off to one side or the other, the difference in the received signals is detected as an FM phase shift and emits an clear tone on the radio.  

The first project that I built was from this article:  http://www.146970.com/PDFs/TDOA%20Phasenpeiler.pdf

See https://youtu.be/VsiD8Vr8Y4I for a demo.  

This project works pretty well, but has one flaw.  You only get a null signal along a line, where the fox could be either in front of you, or behind.  You do not know which.    You need to move to the side some distance and then take a second reading.  

The next project that we are building is an improvement on the TDOA detector, but uses the actual phase of the signal to actually tell you the Direction of the signal.  

I am documenting this build separately because there is a lot of information.  The build article will be complete enough for you to build your own if you wish.