[GreenKeys] OT -- sneak peek -- oops, forgot the pics

[Dave Horsfall]

On Wed, 17 May 2017, Jim Haynes wrote:

> Some guys here made a Doppler direction finder for hidden transmitter 
> hunting.  Consists of a pair of 2-meter dipoles spaced a foot or two 
> apart, and in the middle is a tiny little circuit that switches between 
> them to a cable that goes to a handheld transceiver.  When both antennas 
> are broadside to the signal source you hear nothing, but when they are 
> at different distances from the source you hear a tone.  I haven't got 
> my head around how this works, but the transceiver is FM, so if you hear 
> a tone I guess it's because there is a difference in apparent frequency 
> between the signal from the two antennas.

That's TDA - Time Difference of Arrival.  Basically the two wave-fronts 
arrive slightly out of sync, and they beat together to generate an audio 
tone.  It's really accurate; switching between them sort of simulates a 
rotating array, and with both dipoles the same "distance" away i.e. 
broadside on you get a deep null.

-- 
Dave Horsfall DTM (VK2KFU)  "Those who don't understand security will suffer."