[R-390] HFDF

[Mark Huss]

I checked out the new HFDF Google group that had dropped a message on 
the SWL Google Group a few days ago.
Basic premise is to form a group that combines an HF receiver, a sound 
card, and a GPS receiver module to provide a time tic accurate to 1uSec. 
Combine them, with the receiver recording the audio from a signal, the 
1PPS from the GPS superimposed over it. By finding simultaneous 'events' 
from several such recordings taken by receivers in different locations, 
you can determine the propagation delay to within hundreds of 
microseconds. This will allow you to draw a equidistant circle, figuring 
~930 feet per microsecond. Collecting several of these, subtracting the 
processing delay of the receiver, you will get a fair idea of where the 
transmitter is.
Sounds workable if you are willing to live with errors of maybe 200 
miles or so.
I did something similar back in the early eighties using an R-390A, only 
I did it backwards. The problem I faced was getting a time hack as 
accurately as possible from HF time stations. VLF would have been best, 
but we did not have the capability at the time. Using an Oscilloscope 
triggered from the 1PPS from a Cesium Beam, I monitored several Time 
Standard Stations while watching the IF Out from the R-390A. From this, 
I was able to determine delta time between the received ticks and the 
Standard tick from the Cesium. Subtracting the delay through the 
receiver, and taking into account possible hops through the F-layer 
given distance and the height of the ionosphere, I was able to get 
sufficient accuracy to set the proper delay for the clock I was setting 
from the Cesium. One thing that aided me tremendously was the ability to 
monitor the IF out. And the stability of the R-390A.

Since most of us have R-390's already, as well as computers with sound 
cards, one only needs to buy the GPS module, which is running about $70. 
Connecting the GPS 1PPS to the sound card is as simple as capacitively 
coupling it to the audio out of, say the Diode Load.
Anyway, thought I would bring it to the attention of the group, since we 
do own the *Best* radio for the job. And if nothing else, it will while 
away the long winter months trying to figure out where those pesky 
Numbers Stations are. Or the 'Chinese' Woodpecker.
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Some people are like a Slinky .. not really good for anything,
but you still can’t help but smile when you shove them down the stairs.
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