Intellgence agencies when they did HFDF used huge antenna arrays
called CDAAs (Circular Disposed Antenna Arrays). They were also known
as Wollenwebbers (presumably after an inventor). These arrays were
often very large, and sometimes were affectionally known as "elephant
cages." The diameter of these arrays could be as small as 50-60 feet
for a tactical unit to several hundred feet for large fixed sites in a
secure area. Each circle had many vertical elements.
They measured time difference of arrival by measuring the wavefront
timing on each antenna in the array as the wave passed through the
array. A computer then collated this information to calculate a
bearing. The readouts varied depending upon the era. Some were
digital, some were on an oscillicsope. I'm not quite sure how the
really early ones worked (before my time). The results of several
stations were then combined to get a "fix." Contrary to popular
belief, their accuracy had significant error factors so despite many
requests from military commanders who wanted to drop ordinance on a
target, you really couldn't do that based solely on HFDF. HFDF was
really originally intended for open ocean surveillance against things
like German subs in both WW I and WW II.
You could get a rough idea where a signal was coming from and then
you would have to sent a destroyer or aircraft to actually locate the
sub. There are other types of HFDF antennas as well, but none can
give pin point accuracy unless you are mobile and close in as in
transmitter hunts. As long at there was energy, you could get a
bearing be it a sustained carrier or a single dit. You would have to
rely on other externals (callsigns, radio finger printing etc) to
figure out who was sending a given signal.

W3JT


On 18 Feb 2005 06:29:34 GMT, (Martin Potter)
wrote:


"RB" ) writes:

Wonder if loop antennas were what they really used, and how good they were?
The spies generally used cw. How do you get a fix on the short dots and
dashes?


With AGC turned off, just listen for the null in the signal as the loop is
rotated.