High frequency direction finder, or HF/DF, or Huff-Duff. This is pretty much finding the enemy using his own radio transmissions against him. This is how the Royal Navy was able to locate the Bismarck the day after the Battle of Denmark Straight.

It is actually pretty straight forward. You listen for enemy transmissions. Using multiple sources you listen for the bad guys to broadcast a message. Then while listening to him you use your system to lock in on his signal bearing. When two sources have it you draw a line on that bearing and where they intersect is where the bad guys are at.

The listening device has a movable antenna. The bearing of the signal will be found by slowly rotating the antenna and checking signal strength. When it is strongest you have the bearing. So after you did this on your ship, plane, or ground station you drew a line down that bearing. Then another listening station did the same. Then both stations would use the other bearing and draw the second line. The intersection would give you a very good place to start looking for the target. Doing this is called, triangulation.

In the case of the Bismarck the captain send a 3 minute message back to Berlin telling of his victory the day before. This gave the British plenty of time to track and locate him. Plus when more than two stations give a bearing the location can be more accurately determined.

Just another way that those geeks back in the labs can help counter the brute force of the enemy. We shall return Tuesday!