HF/DF aerials resembled birdcages and were mounted at the top of the mainmast on Allied convoy escorts and some merchant ships as well. The more ships which had HF/DF, the more accurately they could triangulate the location of German U-Boats even if the U-Boat only broadcast for a few seconds. Allied escort ships did this in concert with the large network of Allied onshore HF/DF installations.
Charles McCain is a Washington DC based freelance journalist and novelist. He is the author of "An Honorable German," a World War Two naval epic. You can read more of his work on his website: http://charlesmccain.com/ View all posts by Charles McCain