Oceans are huge. Germany used a lot of radio to organize its U-boats over great distances. At sea, individual submarines announced convoy locations and coordinated attacks over wireless. All of these signals were prime targets for direction finding.
Shore-based fixed DF stations and Ultra did a good job of finding U-boats in general. Convoys were re-routed in real time. However, ship-borne DF was needed to get specific locations and attack formations. This localized direction finding let escorts attack and destroy the U-boats.
Take a look at this video. It provides a great overview of how direction finding was combined with Ultra information in the Battle of the Atlantic.
Wireless Intercept Fighting U-Boats – Technology
As World War 2 started, Allied Admirals had to choose the best technology for spotting U-boats. Should ships be equipped with the newly developed ship-borne radar or the older but proven HF/DF. Both needed antenna locations that were high and well clear of interference from superstructure. These prime masthead locations were scarce. In 1939, many fighting ships had their Huff Duff antennas replaced with radar dishes. Unfortunately, early radar proved tricky to operate and had limited range. It was more useful for spotting aircraft. On the other hand, U-boats were giving off tons of radio signals that could be tracked over great distances. So, the HF/DF was reinstalled by 1941. (Where possible, they kept radar as well.)
Improved Huff-Duff, like the Watson-Watt system, proved effective and relatively easy to operate. Big Adcock antennas did not work on ships. Instead, smaller 3-5 foot crossed-loop antennas did the job. Bearings could be taken visually on an oscilloscope within a few seconds. Operators learned to differentiate between sky and ground waves, the latter indicating a close location. Check out detailed descriptions of the FH3 and FH4 systems used during the Battle of the Atlantic. As well, you might be interested in most naval DF systems from both wars.
There was a problem, though. Uneven amplification and phase delays in the systems, as well as reflections off nearby metal, caused bearing errors. Each ship had to calibrate its HF/DF gear regularly. What’s more, the systems had to be calibrated for every frequency! Fortunately, U-boats typically used only 40-50 different frequencies. Operators had a big book of calibration adjustments. Operating Huff Duff required lots of skill and discipline.
Wireless Intercept Fighting U-Boats – Counter Measures
Towards the end of the war, Germans developed defensive tactics against direction finding. They increased radio silence. U-boats were ordered to submerge and move quickly after each transmission. They were told never to do wireless while refueling. Technically, they tried to jump frequencies. Mainland base stations would transmit on four different frequencies, and send a code word providing an offset frequency for responses. Submarines picked the strongest frequency, and dialed in the offset to respond. The silent listeners figured this out quickly.
At the convoy locations, U-boats adopted VHF R/T for local communications. Allies equipped their escorts with VHF receivers and German speaking operators. Near the end, they tried sending very high speed bursts of Morse Code. At 600 words per minute, this technique worked quite well, but too little, too late.
U-boats also did a lot of signals intelligence. When America entered the war, their shore-based radio discipline was poor. Submarines were able to track convoy formation in or near ports. Eventually, convoy ships were banned from making wireless transmission for two days before sailing. But when they sailed, they would tune up their radios using antennas rather than dummy loads. Finally, convoys and escorts started using VHF for local coordination, not realizing these signals could travel hundreds of miles under the right propagation conditions.