When the war started, Britain’s MI6 ran the Radio Security Service. Its original purpose was to intercept and find transmissions from enemy agents in the homeland. RSS pretty much shut down spy operations, in many cases turning captured spies into double agents by MI5.
Radio Security Service not only did civilian intercept, it used more than 1500 ham radio operators as volunteers. With the cooperation of Radio Society of Great Britain, MI8 and MI6 recruited these Volunteer Interceptors, which became a powerful force. Surprisingly, the activities of the VI remained secret until the 1970’s.
By 1941, Volunteer Interceptors were turning in 10,000 message reports a day. Perhaps their most significant achievement was discovering and then monitoring secret networks run by German War Department Military Intelligence in Europe. In effect, the ham volunteers monitored enemy intelligence.
Most Volunteer Interceptors operated from home using their own equipment. They were assigned frequencies between 3 to 12 MHz and found much enemy activity between 4 to 9 MHz.
As a side note, neighbors often thought the VI were spies, given their strange activities with radios. Each VI carried documents to explain they were on special duty.
Civilian Intercept – FCC Radio Intelligence Division
Established in 1934, the FCC Radio Intelligence Division regulated radio spectrum in the U.S. RID took on a more important role after Pearl Harbor. Almost all FCC civilian intercept was done by amateur radio operators who worked for the FCC. They were organized in three units based in Washington, San Francisco and Hawaii. Facilities included sixty fixed and ninety mobile units.
Initially focused on spies, RID found very little enemy transmissions in the continental United States. More spies were found in the territories and other countries, particularly Central America. Staff made thousands of DF bearings every month. The Washington unit concentrated on European Theater. In 1942, they started cooperating with Britain’s RSS monitoring German spy networks in Europe.
Reception of German military units in North Africa was good on the U.S. East Coast. RID played a role supporting troops in the desert campaigns. They also continuously monitored German weather reports broadcast on frequencies that skipped over England. This meteorological information supported proper planning for Allied bombing in Europe.
Meanwhile, the west coast and Hawaii units specialized in high frequency direction finding of Japanese naval activities in the Pacific. You might enjoy reading The History of the Radio Intelligence Division Before and During World War II by George Sterling.