The Kolster radio compass was a carefully guarded World War I secret. Developed by Swiss-born engineer and inventor Frederick Kolster (1883-1950) while working for the National Bureau of Standards, it played a vital role in the search for German U-boats by U.S. destroyers. In 1921, Kolster joined Federal Telegraph Company, which continued to develop and manufacture his radio compasses for commercial use.
The antenna was either looped around in a circle, or in the shape of a hollow square, and mounted on a ship. As the antenna rotated in a slow circle, the strength or weakness of the signal received indicated the direction from which the signal was coming. A ship lost at sea, or in fog or bad weather, could listen to radio beacons from coastal lighthouses and lightships and find its way. It could also be used for ships to find each other at sea, by taking a bearing on their radio transmitters.