RF Direction Finders
A radio frequency or RF direction finder, as it is commonly known, is a device that allows you to find the geographical direction or bearing of a radio signal. Measuring the direction of a radio source is known as radio direction finding. At its root, this process involves using two or more measurements from separate locations, so as to triangulate the location of an unknown transmitter can be determined.
Low Frequency Systems
An RDF system can be used to find the location and bearing of any radio source, however the size of the receiver antenna, depends on the wavelength of the signal. If the wavelength is very long, and thus has a low frequency, a very large antenna will be required, and is typically only viable in a ground based set up. The reason for using these longer wavelengths are useful for marine communication and navigation because they can travel longer distances, and even over the horizon.
High Frequency Systems
Aerial vehicles, on the other hand, can extend their line of sight and stretch the horizon to hundreds of miles, a higher frequency can allow for smaller antennae to be used on vehicles where space may be at a premium. Today, an automatic bearing finder, which is typically tuned to a commercial AM radio broadcast, is a feature on nearly every modern aircraft.
When it comes to military applications, an RDF system is a primary component of signals intelligence operations and methods. Since World War I, systems intelligence analysts have found the ability to locate the position of an enemy’s radio broadcast invaluable. This capability has played a vital role in modern warfare, from the Battle of the Atlantic, to hunting U-Boats, to the Pacific Theater. Today’s systems use phased array atnennas that allow for more accurate results, and are usually integrated into a greater electronic warfare program.
The technology that allows radio bearing finding to work, is the comparison of different signals relative strength, as received by a directional antenna pointing different directions. Obviously the signal will be stronger when the antenna is pointing directly at the source of the radio broadcast. When first developed, this was widely used by land and sea based radio technicians, using a simple rotating antenna that is connected to a signal degree indicator. Later, this technology was developed from a manual rotation to an automatic direction finding or ADF.
One primary application of an ADF is to use it to home in on a particular station. Homing involves flying an aircraft on a constant heading in order to keep the compass needle pointing directly at 0 degrees (or straight ahead). To home in a station, the radio technician will tune the broadcast, identify the signal, and then the pilot will turn the aircraft until the ADF needle points to the 0 degrees position.
Another Application of ADF is to use it for tracking a desired travel course, and allows for winds that may blow the aircraft off course. A good pilot will calculate the correction angle, after being blown off course, in order to balance any unexpected cross winds.