THE ADF NAVIGATION RADIO
The ADF (Automatic Direction Finder) is used in conjunction with Non Directional Beacons, (N
DBs), sited on the ground. The ground radio stations work in the medium and long wave section of the radio spectrum.
The ground station equipment is simple and cheap to install and thus is used very extensively in developing countries of the world. The more established countries tend to install VOR/DME ground stations at a much greater cost, but with the added security and accuracy that they bring.
The technology for the NDB/ADF system was first used in aircraft between the two World wars. The equipment in the aircraft can detect the direction from which the radio signal is coming. In the early days this was done mechanically by the navigator turning a directional aerial around by hand. The modern equipment can detect the source of the radio signal electronically.
When the equipment detects the direction of the ground station, it will turn a needle on the instrument dial to point at that station with respect to the nose of the aircraft. Therefore, if the aircraft turns onto a new heading, the needle will move around to stay pointing towards the ground station, very much like a compass always pointing towards north.
The system has many disadvantages, probably the worst one is that it can be easily fooled by a thunderstorm. The indicator needle will prefer to point at the storm rather than the relatively low powered ground station, taking the unwary pilot into a dangerous situation!!