A radio direction finder (RDF) is a device for finding the direction to a radio source. Due to radio's ability to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships and aircraft that might be some distance from their destination (see Radio navigation).
Radio Direction Finding works by comparing the signal strength of a directional antenna pointing in different directions. Conventionally the antenna is rotated and the operator listens for the direction in which the signal from a known station comes through most strongly or most weakly. This system was widely used in the 1930s and 1940s. RDF antennas are easy to spot on pre-World War II aircraft, being circular loops under the rear section of the fuselage or above the cockpit. Later installations enclosed the antenna in a teardrop shaped fairing.
In one type of automated system the antenna is spun by a motor. The electronics listen for the trough that occurs when the antenna is at right angles to the signal. A small lamp is attached to a disk that spins at the same speed as the antenna; when trough is detected, the lamp flashes briefly appearing as a single spot of light on a compass rose. Modern Automated Directing Finding, widely used in aircraft navigation, is described below.
RDF was once the primary form of aircraft navigation, and strings of beacons formed "airways" from airport to airport. In the 1950s these systems were augmented by the VOR system, in which the direction to the beacon can be extracted from the signal itself, hence the distinction with non-directional beacons.
Today many NDB have been decommissioned in favour of more accurate and user-friendly GPS systems. However the low cost of ADF systems means they are an efficient backup to GPS.
The operator tunes their ADF receiver to the correct frequency and verifies the identity of the beacon by listening to the Morse code signal transmitted by the NDB. The ADF then automatically moves a compass-like pointer to indicate the direction of the beacon. The pilot may use this pointer to home directly towards the beacon, or may also use the magnetic compass and calculate the direction from the beacon (the radial) at which their aircraft is located.
Unlike early direction finders which were rotated by the navigator as they listened for the direction of the null (the aerial visible as loops above or below the fuselage), ADF operate without direct intervention and continuously display the direction of the tuned beacon. Initially ADF receivers contained a rotating aerial driven by a motor which was controlled by the receiver. More modern ADF contain a small array of fixed aerials and use electronic means to deduce the direction using the strength and phase of the signals from each aerial. In either case, automated ADF use the phase of the signal from a second sense aerial to resolve the correct direction from its opposite.
The ADF's direction needle will always point to the broadcast station, regardless of the aircraft's attitude or heading. An ADF can be used to determine current position, track inbound and outbound, and intercept a desired bearing. These procedures are used to execute holding patterns and non-precision instrument approaches.
|Class of NDB||Transmission Power||Effective Range|
|Locator||below 25 watts||15 NM|
|MH||below 50 watts||25 NM|
|H||50 to 1,999 watts||50 NM|
|HH||2,000+ watts||75 NM|
Most RMI incorporate two direction needles. Often one needle (generally the thin, single-barred needle) is connected to an ADF and the other (thicker and/or double-barred) is connected to a VOR. Using multiple indicators a navigator can accurately fix the position of their aircraft without requiring station passage. There is great variation between models and the operator must take care to that their selection displays information from the appropriate ADF and VOR.