highfrequency direction finder

Radio direction finder

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.

Method of operation

Manual direction finding is performed by rotating a highly directional antenna while the operator listens for a null (the weakest signal) on the already tuned receiver. Listening for the null is easier than listening for a peak, and normally produces a more accurate result.

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.

Usage in navigation

Radio transmitters for air navigation are known as beacons and are the radio equivalent to a lighthouse. The transmitter sends an AM broadcast incorporating the station's identifier in morse code that is used to confirm the station and its operational status. Most modern receivers can also tune in commercial AM radio stations, which can be useful due to their high power and location near major cities. Since the radio signal is broadcast in all directions and does not itself include direction information, these older style beacons are referred to as non-directional beacons, or NDB in the aviation world.

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.


An Automatic Direction Finder (ADF) is an aircraft radio-navigation instrument which displays the relative bearing from the aircraft to a suitable radio station. ADF receivers are normally tuned to aviation Non-directional Beacons (NDB), low-power AM radio transmitters operating in the low frequency band between 190 to 535 kHz. Most ADF receivers can also receive AM broadcast stations but because their location and identity is not controlled like aviation NDB, they are less reliable for navigation.

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.

Typical NDB services ranges

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

Station passage

As an aircraft nears an NDB station, the ADF becomes increasingly sensitive, small lateral deviations result in large deflections of the needle which sometimes shows erratic left/right oscillations. Ideally, as the aircraft overflies the beacon, the needle swings rapidly from directly-ahead to directly-behind. This indicates station passage and provides an accurate position fix for the navigator. Less accurate station passage, passing slightly to one side or another, is shown by slower (but still rapid) swinging of the needle. The time interval from the first indications of station proximity to positive station passage varies with altitude — a few moments at low levels to several minutes at high altitude.


The ADF may be used to home in on a station. Homing is flying the aircraft on the heading required to keep the needle pointing directly to the 0° (straight ahead) position. To home into a station, tune the station, identify the Morse code signal, then turn the aircraft to bring the ADF azimuth needle to the 0° position. Turn to keep the ADF heading indicator pointing directly ahead. Homing is regarded as poor piloting technique because the aircraft may be blown significantly or dangerously off-course by a cross-wind, and will have to fly further and for longer than the direct track.


The ADF may also be used to track a desired course using a ADF and allowing for winds aloft, winds which may blow the aircraft off-course. Good pilotage technique has the pilot calculate a correction angle that exactly balances the expected crosswind. As the the flight progresses, the pilot monitors the direction to or from the NDB using the ADF, adjusts the correction as required. A direct track will yield the shortest distance and time to the ADF location.


A Radio-Magnetic Indicator is an alternate ADF display providing more information than a standard ADF. While the ADF shows relative angle of the transmitter with respect to the aircraft, an RMI display incorporates a compass card, actuated by the aircraft's compass system, and permits the operator to read the magnetic bearing to or from the transmitting station, without resorting to arithmetic.

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.

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