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Automatic Direction Finder(ADF)


It was appreciated quite early in the history of radio that direction finding would be a considerable aid to navigation, both sea and air. Commercial ground station sent quite strong radio signals, and experiments in large aircraft with dedicated operators soon produced acceptable results. It was possible to receive signals from broadcasting beacon, which themselves were non-directional, and determine the direction from which signals were coming.

The early system of airborne direction finding from Non-Directional Beacons (NDBs) has remained basically unchanged, although the airborne equipment has progressed considerably. In fact many NDB have been recently introduced to assist with runway approaches at medium-sized airports, and there are moves to use them to transmit correction signals for the latest satellite navigation system.

Non- Directional Beacon (NDB's) 

The Successors to the old commercial stations are the aeronautical NDB's transmitting Amplitude Modulated (AM) signals in the upper LF and lower MF bands, between 190 kHz and 1750 kHz. The wavelengths of the transmissions, between 1580 m and 170 m, make half-length antennas rather large, so nearly all of the radials used are much shorter, with components included to increase inductance and capacitance to electronically simulate longer antennas.


The advantage of these frequency lie in the diffraction they suffer close to the earth's surface. Aircraft can receive surface waves if the direct waves are disrupted by obstructions or the curvature of the earth. In fact, coastal NDB's can be used by both ships and aircrafts. There are of course disadvantages too.

Beacons are situated along airways to guide controlled air traffic , on ocean coastlines to provide navigation assistance far out to the sea, on airfields to provide homing, and under runway approach paths to lead aircraft safely and speedily on to Instrument Landing System (ILS). Each of these positions requires different characteristics for their transmission, mainly in the signal strength and consequent maximum reception range.


Any modulating a signal requires power, for an AM signal power required is up to 50% above the basic carrier transmission power. For that reason, some NDB's mainly those used for long range navigation at ocean or coasts, are unmodulated, with a short period of keying used to provide an identification message.


These emissions are coded N0N A1A (the second part of the code refers to the keyed part). Unfortunately, to receive any part of the signal, the airborne equipment must employ a beat frequency oscillator (BFO). In addition, during the break in the transmission required for the keying, there is no signal for direction finding. This type of emission is not recommended unless there is no alternative.

To reduce the N0N A1A disadvantages, many NDB's use a different modulation to provide the identification message. The carrier wave continues unmodulated for most of its duration, but when the identification is needed some of the power is use to Amplitude Modulate (AM) the carrier for identification. The resultant emission code is N0N A2A. The BFO is needed only for initial tuning.

NDB's which do not need the maximum signal strength can have their carrier waves Amplitude Modulated (AM) continuously. A BFO is not required to receive or identify this signal, whose emission code is A2A.

Rated Coverage

NDB's are designed for a particular purpose. The area within which their signals must be receivable is calculated, and within that 'rated coverage' the signal strength must be sufficient to give good reception and Direction Finding. 

Types of NBD's

  • Locators : These are low-powered NDB's, usually installed as a supplement to ILS and located together with middle or outer markers. A locator has an average rated coverage of between 10 and 25 Nm. Emission is usually N0N A2A type, and they send identification signals every 10 seconds.

  • Homing and Holding NDB's : These are intended primarily as approach and holding aid in the vicinity of aerodromes, with rated coverage of about 50 Nm. Their emission and 'ident' characteristics are similar to locator.

  • En-route and Long Range NDB's : These provide en-route coverage along airways and a long range bearing facility for ocean tracking and similar operations.

  • Marine NDB's : These are situated along coast to provide navigation assistance to ships. They are generally not used by aircrafts.

Principles of Direction Finding (D/F)

The original method of direction finding involved a 'loop' aerial to receive the signals. If a vertically polarized radio wave approached the loop from one side, as shown in the figure below, each arm of the loop with receive a slightly different strength of signal. This produces a slightly different strength of signal. This produces a slightly different electrical charge in each arm of the loop, and therefore an incentive for electrons to move, producing a same a.c. current.

Loop Antenna.jpg


As show in the figure above a signal reaching a loop oriented in the same direction as the loop maximum current would be induced. If the loop is oriented across the signal, no signal will be received, and no current is induced. This is called as 'null'.

At intermediate angles, the signal and induced current, will be reduced in accordance with the cosine of the angle between the loop and the arriving signal.

As the induced current is A C, there are two direction of 'null', and four for each strength of induced current, as can be seen from the horizontal polar diagram.

This can be resolved by using a second 'sense' dipole antenna to receive the transmitted carrier wave. Adding the amplified current from the loop to the induced current from the dipole produces a horizontal polar diagram as shown in the figure.

The shape of the pattern is known as 'cardioid'. The null in this cardioid is less clearly defined, and occurs when the loop is oriented along the arriving carrier wave, because the signals from the two antennas are out of phase at that point, cancelling each other out. 

Loop and Sense Antenna.jpg


Automatic Direction Finder (ADF)

Older airborne equipment required an operator to turn the loop and determine the original null, then switch in the sense antenna and turn it again to discover the correct direction. Now the total signal from the cardioid can be amplified and fed to a motor which can turn the loop. The direction of the turn depends on the total signal, which will automatically turn the loop towards the null position.

When there is no total signal, the loop and its associated pointers indicate the direction of the incoming signal, relative to the datum of the aircraft's longitudinal axis.

A rotating loop is ungainly, and creates drag. The same effect can be obtained by using a fixed pair of loops arranged at 90 deg to each other. Each arm of the loop will receive a slightly different signal strength.

The difference strength produce a magnetic field inside the the loops, and  a magnetic rotor inside or below the cage formed by the loops will align itself in this magnetic field. This is commonly called as a 'goniometer'. The direction of the rotor can be reproduced by a signal selsyn in a remote indicator, to point towards the incoming signal 



Beat Frequency Oscillator (BFO)

In order for a pilot or operator to actually hear a signal, to either tune it or for identification, it must be at an audio frequency, between 300 to 3000 kHz. The NDB carrier waves are, at a much higher frequency range. Hence a need of BFO arises which can be fitted in a receiver, and can be switched on by the pilot when required.

The BFO is a device which produces a signal inside the receiver at a frequency of about 1000 Hz removed from the received wave. The received wave is compared with the produced signal, and the difference in frequency is converted into the audio signal.

The BFO must be switched on when manually tuning a N0N A1A or N0N A2A signals, and when identifying a N0N A1A signal. For direction finding it should be switched off.


In many equipments, the BFO is controlled by a 'tone' switch.

ADF Controls

ADF controller has many switches which function as follows


Function Switch: OFF-ANT-ADF

In the antenna position (ANT) the receiver operates from the sense antenna only, the bearing pointer being parked at 90º relative bearing. This position may be used from tuning and NDB/ station identification. In the ADF position signals from both loop and sense antenna provide normal ADF operation, the RMI indicate the bearing of the station.

Frequency Select knob

Three knobs are used, to select frequency in 0.5, 10, 100 kHz increments. Digital type frequency display segments indicate the selected frequency.


Beat Frequency Oscillator Switch : TONE

Selects the BFO for use when the NDB is selected is identified by on-off keying of the carrier. 

Some control have an extra switch position in the function switch called as 'LOOP'. This control can be used for manual direction finding, the search coil is rotated until an audio null is achieved or, if provided, a visual tuning indicator indicates a null.

ADF Control Panel.jpg


Relative Bearing Indicator (RBI) vs Radio Magnetic Indicator (RMI)

Relative Bearing Indicator (RBI)

Older equipments used a fixed circular instrument face marked in 360 deg from the nose of the aircraft. The pointer indicating the direction of the signal moves around the dial to indicate the direction relative to the aircrafts heading. The pilot has to then make mathematical calculations to determine the magnetic or true bearing of the aircraft from the beacon in order to plot a position line. Hence it is called as Relative Bearing Indicator (RBI). It is also called as ' FIxed Card Indicator' or 'Radio Compass'.

When using the RBI, the pilot must add the RBI bearing to his own heading to find the true or magnetic bearing to the station.

For example if aircraft heading is 150 deg, and reading on the RBI is 090 deg, the bearing of the station from the aircraft is 090+150 = 240 deg. The bearing of the aircraft from the station is the reciprocal i.e. 240-180 = 060 deg.

Radio Magnetic Indicator (RMI)

The RMI scale is automatically orientated, like a remote indicating compass, to the earth's magnetic field. Again, the needle indicates the actual bearing of the beacon from the aircraft, the tail indicates the bearing of the aircraft from the beacon, and the relative bearing of the beacon can be assessed from the position of the needle relative to the top of the instrument.


Most RMI have two needles, each of which can be selected to show information from an ADF or a VOR equipment.



ADF and NDB errors

ADF and NDB are subjected to a number of errors as follows

Quadrantal Error

The waves arriving at the aircraft strike the whole aircraft, parts of the aircraft reflect the wave and many reflections arrive at the antenna and mixing with the direct waves.

This affects the null signal, and the indication can suffer considerable errors . The greatest of those are called as 'quadrantal error'.

The error is at minimum when the arriving signal are in line with the fuselage. It is also small if the signal comes from a beam. However, a signal coming from 45 deg to the fuselage will produce the greatest quadrantal error, up to + 20 deg. 

When aircraft is banked, the wing area is exposed to the vertically polarised signal, and reflection increased. Therefore, a turning aircraft can produce more error. This error is known as 'dip error' and is compensated on installation.

Sky Wave Interference

Normally, the ADF will receive signals from the NDB as surface waves. The Low frequency as absorbed by the D layer of the ionosphere during day. However, at night the D layer disappears, the other layers thin and rise, and the waves from the beacon can be reflected back to the receiver.

The receiving loop now has a second signal inducing current. Because this signal is received by the top and bottom loop, there is a current even thought the loop is oriented at 90 deg to the incoming signal. The addition of the current induced by the sky waves can cause errors up to 30 deg.

Because the surface waves, by its propogation, is attenuated with range from the transmitter, the sky waves is a greater problem at long range from the beacon, where it is proportionately stronger than the surface waves. Partly because of this, but more because of interference from the other beacons.

Coastal Reflection/Shore-line effect

The amount of diffraction and attenuation of the surface waves changes with the surface on the earth below. That implies that when a wave is over land it has different speed (faster) to that when it is over the sea.

At coasts, the speed change produces a bending of the signal towards the land, where attenuation is greater. This bending does not happen when signal is at 90 deg to the coast, but can have a considerable effect when the signal crosses at 30 deg to the coast line.

The effect is greatest when the waves crosses the coast close to the surface, hence high flying aircrafts will suffer the effect less than low flying ones. If the NDB is on the coast, the reflection will happen immediately after the signals leaves the beacon, and an AFD will continue to give fairly accurate indication. Therefore, to minimize the coastal reflection. aircraft should fly high and use NDB as close to the coast as possible.


Another problem with the chosen frequency band from the NDB's is that of static. Background static from electrical disturbances in the atmosphere can be received at very long range, and there are always thunderstorms taking place somewhere in the world, so as the NDB signal is attenuated, it becomes more difficult to distinguish from the static.

The relation between the signal and the noise from the static is called the signal noise ratio. ICAO recommends the ratio to be minimum of 3:1 within the rated coverage of the station.

Lightning from thunderstorm close to the aircraft can produce signals even stronger than the wave from the NDB. This can confuse the ADF, causing totally incorrect indications.


Most airborne equipment is capable of accuracies in the region of + 2 deg, but the combined accuracy of the system, including of the system, including the NBD's, is + 5 deg.= within the beacon's range.

Factors Affecting ADF accuracy

  • Night Effect : If outside 70 Nm, the sky wave can cause errors of + 30 deg at night.

  • Terrain : In addition to coastal refraction, reflection from hills will change the signal direction. Flying high will reduce both problems.

  • Static : Nearby thunderstorms not only causes  static noise, but their electrical field can be so strong they provide a stronger signal than the NDB transmissions.

  • Station Interference : Do not use a NDB outside its protected range.

  • Quadrantal error : This is calibrated in most aircraft, but will have an effect doing turns.

  • Loop Alignment : If the ADF system is not aligned with the longitudinal axis, errors will result.


A major problem for pilots using the ADF is the lack of any warning device to tell him the system is unreliable or even failed. Even though it is important that pilots using the ADF constantly monitor the identification signal, that is no guarantee either that the carrier wave is strong enough for D/F, or indeed that it is present. There is also no indication that the ADF is functioning properly, although the NDB itself should be monitored by the state in which it is situated.

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