FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Introduction The Automatic Direction Finder (ADF) is a very important and integral part of The Automatic Direction Finder (ADF) is a very important and integral part of radio navigation. radio navigation. The ADF provides the pilot with an indication of the direction of radio signals The ADF provides the pilot with an indication of the direction of radio signals received from selected stations operating in the low and medium frequency received from selected stations operating in the low and medium frequency range of 90 kHz to 1,800 kHz.range of 90 kHz to 1,800 kHz.These stations include: These stations include: – Non-directional beacons (200 kHz to 415 kHz)Non-directional beacons (200 kHz to 415 kHz)– Standard AM broadcast stations (540 kHz to 1,600 kHz). Standard AM broadcast stations (540 kHz to 1,600 kHz).

Non-directional beacons (NDB) are identified by a CW signal modulated with a 1,020-Hz tone that transmits a three-letter identification code. Occasionally, NDBs will interrupt the CW transmission with a voice transmission to provide weather information and flight advisories. When an NDB is used in conjunction with instrument landing system markers, the beacon is referred to as a compass locator.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Compass locators are identified by a continuously transmitted CW two-letter Compass locators are identified by a continuously transmitted CW two-letter identification code. identification code. Standard AM broadcast stations are identified by voice transmission of the Standard AM broadcast stations are identified by voice transmission of the station call letters. station call letters. The concept of ADF navigation is based on the ability of the airborne system: The concept of ADF navigation is based on the ability of the airborne system: – To measure the direction of the arrival of the received signalTo measure the direction of the arrival of the received signal– Provide a relative bearing indication with respect to the centerline of the Provide a relative bearing indication with respect to the centerline of the

aircraft. aircraft. Using the bearing information displayed on the ADF indicator, the pilot can Using the bearing information displayed on the ADF indicator, the pilot can determine the aircraft's position or can fly directly to the NDB or AM broadcast determine the aircraft's position or can fly directly to the NDB or AM broadcast station. station.

To determine the aircraft's position, the pilot simply: To determine the aircraft's position, the pilot simply: – Plots the headings of two different stations on a navigation chartPlots the headings of two different stations on a navigation chart– Triangulates the aircraft location at the point where the two lines intersect. Triangulates the aircraft location at the point where the two lines intersect.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Principles of ADF Navigation

Radio direction finders were developed in the early 1930's as the first radio navigation device to be used for airborne applications. The early devices used an indicator with a left/right needle that would center when the aircraft was pointed toward the station. The radio direction finder has developed into an automatic system that continuously displays the direction to the station by means of a pointer on the ADF indicator. A means is usually provided to manually or automatically rotate the compass card on the ADF indicator to the aircraft's magnetic heading: – The pointer indicates the direction to the station, – The pointer indicates the magnetic heading the aircraft must take to fly

towards the station.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

If the compass card is driven by a synchro, which receives heading information If the compass card is driven by a synchro, which receives heading information from the compass system: from the compass system: – The instrument is known as a Radio Magnetic Indicator (RMI). The instrument is known as a Radio Magnetic Indicator (RMI).

All ADF systems employ the directional characteristics of a loop antenna to find All ADF systems employ the directional characteristics of a loop antenna to find the direction of the NDB or AM broadcast station and non directional sense the direction of the NDB or AM broadcast station and non directional sense antenna to determine where the station is. antenna to determine where the station is.

The directional pattern of the loop antenna is: The directional pattern of the loop antenna is: – If positioned so that the ends of the loop are in alignment with the incidence If positioned so that the ends of the loop are in alignment with the incidence

of the radio wave, the received RF signal will be maximum. of the radio wave, the received RF signal will be maximum.

If the loop is rotated 90If the loop is rotated 90oo from this position, the signal will fade out. from this position, the signal will fade out. – This is known as the "null" position. This is known as the "null" position.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

A non-directional sense antenna is: A non-directional sense antenna is: – To determine which of the two 180To determine which of the two 180oo apart null positions is the correct apart null positions is the correct

bearing to the station.bearing to the station.

Early ADF systems used a rotating loop antenna and a long-wire sense Early ADF systems used a rotating loop antenna and a long-wire sense antenna. antenna.

Modern ADF systems use a goniometer which eliminates the requirement for Modern ADF systems use a goniometer which eliminates the requirement for the loop to rotate. the loop to rotate.

ADF systems with non-rotating loops antennas are packaged in a compact ADF systems with non-rotating loops antennas are packaged in a compact module together with the sense antenna and RF amplifier to afford less drag module together with the sense antenna and RF amplifier to afford less drag and greater reliability. and greater reliability.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

ADF indication with respect to aircraft position

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

The operation of an ADF system is based on the directional characteristics of The operation of an ADF system is based on the directional characteristics of the loop antenna to determine the direction of the incoming RF signal and a the loop antenna to determine the direction of the incoming RF signal and a sense antenna which determine from where the signal is comming. sense antenna which determine from where the signal is comming. The loop antenna consists of a continuously wound coil. The loop antenna consists of a continuously wound coil. When the magnetic lines of force from an incoming RF wave cut across the When the magnetic lines of force from an incoming RF wave cut across the coil, a voltage is induced in the antenna.coil, a voltage is induced in the antenna.Because of the transit time of the wave, the voltage induced at the Because of the transit time of the wave, the voltage induced at the leading leading edgeedge of the loop (relative to the direction of the incoming signal) will of the loop (relative to the direction of the incoming signal) will leadlead the the voltage induced at the voltage induced at the trailing edgetrailing edge. . The algebraic sum of the induced voltages will result in maximum voltage The algebraic sum of the induced voltages will result in maximum voltage when the plane of the loop is aligned to the incoming RF wave. when the plane of the loop is aligned to the incoming RF wave. As the loop is turned 90As the loop is turned 90oo to the direction of the RF wave equal and opposite: to the direction of the RF wave equal and opposite: – Voltages are induced in the sides of the loop which cancel each other to Voltages are induced in the sides of the loop which cancel each other to

result in a zero voltage output. result in a zero voltage output.

ADF Antenna Theory

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

The point of rotation where the resultant output is zero is known as the null The point of rotation where the resultant output is zero is known as the null position of the antenna.position of the antenna.

At the null position, a fairly accurate indication of the station direction can be At the null position, a fairly accurate indication of the station direction can be determined. determined.

ADF loop antennas are automatically rotated to the null position by means of a ADF loop antennas are automatically rotated to the null position by means of a servomotor. servomotor.

The mechanical position of the shaft of the servo used to rotate the loop will The mechanical position of the shaft of the servo used to rotate the loop will reveal the bearing to the station. reveal the bearing to the station.

The shaft is mechanically coupled to synchro which mechanically coupled to The shaft is mechanically coupled to synchro which mechanically coupled to the ADF pointer to provide bearing information. the ADF pointer to provide bearing information.

ADF Antenna Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

The bidirectional The bidirectional fiqure-8 patternfiqure-8 pattern of a loop antenna causes it to null in two of a loop antenna causes it to null in two positions that are 180positions that are 180oo apart. apart. – This condition can result in wrong ADF pointer indication since the pilot This condition can result in wrong ADF pointer indication since the pilot

would not know whether the aircraft was pointed toward the station or away would not know whether the aircraft was pointed toward the station or away from it. from it.

Loop antenna operation

ADF Antenna Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

This problem is eliminated by the use of an omni-directional, open-wire This problem is eliminated by the use of an omni-directional, open-wire sense antenna: sense antenna: – to provide an additional input signal which is 90to provide an additional input signal which is 90oo out-of-phase with the out-of-phase with the

signal received from the loop antenna. signal received from the loop antenna.

The phase of the loop output will always differ by 90The phase of the loop output will always differ by 90oo from that of the sense from that of the sense antenna, antenna, – A 90A 90oo phase shift is added to the loop voltage to cause this voltage to phase shift is added to the loop voltage to cause this voltage to

vary with respect to the constant sense antenna voltage as the loop vary with respect to the constant sense antenna voltage as the loop changes direction. changes direction.

By combining the loop and sense antenna voltages, a By combining the loop and sense antenna voltages, a cardioids directional cardioids directional patternpattern results with only one null position. results with only one null position.

ADF Antenna Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000 Loop and Sense antenna pattern combine to form cardioid's Loop and Sense antenna pattern combine to form cardioid's patternpattern

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000 Typical ADF system block diagram

ADF Circuit Theory

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Typical ADF system block diagram

A typical ADF system A typical ADF system consists of: consists of: – A loop antennaA loop antenna– Sense antennaSense antenna– ReceiverReceiver– Control headControl head– Bearing indicatorBearing indicator

The function of the ADF The function of the ADF control head is to select control head is to select the desired frequency the desired frequency and mode of operation. and mode of operation.

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Typical ADF system block diagram

These modes include: These modes include: – Normal ADF operation using Normal ADF operation using

both the loop and sense both the loop and sense antennasantennas

– Loop-only mode to manually Loop-only mode to manually position the loop antenna to position the loop antenna to its null positionits null position

– Sense-only mode for radio Sense-only mode for radio reception without direction reception without direction finding. finding.

Other functions include a beat Other functions include a beat frequency oscillator switch to frequency oscillator switch to produce a 1,020-Hz tone to produce a 1,020-Hz tone to modulate a CW signal so it is modulate a CW signal so it is audible. audible.

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Typical ADF system block diagram

RF signals induced into the coil RF signals induced into the coil windings of the windings of the loop antennaloop antenna are are fed to the loop amplifier contained fed to the loop amplifier contained within the receiver. within the receiver.

From here, the amplified loop From here, the amplified loop signal is shifted 90signal is shifted 90oo and fed to a and fed to a balanced modulator which is balanced modulator which is used to derive the variable-phase used to derive the variable-phase signal from the loop antenna.signal from the loop antenna.

A fixed-frequency reference A fixed-frequency reference signal from the oscillator is signal from the oscillator is introduced into the balanced introduced into the balanced modulator to modulate the carrier modulator to modulate the carrier signal received from the signal received from the loop loop antennaantenna. .

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Typical ADF system block diagram

As a result, the carrier signal is As a result, the carrier signal is replaced with two sideband replaced with two sideband frequencies, the upper and lower frequencies, the upper and lower sidebands. sidebands. – The The upperupper sidebands are sidebands are

derived from the derived from the sumsum of the of the carrier frequency and the carrier frequency and the reference frequency.reference frequency.

– The The lowerlower sidebands are sidebands are derived from the derived from the differencedifference of of the carrier frequency and the the carrier frequency and the reference frequency.reference frequency.

These sideband products are These sideband products are addedadded to the fixed-phase carrier signal to the fixed-phase carrier signal received from the received from the sense antennasense antenna. .

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Typical ADF system block diagram

The resultant signal is detected The resultant signal is detected and amplified in the and amplified in the superheterodyne receiver.superheterodyne receiver.

The modulation product from The modulation product from one of the sidebands is one of the sidebands is separated from the audio to be separated from the audio to be used as the used as the loop signalloop signal. .

The station is to the right of the The station is to the right of the aircraft if the loop signal will be aircraft if the loop signal will be in-phase with the reference in-phase with the reference signalsignal

The station is to the left of the The station is to the left of the aircraft if the loop signal will be aircraft if the loop signal will be out-of-phase with the reference out-of-phase with the reference signal. signal.

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Typical ADF system block diagram

The loop signal is sent to the phase The loop signal is sent to the phase detector, which outputs the loop drive detector, which outputs the loop drive voltage. voltage. The loop drive voltage positions the The loop drive voltage positions the loop antenna to its null position. loop antenna to its null position. – The loop signal will be zero. The loop signal will be zero.

The loop antenna is driven by a two-The loop antenna is driven by a two-phase induction motor: phase induction motor: – One winding is coupled to the One winding is coupled to the

reference voltagereference voltage– The other winding is coupled to The other winding is coupled to

the signal voltage from the phase the signal voltage from the phase detector, to position the loop detector, to position the loop antenna. antenna.

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

The rotatable loop antennas have since been replaced with stationary loop antennas.

The fixed loop antenna consists of two coils positioned 90o to each other. – Each coil is connected to one of two goniometer windings which are also

90o apart.

Simplified ADF block diagram using a stationary loop antenna

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

The goniometer resides in the ADF receiver and has a rotating winding that positions itself in relation to the induced voltages in the loop antenna.

In recent years: – goniometers have since been replaced with solid-state circuitry– ADF pointers have been replaced with digital readouts, thus eliminating all

moving parts and increasing reliability.

ADF Circuit Theory (cont’d)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

UHF ADF SystemsMilitary aircraft radio direction finding systems use the receiver circuitry in the Military aircraft radio direction finding systems use the receiver circuitry in the UHF communication transceiver in conjunction with direction finding UHF communication transceiver in conjunction with direction finding equipment. equipment. The advantage of using UHF for direction finding is that it is not as susceptible The advantage of using UHF for direction finding is that it is not as susceptible to precipitation static as low- to medium-frequency ADF systems. to precipitation static as low- to medium-frequency ADF systems. – Precipitation static is caused by voltage that accumulates on the aircraft Precipitation static is caused by voltage that accumulates on the aircraft

surface that constantly discharges into the atmosphere. surface that constantly discharges into the atmosphere. The installation of static wicks on the trailing edges of the airframe helps to eliminate this interference, especially for low- to medium-frequency ADF systems.

A typical example of a military direction-finding system using a UHF receiver is the Collins DF-301 UHF ADF.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000 Rockwell Collins DF-301 system operation

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Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

The DF-301 consists of: – A solid-state loop antenna with an associated preamplifier– An audio processing section that includes:

A master clock Antenna switching control

Filter/detectorServo-motor controlPower supply.

– The UHF communications transceiver Broadband RF systemFrequency selector

The antenna output is fed through the preamplifier to an external transfer relay and then to the receiver section of the military UHF communication transceiver. The relay ensures that the transceiver is disconnected from the ADF antenna during the transmit mode. The transceiver ADF audio output is returned to the DF-301 for further processing.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Return audio is amplified and filtered and the bearing information is resolved into its quadrature components. The two components are applied to a resolver and electro-mechanical nulling is accomplished by means of a standard servo-loop arrangement. A torque transmitter follows the resolver angular position to provide a synchro output to the ADF bearing indicator.The DF-301 antenna amplitude-modulates the incident RF signal, and this modulation contains the bearing information. Electrical rotation of the antenna translates the spatial angle of the incoming RF signal to a phase difference between the antenna-modulated output and a reference signal synchronous with antenna rotation. – From this phase difference, the bearing of the incoming signal can be

recovered. Dependable ADF system operation requires proper installation procedures.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Installation Techniques

The first consideration is choosing a location for the ADF antenna that will The first consideration is choosing a location for the ADF antenna that will provide the best possible signal to the antenna at all times. provide the best possible signal to the antenna at all times. On low-wing aircraft, it is best to stay away from the area on the fuselage On low-wing aircraft, it is best to stay away from the area on the fuselage between the leading and trailing edge of the wing. between the leading and trailing edge of the wing. This area exhibits reduced RF field strength. On high-wing aircraft, this area also This area exhibits reduced RF field strength. On high-wing aircraft, this area also exists on the top of the fuselage. exists on the top of the fuselage. In either case, optimum performance will be obtained when the antenna is In either case, optimum performance will be obtained when the antenna is located aft of the wing trailing edge. located aft of the wing trailing edge. This position also minimizes quadrantal error, which is caused by the distortion This position also minimizes quadrantal error, which is caused by the distortion of the radio wave by the aircraft structure. of the radio wave by the aircraft structure. The quadrantal error is maximum at bearings in between the cardinal points of The quadrantal error is maximum at bearings in between the cardinal points of the nose, wingtips, and tail. the nose, wingtips, and tail. A short metal braid or strap should be secured between the loop antenna and A short metal braid or strap should be secured between the loop antenna and the fuselage to provide a good RF ground. the fuselage to provide a good RF ground. Remove paint from the bonding surface of the loop antenna and passivate the Remove paint from the bonding surface of the loop antenna and passivate the cleaned aluminum surface with an alodine to retard corrosion. cleaned aluminum surface with an alodine to retard corrosion.

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

In addition, all antenna connections should be coated with silicon grease.

Noise immunity is one of the most important considerations in ADF system installation.

Generators and alternators should have suitable filters installed and interference must also be suppressed from strobes, inverters, motors, and other electrical equipment.

The proper number of static wicks should be installed at the trailing edges of all airframe surfaces to minimize precipitation static.

The ADF system components must be installed in accordance with the manufacturer's installation instructions and FAA Advisory Circular AC-43.13.

A continuity check of the system wiring should always be performed before installing the equipment and applying power.