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Automatic Direction Finder (ADF)

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Objectives

• The students will be able to• Explains the principles of operation of an

ADF system• Recognize ADF loop and sense antenna• Draw block diagram of an ADF system• Explain installation considerations of ADF

loop and sense antennas• Explain how to flight line test of an ADF

system

ADF

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Introduction to ADF

• It operates in LF and MF band (190-1799 KHz), thus it is based on GROUND WAVE propagation

• Its range is not limited to line-of sight distance

• It can receive on both AM radio stations and NDB (non directional beacons)

• Its operation is similar to listening to a transistor radio.

Introduction to ADF

• As the radio is rotated signal become weaker or stronger. No signal position indicates the direction of the station.

• ADF system consists of two main components :- Ground stations- Aircraft component

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Ground stations

• They transmit omnidirectional (in every direction) signals.

• They are called nondirectional beacons (NDB).

• Stations have a vertical antenna which emits vertically polarized signal, that is magnetic field of the radio wave horizontal to the ground, electrical field is vertical.

Radio wave transmission from an NDB

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Aircraft components of ADF

• Antennas• Receiver• Control head• Indicator

ADF external block diagram

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ADF antennas

Two antennas are used for ADF:• Loop antenna

It indicates the direction of the ground station.

• Sense antennaIt indicates the sense of the station (if station is in front/back or to the right/left of the aircraft)

Loop antenna

• It is a directional antenna• It indicates the direction of the station, but

cannot determine if the station is in front/back or to the right/left of the aircraft.

• It is in the shape of rectangular or circular loop.

• It uses magnetic field component of the radio wave.

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Loop antenna

Loop antenna

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Loop antenna

Loop antenna

• Loop antenna concentrates on magnetic field of the radio wave.

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Loop antenna

• The voltage induced in the loop is proportional to the rate of change of density of magnetic flux.

Loop antenna polar diagram0˚

270˚

180˚

90˚

θ

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Field pattern of loop antenna

• The loop antenna generates two nulls at right angles to its own plane. Thus, station may be in either direction, 180˚ apart.

Loop antenna

• It is not strictly required to rotate the loop antenna to find the null position.

• Modern aircraft use two loops mounted perpendicularly and connected to a device called goniometer.

• By this way, all rotating parts are packaged in receiver box.

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Fixed loop antenna

Sense antenna

• Its function is to indicate if the station is in front / back or to the left/right of the aircraft.

• It is a capacitive antenna.• It couples only with the electrical

component of the radio wave.• It is an omnidirectional antenna, its signal

strength is the same all around.

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Field pattern of sense antenna

Combined field of loop and sense antenna

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Combined field of loop and sense antenna

ADF antennas

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Loop and sense antenna locations(Bottom loop, top sense)

Loop and sense antenna location(Bottom loop, bottom sense)

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Loop and sense antenna location

Loop and sense antenna location

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ADF receiver

ADF Control head

• ANT position :It is used to receive weather information or to listen to other comm. Only sense antenna is in use.

• ADF position : It is used for navigation• BFO position : It is used to identify

stations during no modulation periods.

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ADF Control head

ANT Weather, other comm

ADF Navigation

BFO unmodulatedsignals

FRQ Frequency selector

Control head

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Reminders for some angle & directions

ADF indicator

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Relative Bearing indicator (RBI) & Relative Magnetic indicator (RMI)

RMI

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Operational considerations

• LF/MF band is full of atmospheric and man-made noises

• Courses are not related to the lines on the ground (Cross wind can cause navigation difficulty)

• Unlike VORs, NDB/ADFs do not provide cross wind correction.

• Thunderstorms and reflections can confuse the signal.

NDB on navigation chart

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Cross-wind navigation

Sources of ADF errors

• Night effect• Coastal refraction• Mountain effect• Static interference• Loop alignment error• Polarization error

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Flight line calibration & testing

• The procedure of determining the size of error in an ADF installation is known as loop swing.

• Swing control is performed - When it is required by maintenanceprogram

- After a lightning strike- After modification close the ADF antenna- When a new avionic is installed

Ground swing control (1)

• The ADF is tuned to an NDB with a known magnetic bearing.

• Then various readings are taken from ADF and compass (magnetic heading data) at different position of the aircraft.

• These readings are recorded on a "loop swing record chart"

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Loop swing record chart

Ground swing control (2)

• When completed D is plotted against C • The average of the absolute values of the

peaks gives the amount of correction required, its sign being given the sign of the correction in the first quadrant.

• The loop alignment error is given by the average of the peaks. It should be less then +/-0.25˚.

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Correction against indicated bearing

Loop alignment error

Functional test