Automatic Direction Finder (ADF) 1 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 2 Introduction to ADF • As the radio is rotated signal become weaker or stronger. 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. • ADF system consists of two main components : .Introduction to ADF • It operates in LF and MF band (190-1799 KHz). No signal position indicates the direction of the station.Aircraft component 3 .Ground stations . that is magnetic field of the radio wave horizontal to the ground. • Stations have a vertical antenna which emits vertically polarized signal. electrical field is vertical.Ground stations • They transmit omnidirectional (in every direction) signals. • They are called nondirectional beacons (NDB). Radio wave transmission from an NDB 4 . Aircraft components of ADF • • • • Antennas Receiver Control head Indicator ADF external block diagram 5 . 6 . • Sense antenna It 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. • It is in the shape of rectangular or circular loop. • It uses magnetic field component of the radio wave. but cannot determine if the station is in front/back or to the right/left of the aircraft.ADF antennas Two antennas are used for ADF: • Loop antenna It indicates the direction of the ground station. Loop antenna Loop antenna 7 . Loop antenna Loop antenna • Loop antenna concentrates on magnetic field of the radio wave. 8 . Loop antenna • The voltage induced in the loop is proportional to the rate of change of density of magnetic flux. Loop antenna polar diagram 0˚ θ 270˚ 90˚ 180˚ 9 . station may be in either direction. • Modern aircraft use two loops mounted perpendicularly and connected to a device called goniometer. all rotating parts are packaged in receiver box. Thus. Loop antenna • It is not strictly required to rotate the loop antenna to find the null position. 10 . 180˚ apart. • By this way.Field pattern of loop antenna • The loop antenna generates two nulls at right angles to its own plane. • It is a capacitive antenna. • It couples only with the electrical component of the radio wave.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. 11 . its signal strength is the same all around. • It is an omnidirectional antenna. Field pattern of sense antenna Combined field of loop and sense antenna 12 . Combined field of loop and sense antenna ADF antennas 13 . top sense) Loop and sense antenna location (Bottom loop.Loop and sense antenna locations (Bottom loop. bottom sense) 14 . Loop and sense antenna location Loop and sense antenna location 15 . 16 . Only sense antenna is in use.ADF receiver ADF Control head • ANT position :It is used to receive weather information or to listen to other comm. • ADF position : It is used for navigation • BFO position : It is used to identify stations during no modulation periods. other comm ADF Navigation BFO unmodulated signals FRQ Frequency selector Control head 17 .ADF Control head ANT Weather. Reminders for some angle & directions ADF indicator 18 . Relative Bearing indicator (RBI) & Relative Magnetic indicator (RMI) RMI 19 . NDB/ADFs do not provide cross wind correction. NDB on navigation chart 20 .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. • Thunderstorms and reflections can confuse the signal. Cross-wind navigation Sources of ADF errors • • • • • • Night effect Coastal refraction Mountain effect Static interference Loop alignment error Polarization error 21 . Flight line calibration & testing • The procedure of determining the size of error in an ADF installation is known as loop swing. • These readings are recorded on a "loop swing record chart" 22 .After a lightning strike . • Swing control is performed . • Then various readings are taken from ADF and compass (magnetic heading data) at different position of the aircraft.When it is required by maintenance program .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. It should be less then +/-0. • The loop alignment error is given by the average of the peaks.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. 23 .25˚. Correction against indicated bearing Loop alignment error Functional test 24 .