{"id":14520,"date":"2025-06-07T16:30:04","date_gmt":"2025-06-07T11:00:04","guid":{"rendered":"https:\/\/ibexaviation.com\/pilot-training\/?p=14520"},"modified":"2025-07-20T07:32:20","modified_gmt":"2025-07-20T02:02:20","slug":"automatic-direction-finder","status":"publish","type":"post","link":"https:\/\/ibexaviation.com\/pilot-training\/automatic-direction-finder\/","title":{"rendered":"Automatic Direction Finder (ADF-NDB)"},"content":{"rendered":"\n<h2>Non-Directional Beacon and Automatic Direction Finder<\/h2>\n\n\n\n<h3>Introduction to NDB<\/h3>\n\n\n\n<ul><li>Non-Directional Beacon (NDB) is ground-based transmitter which is used for direction finding by the Automatic Direction Finder(ADF)<\/li><li>NDB is transmits a Vertically polarised interrupted carrier wave signal in the LF or MF bands<\/li><li>Transmission is omni-directional or equally spread in 360 degrees around the ariel<\/li><li>Frequencies allocated to the NDB are from 190 to 1750 kHz in LF and MF bands<\/li><li>NDB Ariels are huge \u00e2\u20ac\u02dcT\u00e2\u20ac\u2122 shaped due to their high wave length<\/li><li>Emission designators of NDB are NON-A1A or NON-A2A depending on the requirement<\/li><li>Range of NDB could be from 25 to 500 NM depending on their purpose<\/li><li>Each NDB transmits a unique three letter identifier in morse code<\/li><\/ul>\n\n\n\n<h3>Principle of ADF<\/h3>\n\n\n\n<ul><li>ADF equipment in the aircraft uses loop antenna principle to locate the NDB<\/li><li>Automatic Direction Finder (ADF) uses NDB beacon to aid in navigation<\/li><li>ADF equipment is capable of selecting from 190 to 1750 kHz<\/li><li>Loop antenna of ADF has two vertical elements receiving the signal<\/li><li>Simultaneous receipt in verticals would result in no phase difference<\/li><li>Detection of same phase in the verticals causes no current to flow in circuit<\/li><li>Zero current indicates that NDB is located perpendicular to the loop<\/li><li>NDBs located in front as well as behind the loop would indicate Null position<\/li><li>Measurement of current in other positions can be resolved to provide directions<\/li><\/ul>\n\n\n\n<h3>Resolution of Null Position in ADF by using Cardioid Polar Diagram<\/h3>\n\n\n\n<ul><li>Loop antenna of the ADF identifies a null position of zero current, but is insufficient to distinguish between Ground Stations ahead and behind the aircraft<\/li><li>Polar diagram of a loop ariel is in the form of a figure of 8 created by rotation of loop through 360 degrees<\/li><li>Datum point of ADF is its null position of zero current, at which the NDB could be either in front or behind the aircraft<\/li><li>A sense antenna with a circular polar diagram is used to resolve this ambiguity<\/li><li>The combined polar diagram by mixing Circular (Sense Ariel)and Figure of \u00e2\u20ac\u02dc8\u00e2\u20ac\u2122 (Loop Ariel) is heart shaped called as Cardioid<\/li><li>Cardioid polar diagram has only one null position which perpendicularly ahead of the loop antenna<\/li><li>Polarity of Sense Antenna is frequently reversed for obtaining better accuracy<\/li><\/ul>\n\n\n\n<h3>ADF Antenna Housing<\/h3>\n\n\n\n<ul><li>Loop and sense antenna are located together in tear drop shaped housing fixed to the underside of fuselage<\/li><li>Modern ADF equipments have two non-rotating loop antennas at right angles<\/li><li>Antenna is connected to a goniometer to measure angles which has a search coil that identifies the null position<\/li><li>The current produced by error signal moves the search coil until it reaches null position to show relative bearing of ground station<\/li><\/ul>\n\n\n\n<h3>Control Panel of ADF<\/h3>\n\n\n\n<ul><li>ADF is controlled using an analogue or digital control panel located in the cockpit with these features<\/li><li>Frequency selector is used to select the required NDB by changing frequency<\/li><li>Antenna (ANT) button is used to check morse code identification of the NDB<\/li><li>Bearings have to be ignored when ANT button is pressed due to disengagement of loop antenna<\/li><li>Test button (TEST) button is a spring loaded switch to check the indicator by moving the needle of RBI by 90 degrees<\/li><li>Digital control panel has a facility to set main and standby frequencies<\/li><\/ul>\n\n\n\n<h3>Beat Frequency Oscillator in ADF<\/h3>\n\n\n\n<ul><li>Beat frequency oscillator (BFO) labelled as TONE \/ VOICE is provided to obtain an audible output from NON-A1A NDB transmissions<\/li><li>Each NDB transmits a three letter unique identification code for positive identification<\/li><li>NON-A1A NDB transmissions are used in long range NDBs since the amplitude modulation of A2A transmissions tends to reduce range<\/li><li>Unlike NON A2A signals, NON-A1A NDB transmissions require BFO for obtaining the transmitted audible identification code<\/li><li>BFO in the ADF has a Heterodyne unit which produces a beat frequency, which is combined with received signal to produce the audible tone<\/li><li>BFO should be selected ON for Tuning, Identification and Monitoring of NON-A1A NDB transmissions but OFF during Bearing Measurement<\/li><\/ul>\n\n\n\n<h3>Relative Bearing Indicator (RBI)<\/h3>\n\n\n\n<ul><li>RBI indicates position of NDB in relation to the longitudinal axis of aircraft<\/li><li>QDM is the magnetic bearing to the station obtained by applying aircraft\u00e2\u20ac\u2122s heading to the relative bearing<\/li><li>If the QDM obtained is more than 360, subtract 360 to get the actual QDM<\/li><li>QDR is the magnetic bearing from the station obtained by adding or subtracting 180 for QDM is lesser or greater than than 180<\/li><li>Moving compass card is useful to set the heading manually to get QDM or QDR in the head or tail of the pointer<\/li><\/ul>\n\n\n\n<h3>Radio Magnetic Indicator (RMI)<\/h3>\n\n\n\n<ul><li>RMI converts relative bearings to magnetic bearings on a moving compass dial by combining aircraft heading with relative bearing<\/li><li>Aircraft\u00e2\u20ac\u2122s heading is indicated on the moving compass dial<\/li><li>While head of the RMI pointer indicates QDM its tail indicates QDR<\/li><li>RMI has two pointers, which can be selected to two NDBs, or two VORs, or a combination of NDB and VOR<\/li><li>True bearings can be obtained from RMI indications by application of Variation and Deviation<\/li><\/ul>\n\n\n\n<h3>Error of Automatic Direction Finder (ADF)<\/h3>\n\n\n\n<ul><li>ADF suffers from errors due to Terrain Effect, Static Interference, Quadrantal Error, Cone of Silence, Dip Error, Station Interference, Coastal Refraction and Night Effect<\/li><li>Terrain effect is caused due to interference from reflected waves which can be minimised by flying higher in mountainous terrain<\/li><li>Static interference is caused due to static and electric discharge from clouds, hence ADF readings have to be treated with caution during thunder storm activity<\/li><li>Quadrantal error is caused due to distortion of radio waves by airframe which can be reduced by electronic devices<\/li><li>Maximum error is observed on quadrantal headings relative to the aircraft&#8217;s longitudinal axis<\/li><li>Cone of silence is located overhead the Non Directional Beacon when the ADF pointer would fluctuates rapidly<\/li><li>Dip error is caused due to tilt of loop antenna during manoeuvres indicating higher value on the side of tilt of loop antenna<\/li><li>Coastal refraction causes an error in NDBs located close to coast line due to bending of radio waves while crossing the coast<\/li><li>Since the speed of radio waves is faster over sea due to change in medium radio waves bend towards land which is the slower speed medium<\/li><li>Coastal refraction is lesser in higher frequencies and therefore coastal NDBs use higher frequency transmissions<\/li><li>Coastal NDBs are designed to transmit crossing perpendicular to coast to minimise Coastal Refraction<\/li><li>Night effect affects accuracy of bearings of ADF at night due to reception and interference between Ground and Sky waves of the same NDB<\/li><li>During day, D layer of ionosphere attenuates sky waves while the D layer disappears a causing sky waves to reach earth<\/li><li>Returning sky waves are out of phase and change in polarity which induce current in the horizontal elements<\/li><li>Night effect is caused due to the interference of sky waves with ground waves when ADF wanders at dawn and dusk time<\/li><li>Designated Operational Coverage (DOC) ensures bearing accuracy during day but are not applicable during night due to night effect<\/li><li>Station interference is caused due to nearby NDB stations transmitting on similar frequencies which can be reduced by judicious frequency allocation<\/li><li>ADF does not have any failure warning mechanism and hence positive identification is mandatory before using ADF Indications<\/li><\/ul>\n\n\n\n<h3>Range and Accuracy of ADF<\/h3>\n\n\n\n<ul><li>ADF Range of is affected by the Power, Frequency, Type of Transmission and Receiver Quality<\/li><li>Minimum signal to noise ratio of three to one is essential for ADF operations<\/li><li>Hilly terrain and Precipitation would reduce the effective range of ADF due to reduction in signal quality<\/li><li>Bearing accuracy of ADF is five degrees within the Designated Operational Coverage (DOC) during day time<\/li><\/ul>\n\n\n\n<h2>Best of luck<\/h2>\n","protected":false},"excerpt":{"rendered":"<p>Non-Directional Beacon and Automatic Direction Finder Introduction to NDB Non-Directional Beacon (NDB) is ground-based transmitter which is used for direction finding by the Automatic Direction Finder(ADF) NDB is transmits a Vertically polarised interrupted carrier wave signal in the LF or MF bands Transmission is omni-directional or equally spread in 360 degrees around the ariel Frequencies allocated to the NDB are from 190 to 1750 kHz in LF and MF bands NDB Ariels are huge \u00e2\u20ac\u02dcT\u00e2\u20ac\u2122 shaped due to their high wave length Emission designators of NDB are NON-A1A or NON-A2A&hellip;<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"disable_featured_image":false},"categories":[326],"tags":[],"_links":{"self":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14520"}],"collection":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/comments?post=14520"}],"version-history":[{"count":0,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14520\/revisions"}],"wp:attachment":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/media?parent=14520"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/categories?post=14520"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/tags?post=14520"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}