Chapter 12

ADF

Automatic Direction Finder

NDB Signal SensingADF works on the Low and Medium Frequency AM bands from 190 to 1750 kHz.

The ADF ground station is known as an NDB (Non-Directional Beacon) because it broadcasts in all directions.

The ADF uses 2 antennas: Loop and sense.The Loop antenna will show the line of position by sensing the difference of signal strength between one side and the other side of the loop.If the loop is facing the station; the signal would be at “null” at this point.

The sense antenna allows the system to tell which side of the station the airplane is on.

By combining the loop and sense signals, the system can compute where the station is relative to the aircraft.

Sense Antenna

Sense Antenna

E Field and H Field

ADF on Fixed Compass Card

The simplest display is a fixed compass card with an ADF pointer.

To fly to the station, the pilot turns right or left and brings the pointer straight up.

Textbook page 81

Very Rare !

ADF on RMI

A vast improvement is the RMI (Radio Magnetic Indicator).

The RMI has a slaved compass card that follows the aircraft heading so the top of the card always indicates aircraft heading.

The RMI has 2 pointers which enable the pilot to select any combinations of ADF and VOR stations.

Textbook page 82

ADF Analog Controls and Indicator

The ADF pointer is pointed to the NDB that is tuned.The Band Selector selects which band is tuned; the First band is 190-430kHz, the Second is 420-850kHz, the Third is 840-1750kHz.The second band is also AM broadcast stations.The Mode Selector allows selection between ADF to receive both loop and sense signals (ADF) or just the sense antenna (REC) to listen to AM radio.TEST will cause the needles to swing 90° to insure the needles are not stuck.

Digital ADF Controls Here is a digital controller for ADF used in General Aviation.The BFO is used for audio station identification in Morse Code.

ADF Controls

The ADF display has 2 sides: one for active and the other for stored frequencies.The indicator light shows which frequency is active.The transfer switch, TFR, allows selection of which frequency is active.The TONE switch is the same as the BFO (Beat Frequency Oscillator) to make an audio ID of the station for stations that don’t broadcast audio.ANT is equivalent to the REC mode in the analog system.

ADF Block Diagram

The goniometer computes angle information which is fed to the ADF receiver.The loop and sense antenna pick up directional signals from the station. The receiver computes bearing information to the station and sends it to the RMI.

The Morse code station ID is sent to the aircraft’s audio control panel.

Review Q & A Chapter 12 ADF12.1 On what bands does the ADF operate?Answer: Low, Medium and High AM bands.12.2 An ADF with a fixed compass card can only indicate ____ bearing to an NDB.Answer: Relative12.3 When the edge of an ADF loop points toward the station, strongest signal is received. This is known as ____.Answer: Peak12.4 When the flat side of the loop faces the station, the received signal is weakest. This is known as?Answer: Null12.5 Which is used by the ADF receiver for determining direction, peak or null? Why?Answer: Null, because it provides a sharper more reliable signal.12.6 What is the purpose of a sense antenna?Answer: To allow computation of bearing to the station.12.7 What is “quadrantal error”?Answer: Radio interference causes by metal mass in the aircraft.12.8 How can the sense antenna be selected by the pilot?Answer: By selecting REC or ANT12.9 How is direction-finding selected?Answer: By selecting ADF12.10 What is the function of the switch marked BFO or TONE?Answer: To provide audio station ID or to aid in tuning.12.11 What methods reduce interference to ADF reception?Answer: Filtering and shielding12.12 What type of interference may occur from distant stations?Answer: Night Effect12.13 What device in an ADF receiver reduces the effect of metal masses on the airplane?Answer: The quadrantal error corrector.