ADF - Automatic Direction Finder

ADF Introduction

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In the first direction finding systems, a pilot or navigator had to manually plot and calculate direction and bearing to a transmitting radio station. A mistake by the pilot or navigator would have had serious results for the flight crew and aircraft. By the Automatic Direction Finding (ADF) system, A pilot has no longer the worry of working out his direction or bearing as it is done automatically. ADF receivers are normally tuned to aviation or marine NDBS operating in the LW band between 190 – 535 khz. Most ADF receivers can also receive medium wave (AM) broadcast stations, though as mentioned, these are less reliable for navigational purposes.

Aircraft components of ADF

Ø Antennas

Ø Receiver

Ø Control Panel

Ø Indicator

ADF RECEIVER

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Controls on the ADF receiver permit the pilot to tune the station desired and to select the mode of operation. When tuning the receiver the pilot must positively identify the station. The low or medium frequency radio beacons transmit a signal with 1,020 Hertz (cycles per second [Hz]) modification keyed to provide continuous identification except during voice communications. All air facilities radio beacons transmit a continuous two- or three-unit identification in Morse code, except for ILS front course radio beacons which normally transmit a continuous one letter identifier in Morse code. The signal is received, amplified, and converted to audible voice or Morse code transmission. The signal also powers the bearing indicator.

ADF ANTENNAE

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The ADF receives signals on both loop and sense antennae. The loop antenna in common use today is a small flat antenna without moving parts. Within the antenna are several coils spaced at various angles. The loop antenna senses the direction of the station by the strength of the signal on each coil but cannot determine whether the bearing is TO or FROM the station. The sense antenna provides this latter information, and also voice reception when the ADF function is not required.

BEARING INDICATOR: As mentioned above, the bearing indicator displays the bearing to the station relative to the nose of the aircraft. If the pilot is flying directly to the station, the bearing indicator points to 0°. An ADF with a fixed card bearing indicator always represents the nose of the aircraft as 0° and the tail as 180°.

Relative Bearing is the angle of intersection of a line drawn through the centerline of the aircraft and a line drawn from the aircraft to the radio station. This angle is always measured clockwise from the nose of the aircraft and is indicated directly by the pointer on the bearing indicator.