AUTOMATIC DIRECTION FINDER

The automatic direction finder (ADF) is the oldest and most widely used radio navigation
system.

The automatic direction finder (ADF) is an airborne system used to determine the relative
bearing from the aircraft to the ground-based transmitter (with respect to the aircraft
centerline). ADF is the oldest of the radio navigation systems and one of the most widely
used throughout the world, because of the availability of numerous ground stations to
tune the ADF to. The concept of the ADF navigation is based on the ability of the
airborne system to provide the bearing indication with respect to the aircraft centerline,
based upon the direction of arrival of radio wave from a selected station. If the indicator
compass card is adjusted so the aircraft's present heading is set below the lubber line
(centerline), then the indicator pointer against the compass card provides a direct
magnetic bearing to the station.

ADF has been in use longer than most radio navigational aids, and its use has become
quite common. ADF was first mandatory aboard commercial air carriers in 1937. The
simplicity of the system and its independence of other systems are two reasons for it's
continued use. ADF is used by itself or in conjunction with a VOR system. Although
VOR systems may be more accurate, ADF has an advantage in that there are more ADF
ground stations (low frequency beacons and standard broadcasting) than there are VOR
stations. This means that finding and ADF station close to the direct line from the city A
to city B is much more likely than finding a VOR station in the same path. ADF is used
for navigation, position fixing, and position holding.

THE ADF SYSTEM:

The airborne portion of the ADF system consists of a receiver, control unit, indicator,
fixed looped antennas, and a sense antenna. The ground facility consists of a transmitter
and an antenna. A typical ground facility used for ADF would be an AM radio station or
a non-directional beacon (NDB).
Quadrantal Error Bearing
Corrector (QEC)
Loop Antenna Indicator

ADF
Sense Antenna
Coupler
Sense Antenna
Receiver Audio to
Interphone
ADF Control Panel System
 BLOCK DIAGRAM OF ADF SYSTEM

ADF SYSTEM COMPONENT LOCATIONS:

There are two ADF systems installed. Each system includes the following components :-
1. Control panel
2. Radio Magnetic Indicator (RMI)
3. Receiver
4. Loop antenna
5. Quadrantal Error Corrector (QEC)
6. Source antenna
7. Sense antenna coupler

CONTROL PANEL

The control panels are located on the off electronic panel. The ADF control unit provides
the control and switching circuits to select the ADF receiver operating mode and
frequency.

RADIO MAGNETIC INDICATOR

There are several types of indicators that can be used with the system. All indicators used
with the ADF system indicate the bearing of the ground station. That is, the needle of the
indicator always points to the station that the receiver is tuned on. An ADF indicator will
have the needle rotating against a fixed azimuth card. This type of indicator was also
called as radio compass indicator.
ADF RECEIVER:

The ADF receiver requires two types of antenna. An omni-directional sense antenna is
required to help tune the receiver and loop antenna is required to provide the bearing.
Depending on the ADF system used, there are different antenna types (older systems
require older antenna types). These include loop antennas that are mechanically rotated,
electrically rotated, or mounted in a fixed position to the aircraft.
The ADF receiver has a sense antenna and a loop antenna corrector located at the front on
the sloping panel. A test button and a self-test indicator are also located on the front
panel.
The receiver contains the circuits for processing the antenna-received signals as selected
by the control panel. The receiver output as connected to the indicators and the audio
selector panels.

ADF LOOP ANTENNA:

The loop antenna is installed on the centerline of the airplane. Attached to the top of the
loop antenna connector is the Quadrantal Error Corrector (QEC).

THEORY OF OPERATION:

The ADF – 700 receivers provides information to the aircraft system for display in radio
magnetic indicator RMI. The ADF 700 is a solid-state receiver using digital techniques to
perform all signal processing, self-test and monitor operations.

Relative bearing angle is sensed and computed electronically. The ADF 700 selected
signals from a sense antenna and two orthogonally polarized, directional loop antenna.

PRINCIPLE OF OPERATION:

ADF – 700 receives frequencies from 190 KHz to 1750 KHz. This range includes
hundreds of broadcast AM stations (500 kHz to 1610 kHz), the Non Directional Beacons
(NDB) in the 190 kHz – 500 kHz range.

The broadcast stations are required to identify themselves periodically and are normally
located in known population centers and may therefore be valuable as a navigation tool.

The low – power NDB have limited range of 150 to 200 miles. Higher powered NDB are
used as outer markers at same locations and generally carry transmitted weather
information as advisories.
Compass locators transmit two-letter ID group. The outer locator transmits the first two
letters and the middle locator the last two letters of the ID group.

TYPICAL OPERATION

1. The navigator selects the transmitting frequency of the ground station in the area of
his flight path.
2. The navigator confirms the identity of the station by listening for the station's Morse
code or voice identification on the cockpit audio system.
3. The navigator uses navigation charts to plot his position, knowing the location of the
ground station and the bearing to the station.
4. The pilot may also use the relative bearing needle to fly to a ground station and the
bearing to the station.
5. The pilot can also use the ADF for flying a holding pattern by using the 180 degrees.
Swing of the relative bearing pointer as an indication of being over the ground
station. Once over the ground station, the pilot can begin a standard turn in the
holding pattern.

The characteristics of the loop antenna are used to determine the bearing to a selected
ground station. Since the loop antenna is directional, the received signal strength is
relative to the position of the antenna with respect to the transmitting ground station. In
the ADF system, antennas are mounted in a fixed position relative to the aircraft.
Therefore the orientation of the antenna with respect to the ground station varies as the
aircraft heading changes.

Signal
strength

Signal
strength
0° 90° 180° 270° 360°
Aircraft heading relative to ground station bearing

LOOP ANTENNA RECEPTION

The other loop antenna signal strength is maximum at 90 degrees and 270 degrees and
minimum at zero and 180 degrees (sine function).this signal is from omni-directional
sense antenna.

The combined sine, cosine and sense antenna signals are detected by ADF-700 and sent
out to the aircraft intercom system as ADF audio. This signal is compared with 96 Hz
signal to determine relative bearing, which is digitized and sent to the aircraft Radio
Magnetic Indicator.