Nav Topic 8 automatic direction finder

Malaysian Institute of Aviation Technology

FOR TRAINING PURPOSE ONLY

Introduction
The Automatic Direction Finder (ADF) is a very important and integral part of
radio navigation.
The ADF provides the pilot with an indication of the direction of radio signals
received from selected stations operating in the low and medium frequency
range of 90 kHz to 1,800 kHz.
These stations include:
– Non-directional beacons (200 kHz to 415 kHz)
– Standard AM broadcast stations (540 kHz to 1,600 kHz).
Non-directional beacons (NDB) are identified by a CW signal modulated with a
1,020-Hz tone that transmits a three-letter identification code.
Occasionally, NDBs will interrupt the CW transmission with a voice
transmission to provide weather information and flight advisories.
When an NDB is used in conjunction with instrument landing system markers,
the beacon is referred to as a compass locator.
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Compass locators are identified by a continuously transmitted CW two-letter
identification code.
Standard AM broadcast stations are identified by voice transmission of the
station call letters.
The concept of ADF navigation is based on the ability of the airborne system:
– To measure the direction of the arrival of the received signal
– Provide a relative bearing indication with respect to the centerline of the
aircraft.
Using the bearing information displayed on the ADF indicator, the pilot can
determine the aircraft's position or can fly directly to the NDB or AM broadcast
station.
To determine the aircraft's position, the pilot simply:
– Plots the headings of two different stations on a navigation chart
– Triangulates the aircraft location at the point where the two lines intersect.
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Principles of ADF Navigation
Radio direction finders were developed in the early 1930's as the first radio
navigation device to be used for airborne applications.
The early devices used an indicator with a left/right needle that would center
when the aircraft was pointed toward the station.
The radio direction finder has developed into an automatic system that
continuously displays the direction to the station by means of a pointer on the
ADF indicator.
A means is usually provided to manually or automatically rotate the compass
card on the ADF indicator to the aircraft's magnetic heading:
– The pointer indicates the direction to the station,
– The pointer indicates the magnetic heading the aircraft must take to fly
towards the station.

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If the compass card is driven by a synchro, which receives heading information
from the compass system:
– The instrument is known as a Radio Magnetic Indicator (RMI).
All ADF systems employ the directional characteristics of a loop antenna to find
the direction of the NDB or AM broadcast station and non directional sense
antenna to determine where the station is.
The directional pattern of the loop antenna is:
– If positioned so that the ends of the loop are in alignment with the incidence
of the radio wave, the received RF signal will be maximum.
If the loop is rotated 90o from this position, the signal will fade out.
– This is known as the "null" position.

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A non-directional sense antenna is:
– To determine which of the two 180o apart null positions is the correct
bearing to the station.
Early ADF systems used a rotating loop antenna and a long-wire sense
antenna.
Modern ADF systems use a goniometer which eliminates the requirement for
the loop to rotate.
ADF systems with non-rotating loops antennas are packaged in a compact
module together with the sense antenna and RF amplifier to afford less drag
and greater reliability.

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ADF indication with respect to aircraft position
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ADF Antenna Theory
The operation of an ADF system is based on the directional characteristics of
the loop antenna to determine the direction of the incoming RF signal and a
sense antenna which determine from where the signal is comming.
The loop antenna consists of a continuously wound coil.
When the magnetic lines of force from an incoming RF wave cut across the
coil, a voltage is induced in the antenna.
Because of the transit time of the wave, the voltage induced at the leading
edge of the loop (relative to the direction of the incoming signal) will lead the
voltage induced at the trailing edge.
The algebraic sum of the induced voltages will result in maximum voltage
when the plane of the loop is aligned to the incoming RF wave.
As the loop is turned 90o to the direction of the RF wave equal and opposite:
– Voltages are induced in the sides of the loop which cancel each other to
result in a zero voltage output.
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ADF Antenna Theory (cont’d)
The point of rotation where the resultant output is zero is known as the null
position of the antenna.
At the null position, a fairly accurate indication of the station direction can be
determined.
ADF loop antennas are automatically rotated to the null position by means of a
servomotor.
The mechanical position of the shaft of the servo used to rotate the loop will
reveal the bearing to the station.
The shaft is mechanically coupled to synchro which mechanically coupled to
the ADF pointer to provide bearing information.

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Loop antenna operation

The bidirectional fiqure-8 pattern of a loop antenna causes it to null in two
positions that are 180o apart.
– This condition can result in wrong ADF pointer indication since the pilot
would not know whether the aircraft was pointed toward the station or away
from it.
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This problem is eliminated by the use of an omni-directional, open-wire
sense antenna:
– to provide an additional input signal which is 90o out-of-phase with the
signal received from the loop antenna.
The phase of the loop output will always differ by 90o from that of the sense
antenna,
– A 90o phase shift is added to the loop voltage to cause this voltage to
vary with respect to the constant sense antenna voltage as the loop
changes direction.
By combining the loop and sense antenna voltages, a cardioids directional
pattern results with only one null position.

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Loop and Sense antenna pattern combine to form cardioid's


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ADF Circuit Theory

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Typical ADF system block diagram



ADF Circuit Theory (cont’d)
A typical ADF system
consists of:
– A loop antenna
– Sense antenna
– Receiver
– Control head
– Bearing indicator
The function of the ADF
control head is to select
the desired frequency
and mode of operation.
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These modes include:
– Normal ADF operation using
both the loop and sense
antennas
– Loop-only mode to manually
position the loop antenna to
its null position
– Sense-only mode for radio
reception without direction
finding.
Other functions include a beat
frequency oscillator switch to
produce a 1,020-Hz tone to
modulate a CW signal so it is
audible.

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RF signals induced into the coil
windings of the loop antenna are
fed to the loop amplifier contained
within the receiver.
From here, the amplified loop
signal is shifted 90o and fed to a
balanced modulator which is
used to derive the variable-phase
signal from the loop antenna.
A fixed-frequency reference
signal from the oscillator is
introduced into the balanced
modulator to modulate the carrier
signal received from the loop
antenna.

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As a result, the carrier signal is
replaced with two sideband
frequencies, the upper and lower
sidebands.
– The upper sidebands are
derived from the sum of the
carrier frequency and the
reference frequency.
– The lower sidebands are
derived from the difference of
the carrier frequency and the
reference frequency.
These sideband products are added
to the fixed-phase carrier signal
received from the sense antenna.


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The resultant signal is detected
and amplified in the
superheterodyne receiver.
The modulation product from
one of the sidebands is
separated from the audio to be
used as the loop signal.
The station is to the right of the
aircraft if the loop signal will be
in-phase with the reference
signal
The station is to the left of the
aircraft if the loop signal will be
out-of-phase with the reference
signal.

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The loop signal is sent to the phase
detector, which outputs the loop drive
voltage.
The loop drive voltage positions the
loop antenna to its null position.
– The loop signal will be zero.
The loop antenna is driven by a twophase induction motor:
– One winding is coupled to the
reference voltage
– The other winding is coupled to
the signal voltage from the phase
detector, to position the loop
antenna.

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The rotatable loop antennas have since been replaced with stationary loop
antennas.
The fixed loop antenna consists of two coils positioned 90o to each other.
– Each coil is connected to one of two goniometer windings which are also
90o apart.

Simplified ADF block diagram using a stationary loop antenna
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The goniometer resides in the ADF receiver and has a rotating winding that
positions itself in relation to the induced voltages in the loop antenna.
In recent years:
– goniometers have since been replaced with solid-state circuitry
– ADF pointers have been replaced with digital readouts, thus eliminating all
moving parts and increasing reliability.

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UHF ADF Systems
Military aircraft radio direction finding systems use the receiver circuitry in the
UHF communication transceiver in conjunction with direction finding
equipment.
The advantage of using UHF for direction finding is that it is not as susceptible
to precipitation static as low- to medium-frequency ADF systems.
– Precipitation static is caused by voltage that accumulates on the aircraft
surface that constantly discharges into the atmosphere.
The installation of static wicks on the trailing edges of the airframe helps to
eliminate this interference, especially for low- to medium-frequency ADF
systems.
A typical example of a military direction-finding system using a UHF receiver is
the Collins DF-301 UHF ADF.

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Rockwell Collins DF-301 system operation



The DF-301 consists of:
– A solid-state loop antenna with an associated preamplifier
– An audio processing section that includes:
 A master clock
 Antenna switching control
Filter/detector
Servo-motor control
Power supply.
– The UHF communications transceiver
Broadband RF system
Frequency selector
The antenna output is fed through the preamplifier to an external transfer relay
and then to the receiver section of the military UHF communication transceiver.
The relay ensures that the transceiver is disconnected from the ADF antenna
during the transmit mode.
The transceiver ADF audio output is returned to the DF-301 for further
processing.
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Return audio is amplified and filtered and the bearing information is resolved
into its quadrature components.
The two components are applied to a resolver and electro-mechanical nulling is
accomplished by means of a standard servo-loop arrangement.
A torque transmitter follows the resolver angular position to provide a synchro
output to the ADF bearing indicator.
The DF-301 antenna amplitude-modulates the incident RF signal, and this
modulation contains the bearing information.
Electrical rotation of the antenna translates the spatial angle of the incoming RF
signal to a phase difference between the antenna-modulated output and a
reference signal synchronous with antenna rotation.
– From this phase difference, the bearing of the incoming signal can be
recovered.
Dependable ADF system operation requires proper installation procedures.
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Installation Techniques
The first consideration is choosing a location for the ADF antenna that will
provide the best possible signal to the antenna at all times.
On low-wing aircraft, it is best to stay away from the area on the fuselage
between the leading and trailing edge of the wing.
This area exhibits reduced RF field strength. On high-wing aircraft, this area also
exists on the top of the fuselage.
In either case, optimum performance will be obtained when the antenna is
located aft of the wing trailing edge.
This position also minimizes quadrantal error, which is caused by the distortion
of the radio wave by the aircraft structure.
The quadrantal error is maximum at bearings in between the cardinal points of
the nose, wingtips, and tail.
A short metal braid or strap should be secured between the loop antenna and
the fuselage to provide a good RF ground.
Remove paint from the bonding surface of the loop antenna and passivate the
cleaned aluminum surface with an alodine to retard corrosion.
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In addition, all antenna connections should be coated with silicon grease.
Noise immunity is one of the most important considerations in ADF system
installation.
Generators and alternators should have suitable filters installed and
interference must also be suppressed from strobes, inverters, motors, and
other electrical equipment.
The proper number of static wicks should be installed at the trailing edges of all
airframe surfaces to minimize precipitation static.
The ADF system components must be installed in accordance with the
manufacturer's installation instructions and FAA Advisory Circular AC-43.13.
A continuity check of the system wiring should always be performed before
installing the equipment and applying power.

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Nav Topic 8 automatic direction finder

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