Instrument Flying Handbook Menu >Navigation
Radio Beacon (NDB)>ADF Components
The airborne equipment includes two antennas,
a receiver, and the indicator instrument. The “sense”
antenna (nondirectional) receives signals with nearly equal
efficiency from all directions. The “loop” antenna
receives signals better from two directions (bidirectional).
When the loop and sense antenna inputs are processed together
in the ADF radio, the result is the ability to receive a radio
signal well in all directions but one, thus resolving all directional
The indicator instrument can be one of three
kinds: the fixedcard ADF, movable-card ADF, or the radio magnetic
indicator (RMI). The fixed-card ADF (also known as the relative
bearing indicator (RBI)), always indicates zero at the top of
the instrument, and the needle indicates the RB to the station.
Figure 7-3 indicates an RB of 135°, and if the MH is 45°,
the MB to the station is 180°. (MH + RB = MB to the station.)
Figure 7-3. Relative bearing (RB) on
a fixed-card indicator.
The movable-card ADF allows the pilot to rotate
the aircraft’s present heading to the top of the instrument
so that the head of the needle indicates MB to the station,
and the tail indicates MB from the station. Figure 7-4 indicates
a heading of 45°, the MB to the station is 180°, and
the MB from the station is 360°.
Figure 7-4. Relative bearing (RB) on
a movable-card indicator.
The RMI differs from the movable-card ADF in
that it automatically rotates the azimuth card (remotely controlled
by a gyrocompass) to represent aircraft heading. The RMI has
two needles, which can be used to indicate navigation information
from either the ADF or the VOR receivers. When a needle is being
driven by the ADF, the head of the needle indicates the MB TO
the station tuned on the ADF receiver. The tail of the needle
is the bearing FROM the station. When a needle of the RMI is
driven by a VOR receiver, the needle indicates where the aircraft
is radially with respect to the VOR station. The needle points
to the bearing TO the station, as read on the azimuth card.
The tail of the needle points to the radial of the VOR the aircraft
is currently on or crossing. Figure 7-5 indicates a heading
of 005°, the MB to the station is 015°, and the MB from
the station is 195°.
Figure 7-5. Radio magnetic indicator