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ADF Components


Instrument Flying Handbook Menu >Navigation Systems >Nondirectional 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 ambiguity.

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 (RMI).

NDB Components
Function of ADF
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