Non-Directional Beacons (NDBs)
A non-directional beacon is basically an AM radio station used for aircraft navigation. NDBs transmit on a frequency range of 190 to 535 kHz. The AM radio band is from 550 to 1650 kHz, but these frequencies are much more susceptible to sky wave propagation.
Sky wave propagation occurs when a transmitted signal is reflected by the ionosphere back toward the ground. This allows radio signals to be received at great distances well beyond normal line of sight reception range. Sky wave propagation is much more prominent at night, when there is no interference from the sun.
The automatic direction finder, ADF, inside the airplane receives the NDB signal. The ADF uses two antennas, a sense antenna and a loop antenna, which is a directional antenna. The two signals are processed by the ADF to determine the bearing to the station.
Most ADF receivers can be tuned from 190 kHz through 1650 kHz. However, AM radio stations are not usable for navigation, because they broadcast no station identifier.
Tuning and Identifying an NDB
As with a VOR, select the appropriate frequency and configure the airplane's audio system so that the NDB's Morse code identifier is audible. Each NDB has a unique two or three letter identifier. Since the ADF has no flag to indicate reception of an NDB, the pilot must continuously monitor the Morse code identifier whenever the NDB station is being used.
Types of Bearing
The ADF needle points directly toward the NDB, displaying a bearing relative to the nose of the airplane. This relative bearing is simply the angular difference between the nose of the airplane and the station's location. Turning the airplane until the needle is pointing directly ahead results in the airplane being pointed directly at the station.
The magnetic bearing is much of the time a more usable number, since it tells you what the magnetic direction to the station is from the airplane. To find the magnetic bearing, add the airplane's heading to the relative bearing displayed by the ADF needle. For example, if the ADF needle showed a relative bearing of 150 degrees, and the airplane was on a 030 heading, then the magnetic bearing to the station is 180 degrees. This means a 180 degree heading would point the airplane at the NDB, so the NDB is south of the airplane.
Adding or subtracting 180 degrees from the magnetic bearing to the NDB gives the magnetic bearing from the station. In our example, 180 plus 180 gives us 360 degrees from the station. This means the airplane is north of the station.
Most pilots memorize this formula to aid them in learning to use NDB's: MH + RB = MB, or magnetic heading plus relative bearing equals magnetic bearing.