Any ground antenna transmitting LF or MF radio waves in  range of the aircraft receiver’s tuning capabilities can be  used for ADF. This includes those from AM radio stations.  Audible identifier tones are loaded on the NDB carrier waves.  Typically, a two-character Morse code designator is used.  With an AM radio station transmission, the AM broadcast is  heard instead of a station identifier code. The frequency for  an NDB transmitter is given on an aeronautical chart next  to a symbol for the transmitter. The identifying designator  is also given.  [Figure 11-106] ADF receivers can be mounted in the flight deck with the  controls accessible to the user. This is found on many general  aviation aircraft. Alternately, the ADF receiver is mounted in  a remote avionics bay with only the control head in the flight  deck. Dual ADF receivers are common. ADF information  can be displayed on the ADF indicators mentioned or it can  be digital. Modern, flat, multipurpose electronic displays  usually display the ADF digitally.  [Figure 11-107]  When  ANT is selected on an ADF receiver, the loop antenna is  cut out and only the sense antenna is active. This provides  better multi-directional reception of broadcasts in the ADF  frequency range, such as weather or AWAS broadcasts.  When the best frequency oscillator (BFO) is selected on an  ADF receiver/controller, an internal beat frequency oscillator  is connected to the IF amplifier inside the ADF receiver. This  is used when an NDB does not transmit a modulated signal.  Continued refinements to ADF technology has brought it to  its current state. The rotating receiving antenna is replaced  by a fixed loop with a ferrite core. This increases sensitivity  and allows a smaller antenna to be used. The most modern  ADF systems have two loop antennas mounted at 90° to  each other. The received signal induces voltage that is sent  to two stators in a resolver or goniometer. The goniometer  stators induce voltage in a rotor that correlates to the signal  of the fixed loops. The rotor is driven by a motor to seek the  null. The same motor rotates the pointer in the flight deck  indicator to show the relative or magnetic bearing to the  station.  [Figure 11-108]  Technicians should note that the installation of the ADF  antenna is critical to a correct indication since it is a  directional device. Calibration with the longitudinal axis of  the fuselage or nose of the aircraft is important. A single null  reception area must exist in the correct direction. The antenna  must be oriented so the ADF indicates station location when  the aircraft is flying toward it rather than away. Follow all  manufacturer’s instructions.  11-48 Figure 11-106.  Nondirectional broadcast antenna in the LF and  medium frequency range are used for ADF navigation.  Figure 11-107.  A cockpit mountable ADF receiver used on general  aviation aircraft. 