Generally, space waves are “line of sight” receivable, but  those of lower frequencies “bend” somewhat over the  horizon. The VOR signal at 108 to 118 MHz is a lower  frequency than distance measuring equipment (DME) at 962  to 1213 MHz. Therefore, when an aircraft is flown “over the  horizon” from a VOR/DME station, the DME is normally  the first to stop functioning. Static distorts the radio wave and interferes with normal  reception of communications and navigation signals. Low- frequency airborne equipment, such as automatic direction  finder (ADF) and LORAN (LOng RAnge Navigation) ,are particularly subject to static disturbance. Using very  high frequency (VHF) and ultra-high frequency (UHF)  frequencies avoids many of the discharge noise effects.  Static noise heard on navigation or communication radio  frequencies may be a warning of interference with navigation  instrument displays. Some of the problems caused by  precipitation static (P-static) are: The ground equipment, the NDB, transmits in the frequency  range of 190 to 535 kHz. Most ADFs also tune the AM  broadcast band frequencies above the NDB band (550 to  1650 kHz). However, these frequencies are not approved  for navigation because stations do not continuously identify  themselves, and they are much more susceptible to sky wave  propagation especially from dusk to dawn. NDB stations  are capable of voice transmission and are often used for  transmitting the Automated Weather Observing System  (AWOS). The aircraft must be in operational range of the  NDB. Coverage depends on the strength of the transmitting  station. Before relying on ADF indications, identify the  station by listening to the Morse code identifier. NDB stations  are usually two letters or an alpha-numeric combination. The nondirectional radio beacon (NDB) is a ground-based  radio transmitter that transmits radio energy in all directions.  The ADF, when used with an NDB, determines the bearing  from the aircraft to the transmitting station. The indicator  may be mounted in a separate instrument in the aircraft  panel.  [Figure 9-2] The ADF needle points to the NDB  ground station to determine the relative bearing (RB) to the  transmitting station. It is the number of degrees measured  clockwise between the aircraft’s heading and the direction  from which the bearing is taken. The aircraft’s magnetic  heading (MH) is the direction the aircraft is pointed with  respect to magnetic north. The magnetic bearing (MB) is the  direction to or from a radio transmitting station measured  relative to magnetic north.  The airborne equipment includes two antennas: a receiver  and the indicator instrument. The “sense” antenna (non- directional) 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 four kinds: fixed- card ADF, rotatable compass-card ADF, or radio magnetic  $^{ }$Traditional Navigation Systems Nondirectional Radio Beacon (NDB) ADF Components NDB Components Figure 9-2.  ADF indicator instrument and receiver. • Complete loss of VHF communications. • Erroneous magnetic compass readings. • Aircraft flying with one wing low while using  the autopilot. • High-pitched squeal on audio. • Motorboat sound on audio. • Loss of all avionics. •  Inoperative very-low frequency (VLF) navigation  system. • Erratic instrument readouts. • Weak transmissions and poor radio reception. • St. Elmo’s Fire. Disturbances to Radio Wave Reception 9-3 33  30  24  21  15  12  6  3  W  S  E  N HDG OFF  VOL  RST  SET  ET  FLT  FRQ  BFO  ADF  A D F F R Q ADF  KR 87 TSD 