Generally, space waves are 'line of sight" receivable, but
those of lower frequencies will "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 will normally
be the first to stop functioning.

Disturbances to Radio Wave Reception
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 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 he a warning of
interference with navigation instrument displays. Some of
the problems caused by precipitation static (P-static) are:

• 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.

Traditional Navigation Systems

Nondirectional Radio Beacon (NDB)
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 hearing
from the aircraft to the transmitting station. The indicator
may be mounted in a separate instrument in the aircraft
panel. [Figure 7.2] 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.

NDB Components
The ground equipment, the NDB, transmits in the frequency
range of 190 to 535 kHz. Most ADFs will 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 he 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 alphanumeric combination.

ADF Components
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 hut 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
indicator (RMI) with either one needle or dual needle. Fixed-
card ADF (also known as the relative bearing indicator (RBI))
always indicates zero at the top of the instrument, with the
needle indicating the RB to the station. Figure 7-3 indicates
an RB of 135°; if the MH is 045°, the MB to the station is
280°. (MH + RB = MB to the station.)

Figure 7-2. ADF Indicator Instrument and Receiver.