AI-3
DISCRIMINATOR—A circuit in which amplitude variations are derived in response to phase or
frequency variations.
DISTILLED WATER—Water that has been purified through a process of evaporation and
condensation.
DOPPLER EFFECT—In radar, the change in frequency of a received signal caused by the relative
motion between the radar and the target.
DOPPLER FREQUENCY—Difference between transmitted and reflected frequencies; caused by the
Doppler effect.
DOUBLE-MODING—In a transmitter output tube, the abrupt and random change from one frequency to
another.
DRY-AIR SYSTEM—Provides dehumidified air for electronic equipment that is moisture critical.
DUCTING—Trapping of an rf wave between two layers of the earth's atmosphere or between an
atmospheric layer and the earth.
DUPLEXER—A radar device that switches the antenna from the transmitter to the receiver and vice
versa.
DUTY CYCLE—In a transmitter, ratio of time on to time off.
ECHO—The rf signal reflected back from a radar target.
ECHO BOX—A resonant cavity device that is used to check the overall performance of a radar system.
It receives a portion of the transmitted pulse and retransmits it back to the receiver as a slowly
decaying transient.
ELECTRICAL POWER SYSTEM—Provides the necessary input power.
ELECTRONIC COUNTER-COUNTERMEASURES (ECCM) CIRCUITS—See Antijamming
Circuits.
ELECTRONIC EQUIPMENT DEHYDRATOR—Provides alternate dry-air input in the event of
failure of the central dry-air system. May include a compressor.
ELECTRONIC FREQUENCY COUNTER—An instrument that counts the number of cycles (pulses)
occurring during a precise time interval.
ELECTRONIC SCANNING—Scanning in which the axis of the beam is moved, relative to the antenna
axis, in a desired pattern.
ELECTRONICS DRY-AIR BRANCH—A common line for providing dry air to various electronic
equipment, such as search radar, fire-control radar, and repeaters.
ELEVATION ANGLE—The angle between the horizontal plane and the line of sight.
EMERGENCY POWER—Temporary source of limited electrical power used upon the loss of the
normal power source.
EXTERNALLY SYNCHRONIZED RADAR—Radar system in which timing pulses are generated by a
master oscillator external to the transmitter.
