Module 18 − Radar Principles
Index−1 to 3
A-SCOPE - a radar display on which slant
range is shown as the distance along a horizontal trace.
ACQUIsITION - Operational phase of a fire- control
or track radar during which the radar system searches a small volume of space in a prearranged pattern.
AIR-CONTROL PANEL - Panel that monitors the dry-air input at each user equipment.
ALTITUDE - Vertical distance
of an aircraft or object above a given reference, such as ground or sea level.
AMBIGUOUS RETurns - Echoes
that exceed the PRT of a radar and appear at incorrect ranges.
AMPLITRON - See Crossed-Field Amplifier.
Antenna BEAM WIDTH - Width of a radar beam measured between half-power points
Antenna System - Routes RF
energy from the transmitter, radiates the energy into space, receives echoes, and routes the echoes to the
ANTIJAMMING Circuit - Electronic circuit used to minimize the effects of enemy countermeasures,
thereby permitting radar echoes to be visible on the indicator.
ANTITRANSMIT-RECEIVE TUBE (ATR) - Tube that
isolates the transmitter from the antenna and receiver. Used in conjunction with TR tube.
TRANSMIsSION LINE - An LC network that is designed to simulate characteristics of a transmission line.
ASYMMETRICAL MULTIVIBRATOR - Multivibrator that generates rectangular waves.
AUTOMATIC Gain CONTROL - Circuit
used to vary radar receiver gain for best reception of signals that have widely varying amplitudes.
AVERAGE Power - Output power of a transmitter as measured from the start of one pulse to the start of the next
AZIMUTH - Angular measurement in the horizontal plane in a clockwise direction.
MIXER - Waveguide arrangement that resembles a T and uses crystals for coupling the output to a balanced
BEAM - See Lobe.
BEARING RESOLUTION - Ability of a radar to distinguish between targets
that are close together in bearing.
BEAT FREQUENCIES - Difference and sum frequencies which result from
combining two different frequencies.
BLIP - See Pip.
BlockING - a condition in an amplifier, caused by overdriving one or more stages,
in which the amplifier is insensitive to small signals immediately after reception of a large signal.
BROADSIDE ARRAY - An antenna array in which the direction of maximum radiation is perpendicular to the plane of the
BUFFER Amplifier Stage - Amplifier stage that isolates one circuit from another.
CARRIER Frequency - The
frequency of an unmodulated transmitter output.
CARRIER-Controlled APPROACH - Shipboard radar system used to
guide aircraft to safe landings in poor visibility conditions.
CLUTTER - Confusing, unwanted echoes that
interfere with the observation of desired signals on a radar indicator.
COHERENCE - a definite phase
relationship between two energy waves, such as transmitted frequency and reference frequency.
Oscillator - In CW radar this oscillator supplies phase references to provide coherent video from target returns.
CONICAL SCANNING - Scanning in which the movement of the beam describes a cone, the axis of which coincides with
that of the reflector.
CONTACT - In radar, an object that reflects RF energy; target.
Reflector - Two flat reflectors that meet at an angle and are normally fed by a half-wave radiator.
CROSSED-FIELD Amplifier - High-power electron tube that converts dc to microwave power by a combination of crossed
electric and magnetic fields.
CYLINDRICAL PARABOLIC Reflector - a parabolically shaped reflector that
resembles part of a cylinder.
DEFLECTION Coils - In a cathode-ray tube, coils used to bend an electron beam
a desired amount.
DEIONIZATION TIME - In a spark gap, the time required for ionized gas to return to its
neutral state after the spark is removed.
DESIGNATION - Operational phase of a fire- control or track radar
during which the radar is directed to the general direction of a desired target.
Difference Frequency - See Beat Frequency.
DIODE DETECTOR - a demodulator that uses one or more diodes
to provide a rectified output with an average value that is proportional to the original modulation.
DirectIONAL Antenna - An antenna that radiates most effectively in only one direction.
DirectIVITY - Ability
of an antenna to radiate or receive more energy in some directions than in others. The degree of sharpness of the
DIsCRIMINATOR - a circuit in which amplitude variations are derived in response to phase or frequency
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
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
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
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
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.
FAST-TIME-CONSTANT Circuit - Differentiator circuit in the first video amplifier that allows only the
leading edges of target returns, no matter how small or large, to be used.
Feedback - The return of a
portion of the output of a circuit to its input.
FEEDHORN - a horn radiator used to feed a reflector.
First DETECTOR - See Mixer.
Frequency Compensation Network - Circuit modification used to improve or broaden
the linearity of its frequency response.
Frequency SCANNING - Varying the output frequency to achieve
Frequency SPECTRUM - In a radar, the entire range of frequencies contained in an RF pulse or signal.
Frequency Synthesizer - a bank of oscillators in which the outputs can be mixed in various combinations to produce a
wide range of frequencies.
Gain - Any increase in the strength of a signal.
GATED AGC - Circuit that
permits automatic gain control to function only during short time intervals.
GLow DIsCHARGE - Discharge of
electricity through a gas in an electron tube.
Ground CLUTTER - Unwanted echoes from surrounding land masses
that appear on a radar indicator.
Ground RANGE - The distance on the surface of the earth between a radar
and its target. Equal to slant range only if both radar and target are at the same altitude.
APPROACH - Radar system used to guide aircraft to safe landings in poor visibility conditions.
RADAR - System which provides information that is used to guide a missile to a target.
Half-Power Point - a
point on a waveform or radar beam that corresponds to half the power of the maximum power point.
MODULATOR - a high-vacuum electron tube modulator that uses a driver for pulse forming.
RADAR - Radar that provides target altitude, range, and bearing data.
HITS PER SCAN - The number of times an
RF beam strikes a target per antenna revolution.
HORIZONTAL Plane - Imaginary plane that is tangent (or
parallel) to the earth's surface at a given location.
HORN Antenna - See Horn Radiator.
Radiator - a tubular or rectangular microwave antenna that is tapered and is widest at the open end.
HYBRID RING - a circular waveguide arrangement with four branches. When properly terminated, energy is
transferred from any one branch into any two of the remaining three branches.
HYBRID MIXER - See Balanced
IF Amplifier - Usually a narrow-bandwidth IF amplifier that is tuned to one of the output frequencies produced
by the mixer.
Index of REFRACTION - The degree of bending of an RF wave when passing from one medium to
INDICATOR - Equipment that provides a visual presentation of target position information.
INSTANTANEOUS AUTOMATIC Gain CONTROL (IAGC) - Circuit that can vary the gain of the radar receiver with each input
pulse to maintain the output peak amplitude nearly constant.
INTERMEDIATE Frequency (IF) - a lower frequency
to which an RF echo is converted for ease of amplification.
KEEP-ALIVE CURRENT - See Keep-Alive Voltage.
KEEP-ALIVE Voltage - Dc voltage applied to a TR gap electrode to produce a glow discharge that allows the tube to
ionize faster when the transmitter fires.
KEYED-Oscillator TRANSMITTER - a transmitter in which one stage is
used to produce the RF pulse.
KEYER - See Synchronizer.
KLYSTRON Power Amplifier - Multicavity microwave electron tube that uses
LIN-LOG Amplifier - Amplifier in which the response is linear for weak signals and
logarithmic for large signals.
LINE of SIGHT - Straight line from a radar antenna to a target.
LINE-PULSING MODULATOR - Circuit that stores energy and forms pulses in the same circuit element, usually the
pulse-forming network (PFN).
LIQUID-COOLING System - Source of cooling for high-heat producing equipments, such as microwave components, radar
repeaters, and transmitters.
LOBE - An area of greater signal strength in the transmission pattern of an
LOGARITHMIC RECEIVER - Receiver that uses a linear logarithmic amplifier (lin-log) instead of a normal linear
Low-NOIsE Amplifier - See Preamplifier.
MAGIC T - See Balanced Mixer.
Oscillator - Electron tube that provides a high power output. Theory of operation is based on interaction of
electrons with the crossed electric and magnetic fields in a resonant cavity.
MASTER Oscillator - In a transmitter, the oscillator that establishes the carrier frequency of the
MECHANICAL SCANNING - The reflector, its feed source, or the entire antenna is moved in a desired pattern.
MINIMUM DIsCERNIBLE Signal (MDS) - The weakest signal that produces a usable signal at the output of a
receiver. The weaker the signal, the more sensitive the receiver.
MIXER - In radar, a circuit that combines
the received RF signal with a local-oscillator signal to effectively convert the received signal to a lower IF
Mode SHIFTING - In a magnetron, shifting from one mode to another during a pulse.
SKIPPING - Rather than firing on each successive pulse as desired, the magnetron fires randomly.
Modes - Operational phases (of a radar).
MODULATOR Switching DEVICE - Controls the on (discharge) and off
(charge) time of the modulator.
MODULATOR - Produces a high-voltage pulse that turns the transmitter on and off.
(SIMULTANEOUS) LOBING - Radar receiving method using two or more (usually four) partially overlapping lobes. Sum and
difference channels locate the target with respect to the axis of the antenna.
MONOPULSE RADAR - a radar
that gets the range, bearing, and elevation position data of a target from a single pulse.
RECEIVER - See Monopulse Lobing.
MOIsTURE LAPSE - Abnormal variation of moisture content at different altitudes because of high moisture located
just above large bodies of water.
MOVING TARGET INDICATOR - a device that limits the display of radar
information to moving targets.
NAUTICAL MILE - The length of a minute of arc of a great circle of the earth
NAUTICAL RADAR MILE - See Radar Mile.
NOIsE - In radar, erratic or random deflection or intensity of
the indicator sweep that tends to mask small echo signals.
NOIsE FIGURE - The ratio of output noise to input
noise in a receiver.
NUTATING - Moving an antenna feed point in a conical pattern so that the polarization of
the beam does not change.
OMNIDirectIONAL Antenna - An antenna that radiates equally in all directions
ORANGE-PEEL PARABOLOID - a section of a complete circular paraboloid that is narrow in the horizontal plane
and wide in the vertical plane.
PARABOLIC Reflector - An antenna reflector in the shape of a parabola. It converts spherical wavefronts
from the radiating element into plane wavefronts.
PARALLEL-CONNECTED DUPLEXER - Configuration in which the
TR spark gap is connected across the two legs of the transmission line one-quarter wavelength from the Tjunction.
PARASITIC ARRAY - An antenna array containing one or more elements not connected to the transmission line.
PEAK Power - Maximum power of the RF pulse from a radar transmitter. PERSIsTANCE - The length of time a phosphor
dot glows on a CRT before disappearing. PHANTASTRON - a variable-length sawtooth generator used to produce a sweep
on an A-scope. PIP (BLIP) - On a CRT display, a spot of light or a base-line irregularity representing the radar
Plane WAVEFRONTS - Waves of energy that are flat, parallel planes and perpendicular to the direction of
PLANNED-POSITION INDICATOR - a radar display in which range is indicated by the distance of a bright spot or
pip from the center of the screen and the bearing is indicated by the radial angle of the spot.
Gain - In an antenna, the ratio of its radiated power to that of a reference.
TRANSMITTER - Transmitter that uses a series of power amplifiers to create a high level of power.
PREAmplifier (PREAMP) - An amplifier that raises the output of a low-level source for further processing without
appreciable degradation of the signal-to-noise ratio.
PRIMARY Loop - In a cooling system, the primary source
of cooling for the distilled water.
PROBE COUPLER - a resonant conductor placed in a waveguide or cavity to
insert or extract energy.
PULSE WIDTH - Duration of time between the leading and trailing edges of a pulse.
PULSE-forMING Network (PFN) - An LC network that alternately stores and releases energy in an approximately
PULSE-REPETITION RATE (PRR) - Average number of pulses per unit of time; pulse rate.
PULSE-REPETITION Frequency (PRF) - The rate at which pulses are transmitted, given in hertz or pulses per second;
reciprocal of pulse-repetition time.
PULSE-REPETITION TIME (PRT) - Interval between the start of one pulse
and the start of the next pulse; reciprocal of pulse-repetition frequency.
RADAR - An acronym for RAdio Detecting And Ranging.
RADAR ALTIMETER - Airborne radar that measures the
distance of the aircraft above the ground.
RADAR BEAM - The space in front of a radar antenna where a target
can be effectively detected or tracked.
RADAR DIsTRIBUTION SwitchBOARD - An electrical switching panel used to connect inputs from any of
several radars to repeaters.
RADAR MILE - Time interval (12.36 microseconds) for RF energy to travel out
from a radar to a target and back to the radar; radar nautical mile.
RADAR TEST SET - Combination of several
test circuits and equipment used to test various characteristics of a radar.
RANGE - The length of a
straight line between a radar set and a target.
RANGE-HEIGHT INDICATOR - a radar display on which slant
range is shown along the X axis and height along the Y axis.
RANGE-GATE - a movable gate used to select
radar echoes from a very short-range interval.
RANGE MARKER - a movable vertical pulse on an A-scope or ring
on a PPI scope used to measure the range of an echo or to calibrate the range scale.
RESOLUTION - Ability of a radar to distinguish between targets that are close together. RANGE STEP - On an A-scope
sweep, vertical displacement used to measure the range of an echo. RECEIVER - In radar, a unit that converts RF
echoes to video and/or audio signals.
RECEIVER SENSITIVITY - The degree to which a receiver can usefully
detect a weak signal; the lower limit of useful signal input to the receiver.
RECOVery TIME - In a radar, the time interval between the end of the transmitted pulse and the time when echo
signals are no longer attenuated by the TR gap.
REFLECTING OBJECT - In radar, an air or surface contact that
provides an echo.
REFLEX KLYSTRON - a microwave oscillator that is tuned by changing the repeller voltage.
REFRACTION - The bending of RF waves as the waves pass through mediums of different density.
Index - In a wave-transmission medium, the ratio between the phase velocity in free space and in the medium.
REGENERATION - See Feedback.
RELATIVE BEARING - Bearing of target measured in a clockwise direction from "dead ahead" of a ship or plane.
RESONANCE CHAMBER - See Echo Box.
RETurn - The RF signal reflected back from a radar target; echo.
Radiation HAZARD - Health hazard caused by exposure to electromagnetic radiation or high- energy particles (ions).
RING TIME - In radar, the time during which the output of an echo box remains above a
RINGING - Rf oscillations caused by shock excitation of a resonant circuit (cavity).
SCANNING - Systematic movement of a radar beam to cover a definite pattern or area in space.
CLUTTER - Unwanted echoes from the irregular surface of the sea that appear on a radar indicator.
RADAR System - Early-warning device that searches a fixed volume of space.
SECOND DETECTOR DEMODULATOR - The
part of the receiver that separates the audio or video component from the modulated intermediate frequency.
SECOND-SWEEP ECHOES - See Ambiguous Returns.
SECONDARY Loop - In a cooling system, the loop that transfers the
heat from the heat source (electronic equipment) to the primary loop; usually distilled water.
SELF-SYNCHRONIZED RADAR - a type of radar in which the timing pulses are generated within the transmitter.
SENSITIVITY TIME CONTROL (STC) - a circuit that varies the gain of a receiver as a function of time.
Series-CONNECTED DUPLEXER - Configuration in which the TR spark gap is connected in series in one leg of the
transmission line one-half wavelength away from the T- junction.
SHADOW - a dead spot (minimum radiation)
caused by the physical obstruction of transmitted waves by a feed horn.
Single-ENDED MIXER - See Unbalanced
Single, STATIONARY-LOBE SCANNING System - Antenna (with a single, stationary beam) that is
rotated to obtain 360-degree coverage.
SLANT RANGE - See Range.
SPECTRUM ANALYZER - a test instrument
that provides a visual display of the frequency distribution of a transmitter output.
WAVEFRONTS - Waves of energy that spread out in concentric circles. STABILITY - In a magnetron, the ability to
maintain normal operating characteristics. STATUTE MILE - I5,280 ft.
STUB - a short section of transmission
line connected in parallel with the main transmission line.
SCINTILLATION - Apparent change in target
reflectivity. Motion of the target causes radar pulses to bounce off different parts of the target, such as
fuselage and wingtip.
SUPERHETERODYNE RECEIVER - a type of receiver that uses a mixer to convert the RF echo
to an IF signal for amplification.
SUPPORT System - For a radar, a system that provides an auxiliary input,
such as dry air, electrical power, or liquid cooling.
SYMMETRICAL MULTIVIBRATOR - Circuit that generates
SYNCHRONIZER - Circuit that supplies timing signals to other radar components.
TARGET - In radar,
a specific object of radar search or detection.
TARGET RESOLUTION - The ability of a radar to distinguish
between two or more targets that are close to each other.
THREE-DIMENSIONAL RADAR (3D) - Measures the range,
bearing, and altitude of a target.
THYRATRON - Gas tube used as a modulator switching device.
TIMER - See Synchronizer.
TR RECOVery TIME - Time required for a fired TR or ATR tube to deionize to a normal level of conductance.
TRACK - Operational phase of a fire-control or track radar during which the radar beam is kept on the
TRACK RADAR - Radar that provides continuous range, bearing, and elevation data by keeping the RF beam on
TRANSMIT-RECEIVE TUBE (TR) - Gas-filled RF switch that is used as a duplexer.
TRANSMITTER - Equipment that generates, amplifies, and modulates electromagnetic energy.
Frequency (CARRIER Frequency) - The frequency of the unmodulated output of a transmitter.
Signal - In a monopulse radar system, the combination of individual lobe signals that represents target offset
direction and amplitude from the antenna axis.
TRIGGER GENERATOR - See Synchronizer.
PULSES - In radar, pulses that are used to initiate specific events.
TRUE BEARING - Angle between a target and
true north measured clockwise in the horizontal plane.
TRUE NORTH - Geographic north.
TRUNCATED PARABOLOID - a paraboloid reflector that has been cut away at
the top and bottom to increase beam width in the vertical plane.
TWO-DIMENSIONAL RADAR (2D) - Measures the
range and bearing to a target.
UNBALANCED Crystal MIXER - Circuit consisting of a section of coaxial
transmission line one- half wavelength long that is tuned to the difference (intermediate) frequency between the
local oscillator and RF echo signals.
VERTICAL Plane - Imaginary plane that is perpendicular to the
Video ENHANCEMENT FEATURES - See Antijamming Circuits.
Voltage STandING WAVE RATIO (VSWR) - In a
waveguide, the ratio of the electric field at a maximum point to that of an adjacent minimum point.
WAVEGUIDE DUPLEXER - Consists of TR and ATR tubes housed in a resonant cavity attached to a waveguide
WAVEMETER - An instrument for measuring the wavelength of an RF wave.
X-RAY EMIsSION - Penetrating
radiation similar to light, but with shorter wavelength, that can penetrate human tissue.
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