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Module 18 - Radar Principles
Navy Electricity and Electronics Training Series (NEETS)
Chapter 4:  Pages AI-1 through AI-11

Module 18 - Radar Principles

Pages i, 1-1, 1-11, 1-21, 1-31, 1-41, 2-1, 2-11, 2-21, 2-31, 2-41, 3-1, 3-11, 3-21, 4-1, 4-11, 4-21, AI-1, AII-1, Index-1 to 3


Appendix I



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 receiver.

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.

ARTIFICIAL 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 pulse.

AZIMUTH - Angular measurement in the horizontal plane in a clockwise direction.

BALANCED MIXER - Waveguide arrangement that resembles a T and uses crystals for coupling the output to a balanced transformer.

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 array.

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.

Coherent 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.

CORNER 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 antenna beam.


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 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.


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 electronic scanning.

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.

Ground-Controlled APPROACH - Radar system used to guide aircraft to safe landings in poor visibility conditions.

GUIDANCE 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.

HARD-TUBE Modulator - a high-vacuum electron tube modulator that uses a driver for pulse forming.

HEIGHT-FINDING 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.

HORN 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 Mixer.

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 another.

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 velocity modulation.

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 antenna.

LOGARITHMIC RECEIVER - Receiver that uses a linear logarithmic amplifier (lin-log) instead of a normal linear amplifier.

Low-NOIsE Amplifier - See Preamplifier.

MAGIC T - See Balanced Mixer.

MAGNETRON 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 output.

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 frequency signal.

Mode SHIFTING - In a magnetron, shifting from one mode to another during a pulse.

Mode 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.

MONOPULSE (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.

MONOPULSE 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 (6,076 ft.)

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 (nondirectional).

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 echo.

Plane WAVEFRONTS - Waves of energy that are flat, parallel planes and perpendicular to the direction of propagation.

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.

Power Gain - In an antenna, the ratio of its radiated power to that of a reference.

Power-Amplifier (CHAIN) 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 rectangular wave.

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.

RANGE 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.

REFRACTIVE 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.


RETurn - The RF signal reflected back from a radar target; echo.

RF Radiation HAZARD - Health hazard caused by exposure to electromagnetic radiation or high- energy particles (ions). Abbreviated RADHAZ.

RING TIME - In radar, the time during which the output of an echo box remains above a specified level.

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.

SEA CLUTTER - Unwanted echoes from the irregular surface of the sea that appear on a radar indicator.

SEARCH 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 Crystal Mixer.

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.

SPHERICAL 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 square waves.


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 target.

TRACK Radar - Radar that provides continuous range, bearing, and elevation data by keeping the RF beam on the target.

TRANSMIT-RECEIVE TUBE (TR) - Gas-filled RF switch that is used as a duplexer.

Transmitter - Equipment that generates, amplifies, and modulates electromagnetic energy.

Transmitter Frequency (CARRIER Frequency) - The frequency of the unmodulated output of a transmitter.

TRAVERSE (BEARING) 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.

TRIGGER 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 horizontal plane.

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 system.

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.


NEETS Modules
- Matter, Energy, and Direct Current
- Alternating Current and Transformers
- Circuit Protection, Control, and Measurement
- Electrical Conductors, Wiring Techniques, and Schematic Reading
- Generators and Motors
- Electronic Emission, Tubes, and Power Supplies
- Solid-State Devices and Power Supplies
- Amplifiers
- Wave-Generation and Wave-Shaping Circuits
- Wave Propagation, Transmission Lines, and Antennas
- Microwave Principles
- Modulation Principles
- Introduction to Number Systems and Logic Circuits
- - Introduction to Microelectronics
- Principles of Synchros, Servos, and Gyros
- Introduction to Test Equipment
- Radio-Frequency Communications Principles
- Radar Principles
- The Technician's Handbook, Master Glossary
- Test Methods and Practices
- Introduction to Digital Computers
- Magnetic Recording
- Introduction to Fiber Optics
Note: Navy Electricity and Electronics Training Series (NEETS) content is U.S. Navy property in the public domain.
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