Appendix A: Glossary of Terms

A reference glossary of 426 radar and microwave terms drawn from the source course material. Use the letter index to jump to a section. This appendix is supplementary reference material; for the curated short glossary of the core terms used across the chapters, see Appendix G.

Flagged for review. These definitions were extracted from the course material and lightly cleaned. Verify any specification, frequency, or model reference against a current authoritative source before reuse.

A · B · H · V · U · L · S · C · X · K · D · E · F · # · G · I · J · M · N · O · P · Q · R · T · W · Y · Z

A

Absolute Altitude: The altitude of an object relative to the ground beneath it.
Acceptable Degradation: The allowable reduction in system performance. For a fire control radar, the acceptable degradation is usually expressed as a reduction in range; for example, the maximum lock-on range might be degraded by 25 percent without loss of essential defense capability.
Acquisition: The process between the initial location of a target and the final alignment of the tracking radar on the target. Operational phase of a fire-control or track radar during which the radar system searches a small volume of space in a pre-arranged pattern. A procedure by which a fire control tracking radar attains initial lock-on. Usually, the approximate target coordinates are supplied to the tracking radar and it searches a predetermined volume of space to locate the target.
Active Electronically Scanned Array (AESA): An active phased array radar.
AFC (Automatic Frequency Control): An arrangement whereby the frequency of an oscillator or the tuning of a circuit is automatically maintained within specified limits with respect to a reference frequency. A magnetron drifts in frequency over a period of time. The AFC of a radar makes the local oscillator shift by an equal amount so the IF frequency will remain constant.
AGC (Automatic Gain Control): A method for automatically obtaining an essentially constant receiver output amplitude. The amplitude of the received signal in the range gate determines the AGC bias (a DC voltage) which controls the receiver gain so as to maintain a nearly constant output even though the amplitude of the input signal changes.
Air-Control Panel: Panel that monitors the dry-air input at each users' 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.
Amplifier: An electronic device used to increase signal magnitude or power. See also GaAs FET Amplifier, Klystron Amplifier, Traveling-Wave Tube Amplifier.
Amplitude Modulation (AM): A method of impressing a message on a carrier wave by causing the carrier amplitude to vary proportionally to the message waveform.
Amplitude Shift Keying (ASK): A method of impressing a digital signal upon a carrier signal by causing the carrier amplitude to take different values corresponding to the different values of the digital signal.
Angle Jamming: ECM technique, when azimuth and elevation information from a scanning fire control radar is jammed by transmitting a jamming pulse similar to the radar pulse, but with modulation information out of phase with the returning target angle modulation information.
Angle of Reflection: The angle between the reflected wave and the normal.
Angular Error: The error in angle of an object reported by the antenna.
Angular Rate: The rate of change of the bearing of an object.

B

Backscattering: Radio wave propagation in which the direction of the incident and scattered waves, resolved along a reference direction (usually horizontal) are oppositely directed. A signal received by backscattering is often referred to as backscatter. In optics, the scattering of light into a direction generally opposite to the original one.
Backward Wave Oscillator (BWO): A cross-field device in which an electron stream interacts with a backward wave on a non-reentrant circuit. This oscillator may be electronically tuned over a wide range of frequencies, is relatively unaffected by load variations and is stable.
Balanced Mixer: The two most frequently encountered mixer types are single-balanced and double-balanced. In a double-balanced mixer, four Schottky diodes and two wideband transformers are employed to provide isolation of all three ports. A Waveguide arrangement that resembles a T and uses crystals for coupling the output to a balanced transformer.
Bandpass Filter: A type of frequency discrimination network designed to pass a band or range of frequencies and produce attenuation to all other frequencies outside of the pass region. The upper and lower frequencies are usually specified to be the half power (-3 dB) or half voltage points (-6 dB). Bands [Frequency]
Bandwidth: An expression used to define the actual operational frequency range of a receiver when it is tuned to a certain frequency. For a radar receiver, it is the difference between the two frequencies at which the receiver response is reduced to some fraction of its maximum response (such as 3 dB, 6 dB, or some other specified level). The frequencies between which "satisfactory" performance are achieved.
Barker Codes: Binary codes used in pulse compression.
Barrage Noise Jamming: Noise jamming spread in frequency to deny the use of multiple radar frequencies to effectively deny range information. Although this is attractive because it enables one jammer to simultaneously jam several radar of different frequencies, it does have the inherent problem that the wider the jamming spread, the less jamming power available per radar; that is, the watts per MHz bandwidth is low.
Bayonet Connector: A connector using some number of locking posts or slots so that the two ends of the connectors will mate and lock together with a twisting motion. There are many different families of connectors that are bayonet locking, BNC being the most common.
Beacon: A system wherein a transponder in a missile receives coded signals from a shipboard radar guidance transmitter and transmits reply signals to a shipboard radar beacon receiver to enable a computer to determine missile position. The missile beacon transmitter and shipboard radar beacon receiver are tuned to a frequency different from that of the guidance transmitter.

H

HF 3 MHz: 30 MHz
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.
Harmonic: A sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency. For example, a component that is twice the fundamental frequency is called the second harmonic. (The fundamental is the first harmonic.)
Height-Finding Radar: A radar that provides target altitude, range, and bearing data.
Helical Scanning: Radar scanning in which the RF beam follows a distorted spiral motion.
Hertz (Hz): The unit of frequency equal to one cycle per second.
Heterodyne: To generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer.
Heterodyne Repeater: In radio reception and retransmission, a repeater that converts the original band of frequencies of the received signal to a different frequency band for retransmission after amplification.
HF (High Frequency): The band of frequencies between 3 MHz and 30 MHz.
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: A flared, open-ended section of waveguide used to radiate the energy from a waveguide into space. Also termed "horn" or "horn radiator." Usually linearly polarized, it will be vertically polarized when the feed probe is vertical, or horizontally polarized if the feed is horizontal. Circular polarization can be obtained by feeding a square horn at a 45E angle and phase shifting the vertical or horizontal excitation by 90E.
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.

V

VHF 30 MHz: 300 MHz
Varactor Diode: A P-N junction employing an external bias to create a depletion layer containing very few charge carriers. The diode effectively acts as a variable capacitor.
Varactor Tuned Oscillator: A varactor diode serves as a voltage-controlled capacitor in a tuned circuit to control the frequency of a negative resistance oscillator. The major feature of this oscillator is its extremely fast tuning speed. A limiting factor is the ability of the external voltage driver circuit to change the voltage across the varactor diode, which is primarily controlled by the driver impedance and the bypass capacitors in the tuning circuit.
Velocity Gate Pull-off (VGPO): Method of capturing the velocity gate of a Doppler radar and moving it away from the skin echo. Similar to the RGPO, but used against CW or Doppler velocity tracking radar systems. The CW or pulse Doppler frequency, which is amplified and retransmitted, is shifted in frequency (velocity) to provide an apparent rate change or Doppler shift.
Velocity Modulation: Velocity modulation is modification of the velocity of an electron beam by alternately accelerating and decelerating the electrons at a frequency equal to the input frequency. Thus, the electrons are segregated in bunches, each bunch causing a cycle or current as it passes an output electrode. The velocity of the electrons is thus a function of the modulation voltage. See also Klystron, Multicavity and Klystron, Reflex.
Vertical Plane: Imaginary plane that is perpendicular to the horizontal plane.
Victim: A receiver (radar or missile) that suffers degradation due to ECM or EMI effects.
Video: Receiver RF signals that have been converted (post-detection) into a pulse envelope that can be seen when applied to some type of radar visual display; also used to describe the actual display itself (such as the video on an A-scope).
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.

U

UHF 300 MHz: 1.0 GHz
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.
Unbalanced Modulator: A modulator in which the modulation factor is different for the alternate half-cycles of the carrier.
Unbalanced Transmission Line: A transmission line one of whose conductors is grounded. A standard condition that occurs with the usage of Coaxial Cable implementations.
Undesired Signal: Any signal that tends to produce degradation in the operation of equipment or systems. Noise.
Unidirectional: In one direction only.
Unidirectional Array: An array that radiates in only one general direction.
Uniform Linear Array: An antenna composed of a relatively large number of usually identical elements arranged in a single line or in a plane with uniform spacing and usually with a uniform feed system.
Uniform-Spectrum Random Noise: Random noise
Unit Impulse: A mathematical artifice consisting of an impulse of infinite amplitude and zero width, and having an area of unity. Note: the unit impulse is useful for the mathematical expression of the impulse response, i.e., the transfer function, of a device.
Up-Converter: A device that translates frequencies from lower to higher frequencies.

L

L 1.0 GHz: 2.0 GHz
L-Band: A contiguous range of frequencies between 1,000 MHz and 2,000 MHz. Frequencies between 1,700 and 1850 MHz may be referred to as the upper L-band.
Leakage: Undesired radiation or conduction of RF energy through the shielding of an enclosed area or of an electronic device.
Lens, Radar: The purpose of any such lens is to refract (focus) the diverging beam from an RF feed into a parallel beam (transmitting) or vice versa (receiving). The polarization is feed dependent.
Light Amplification by Stimulated Emission of Radiation (LASER): A process of generating coherent light. The process utilizes a natural molecular (and atomic) phenomenon whereby molecules absorb incident electromagnetic energy at specific frequencies, store this energy for short but usable periods, and then release the stored energy in the form of light at particular frequencies in an extremely narrow frequency band.
Limiting: A term to describe that an amplifier has reached its point of saturation or maximum output voltage swing. Deliberate limiting of the signal is used in FM demodulation so that AM will not also be demodulated.
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 equipment, such as microwave components, radar repeaters, and transmitters.
Littoral: Near a shore.
Load Termination: A termination is designed to act as a load and absorb microwave energy when the actual load is not present. Also called a Dummy Load. An example load termination for a waveguide is shown below.
Lobe: An area of greater signal strength in the transmission pattern of an antenna.
Lobe, Antenna: Various parts of the antenna's radiation pattern are referred to as lobes, which may be sub-classified into major and minor lobes. The major lobe is the lobe of greatest gain and is also referred to as the main lobe or main beam. The minor lobes are further sub-classified into side and back lobes as indicated in the figure to the right. The numbering of the sidelobes is from the main lobe to the back lobe.
Local Oscillator Frequency: An internally generated frequency in a superheterodyne receiver. The local oscillator frequency differs from the receiver operating frequency by an amount equal to the intermediate frequency (IF) of the receiver. The local oscillator frequency may be either above or below the incoming signal frequency.
Logarithmic Receiver: A receiver that uses a linear logarithmic amplifier (lin-log) instead of a linear amplifier.
Log Video: This receiver process, generally implemented in the IF, compresses the dynamic range of the signal so both weak and strong signals are displayed without changing the gain setting. Output voltage can be calibrated in volts/dB of input power.
Long Pulse Mode: A long pulse mode is used to obtain high average power for long-range search or tracking, while a short pulse mode gives low average power for short-range, high-definition tracking or search.
LORO (Lobe-On-Receive-Only): A mode of operation generally consisting of transmitting on one non-scanning antenna system and receiving the reflected energy on another scanning system.
Low-Noise Amplifier (LNA): A circuit designed to amplify low-level signals without adding an undo amount of noise.

S

S 2.0 GHz: 4.0 GHz
Stagger Pulse Repetition Frequency (PRF): The rate at which pulses are transmitted, given in hertz or pulses per second; reciprocal of pulse repetition time. The technique of switching PRF (or PRI) to different values on a pulse-to-pulse basis such that the various intervals follow a regular pattern. This is useful in compensating for blind speeds in pulsed MTI radar. Interpulse intervals that differ but follow a regular pattern.
S-Band: A range of frequencies between 2,000 MHz and 4,000 MHz.
Scan: To transverse or sweep a sector or volume of airspace with a recurring pattern, by means of a controlled directional beam from a radar antenna. See also Antenna, nutating.
Scanning: Systematic movement of a radar beam to cover a definite pattern or area in space.
Scattering: Of a wave propagating in a material medium, a phenomenon in which the direction, frequency, or polarization of the wave is changed when the wave encounters discontinuities in the medium, or interacts with the material at the atomic or molecular level.
Scattering Loss: The part of the transmission (power) loss that results from scattering within a transmission medium or from roughness of a reflecting surface.
Schottky Barrier Diode: The Schottky barrier diode is a simple metal-semiconductor boundary with no P-N junction. A depletion region between the metal contact and the doped semiconductor region offers little capacitance at microwave frequencies. This diode finds use as detectors, mixers, and switches.
Schottky Diode Mixer: The mixer is a critical component in modern RF systems. Any nonlinear element can perform the mixing function, but parameters determining optimal mixing are noise figure, input admittance, and IF noise and impedance. The Schottky diode is particularly effective because of its low-noise figure and nearly square law characteristics.
Schottky Diode Switch: Standard P-N diodes are limited in switching ability at high frequencies because of capacitance provided by the minority carriers. The Schottky diode overcomes this problem by use of the metal-semiconductor junction with inherently low carrier lifetimes, typically less than 100 picoseconds.
Scintillation: Apparent change in target reflectivity. Motion of the target causes successive radar pulses to bounce off different parts of the target, such as fuselage and wingtip. In electromagnetic wave propagation, a small random fluctuation of the received field strength about its mean value.
Search Radar: A radar whose prime function is to scan (search) a specified volume of space and indicate the presence of any targets on some type of visual display, and, in some cases, to provide coordinates of the targets to a fire control system to assist in target acquisition and tracking.
Secondary Radar: A radio-determination system based on the comparison of reference signals with radio signals retransmitted from the position to be determined.
Seeker: The seeker consists of circuitry in a homing missile that detects, electronically examines, and tracks the target; provides data for controlling the flight path of the missile; and provides signals for destroying the missile or for detonating it at intercept. (The seeker function is similar to that of an interferometer.)
Self-Synchronized Radar: A type of radar in which the timing pulses are generated within the transmitter.
Sensitivity: The sensitivity of a receiver is taken as the minimum signal level required to produce an output signal having a specified signal-to-noise ratio.
Sensor: The receiver portion of a transmitter/receiver pair used to detect and process electromagnetic energy.
Shadow: A dead spot (minimum radiation) caused by the physical obstruction of transmitted waves by a feed horn.
Shielding: The physical arrangement of shields for a particular component, equipment, or system. (A shield is a housing, screen, or other material, usually conducting, that substantially reduces the effect of electric or magnetic fields on one side of the shield upon devices or circuits on the other side.) Examples are tube shields, a shielded enclosure or cabinet for a radar receiver, and the screen around a screen room.
Sideband: A signal either above or below the carrier frequency, produced by the modulation of the carrier wave by some other wave.
Signal Strength: The magnitude of a signal at a particular location. Units are volts per meter or dBV/m.
Signature: The set of parameters that describe the characteristics of a radar target or an RF emitter and distinguish one emitter from another. Signature parameters include the radio frequency of the carrier, the modulation characteristics (typically the pulse modulation code), and the scan pattern.
Silicon Controlled Switch: A P-N-P-N device able to operate at sub-microsecond switching speeds by the application of gate signals. Because it is a four-layer device, this switch is also known as a tetrode thyristor.
Slant Polarization: Technique of rotating a linear antenna 45E so it can receive or jam both horizontal and vertical polarization although there is a 3 dB loss.
Spectrum: The distribution of power versus frequency in an electromagnetic wave. See also Spectrum Signature Analysis and illustrations under Sideband.
Spectrum Analyzer: An electronic device for automatically displaying the spectrum of the electromagnetic radiation from one or more devices. A cathode ray tube display is commonly used to display this power-versus frequency spectrum.
Spectrum Signature Analysis: The analysis of the electromagnetic radiation from an electronic device to determine the relative power in each sideband, harmonic, and spurious emission compared to the carrier frequency.
Spectrum Width: The difference between the frequency limits of the band, which contains all the spectrum frequency components of significant magnitude.
Spherical Wavefronts: Waves of energy that spread out in concentric circles.
Spoiler Antenna: An antenna used to change (spoil) the antenna pattern of a second antenna so as to reduce the nulls in the pattern of the second antenna.
Spoking: Periodic flashes of the rotating radial radar display. Sometimes caused by mutual interference.
Spot Jamming: Narrow frequency band jamming concentrated against a specific radar at a particular frequency. The jamming bandwidth is comparable to the radar bandpass. Can deny range and angle information.
Spurious Emission: Electromagnetic radiation transmitted on a frequency outside the bandwidth required for satisfactory transmission of the required waveform. Spurious emissions include harmonics, parasitic emissions, and intermodulation products but exclude necessary modulation sidebands of the fundamental carrier frequency.
Squint Angle: The angular difference between the axis of the antenna main lobe and the geometric axis of the antenna reflector, such as the constant angle maintained during conical scan as the main lobe rotates around the geometric axis of the reflector.
Staggered PRF: Staggered PRF allows an increase in MTI blind speeds such that no zeros exist in the velocity response at lower velocities. In a two-period mode, the usual "blind speed" or occurrence of a zero in the velocity response is multiplied by a factor that is a function of the ratio of the two repetition periods.
STAMO (Stable Master Oscillator): A very stable (drift-free) oscillatory used to provide a precise frequency for transmission and for comparison with the reflected radar signal returned to the receiver, such as in a Doppler radar where a precise difference between transmitted and received signals must be measured to determine the Doppler frequency accurately.
Stand-Forward Jamming: A method that places the jamming vehicle between the enemy sensors and attack aircraft.
STC (Sensitivity Time Control): Gain control that reduces the radar receiver gain for nearby targets as compared to more distant targets. STC prevents receiver saturation from close-in targets.
Step Recovery Diode: A charge-controlled switch that ceases current conduction so rapidly that it can be used to produce an impulse. Cyclic operation of the diode can produce a train of impulses, which when used with a resonant circuit can produce a single frequency output at any harmonic of the pulse frequency.
Steradian: Unit of solid angle. An entire sphere has 4B steradians.
Stub: A short section of transmission line connected in parallel with the main transmission line.
Subharmonic: A frequency that is an integral submultiple of another frequency. For example, a sine wave whose frequency is one-third of the frequency of another sine wave is called the third subharmonic. (3 MHz is the third sub-harmonic of 9 MHz.)
Superheterodyne Receiver: A receiver that mixes the incoming signal with a locally generated signal (local oscillator) to produce a fixed, low intermediate frequency (IF) signal for amplification in the IF amplifiers. Refer to the general engineering dictionary for an AM Receiver Diagram.
Suppression: Elimination or reduction of any component of an emission, such as suppression of a harmonic of a transmitter frequency by band rejection filter.
Susceptibility: The degree to which an equipment or a system is sensitive to externally generated interference.
Sychrodyne: A klystron mixer amplifier stage in a transmitter, where two signal frequencies are applied as inputs and a single amplified signal is taken out.

C

C 4.0 GHz: 8.0 GHz
Capture Beam: A wide beam incorporated in capture transmitters of beam rider missile systems to facilitate gaining initial control of a missile immediately after launch.
Carrier-Controlled Approach: A shipboard radar system used to guide aircraft to safe landings in poor visibility conditions.
Carrier Frequency: The frequency of an unmodulated transmitter output. The basic radio frequency of the wave upon which modulations are impressed. Also called "Carrier" or Fc.
Carrier Power: The average power supplied to the antenna transmission line by a transmitter during one radio frequency cycle taken under the condition of no modulation. The average un-modulated power supplied to a transmission line.
Cassegrain Antenna: An antenna in which the feed radiator is mounted at or near the surface of a concave main reflector and is aimed at a convex secondary reflector slightly inside the focus of the main reflector.
Cathode Ray Tube: An electron tube that has an electron gun, a deflection system, and a screen.
Cavity: A space enclosed by a conducting surface used as a resonant circuit at microwave frequencies. Cavity space geometry determines the resonant frequency. A storage area for oscillating electromagnetic energy.
C-Band: A frequency band between 4 GHz and 6 GHz used in satellite communications. Note: "Letter designators of radio frequency bands are imprecise, deprecated, and legally obsolete."
Center Frequency: The tuned or operating frequency. Also referred to as center operating frequency. In frequency diversity systems, the midband frequency of the operating range. See also Carrier Frequency.
Chaff: Ribbon-like pieces of metallic materials or metalized plastic that are dispensed by aircraft or ships to mask or screen other "targets." The radar reflections off the chaff may cause a tracking radar to break lock on the target. The foil materials are generally cut into small pieces for which the size is dependent upon the radar interrogation frequency (approximately 1/2 wavelength of the victim radar frequency). Being this length, chaff acts as a resonant dipole and reflects much of the energy back to the radar.
Channel: A frequency or band of frequencies. An assigned center frequency and a fixed bandwidth around it. Designates operating frequency of track radar and frequency/code assignments of X-band CW illuminators.
Chirp: A pulse compression technique that uses frequency modulation (usually linear) on pulse transmission.
Circularly Polarized Jamming: The technique of radiating jamming energy in both planes of polarization simultaneously. With this method, there is a loss of 3 dB of effective power in either linear plane, and substantial loss if the opposite sense of circular polarization is used (i.e., left versus right).
Clutter: Confusing, unwanted echoes that interfere with the observation of desired signals on a radar indicator. Undesired radar returns or echoes resulting from man-made or natural objects including chaff, sea, ground, and rain, which interfere with normal radar system observations.
Coaxial Cable: A cable with one conductor completely surrounded by another conductor, separated by a dielectric.
Co-Channel: Indicates that two or more pieces of equipment are operating on the same frequency.
Coherent: A definite phase relationship between two energy waves, such as transmitted frequency and reference frequency. Two signals that have a set (usually fixed) phase relationship.
Coherent Oscillator: An oscillator that supplies phase references to provide coherent video from target returns, in CW (Continuous Wave) radar.

X

X 8.0 GHz: 12.0 GHz
X-Band: A range of frequencies between 8 GHz and 12 GHz.
XCO: Abbreviation for a crystal-controlled oscillator. An oscillator that uses a crystal to control the oscillation frequency.
XO: Abbreviation for crystal oscillator.
X-Ray Emission: Penetrating radiation similar to light, but with shorter wavelength, that can penetrate human tissue.
X-Y Mount: A style of antenna mounting that uses the X-axis as the primary mount and the Y-axis as the secondary mount.

K

Ku 12.0 GHz: 18.0 GHz
K 18.0 GHz: 27.0 GHz
Ka 27.0 GHz: 40.0 GHz
K-Band: A band of frequencies ranging between 18 GHz and 27 GHz
Keyed-Oscillator Transmitter: A transmitter in which one stage is used to produce the RF pulse, which is keyed (or modulated) by a DC pulse from a modulator.
KILO: A prefix meaning 103 (times one thousand). i.e., kilohertz.
Klystron Amplifier: An electron beam device that achieves amplification by the conversion of periodic velocity variations into conduction-current modulation in a field-free drift region. Velocity variations are launched by the interaction of an RF signal in an input resonant cavity and are coupled out through an RF output cavity. Several variations including reflex, pulse and multicavity klystrons are used.
Klystron, Multicavity: An electron tube that employs velocity modulation to generate or amplify electromagnetic energy in the microwave region.
Klystron, Reflex: A klystron that employs a reflector electrode in place of a second resonant cavity to redirect the velocity-modulated electrons through the resonant cavity. The repeller causes one resonant circuit to serve as both input and output, which simplifies the tuning operation.
Klystron Power Amplifier: A multicavity microwave electron tube that uses velocity modulation.

D

Decibel (dB): A dimensionless unit for expressing the ratio of two values of power, current, or voltage. The number of decibels being equal to: dB = 10 log P2/P1 = 20 log V2/V1 = 20 log I2/I1. Normally, used for expressing transmission gains, losses, levels, and similar quantities.
Deception: The deliberate radiation, re-radiation, alteration, absorption or reflection of electromagnetic energy in a manner intended to mislead the enemy interpretation or use of information received by his electronic systems.
dBc: Decibels referenced to the carrier signal.
dBi: Decibels referenced to an isotropic radiator. (dBLi indicating linear isotropic radiator is sometimes used).
dBm: Decibels relative to 1 mW. dBm is calculated by using the ratio of some power (expressed in mW) to 1 mW. For example, 1 mW is 0 dBm and 10 mW is +10 dBm.
dBsm: Decibel referenced to one square meter.
dBv / dBFv: Decibels referenced to one volt or microvolt, i.e., 0 dBv is 1 volt or 120 dBuv.
dBW / dBuW: Decibels referenced to 1 watt or one microwatt, i.e., 0 dBW is 1 watt or 30 dBm or 60 dBuW.
Dead Space: The area, zone, or volume of space that is within the expected range of a radio, radar, or other transmitted signal but in which the signal is not detectable and therefore cannot be received.
Deflection: A change in the direction of a traveling particle, usually without loss of particle kinetic energy, representing a change in velocity without a change in the scalar speed of the particle. A change in the direction of a wave, beam, electron, or other entity, such as might be accomplished by an electric or magnetic field.
Deflection Coil: 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.
Demodulator: A device employed to separate the modulation signal from its associated carrier, also called Second Detector. See also Detection.
Departure Angle: The angle between the axis of the main lobe of an antenna pattern and the horizontal plane at the transmitting antenna. Also known as takeoff angle.
Designation: The assignment of a fire control radar to a specific target by supplying target coordinate data to the radar system. Operational phase of a fire-control or track radar during which the radar is directed to the general direction of a desired target.
Detection: Usually refers to the technique of recovering the amplitude modulation signal (envelope) superimposed on a carrier.
Dielectrically Stabilized Oscillator: The DSO uses a dielectric resonator as the frequency-determining element. When the dielectric material is properly selected and used, the variations in dielectric constant versus temperature and the dimensions of the resonant structure versus temperature tend to cancel out, providing relatively good frequency versus temperature stability. The DSO offers frequency accuracy and stability, low power consumption and high reliability. Some of the commonly used materials are barium, zirconium, or tin tinates. The composition of these materials may be controlled to achieve any frequency variation with temperature with close tolerances.
Dielectric Waveguide: A waveguide that consists of a dielectric material surrounded by another dielectric material, such as air, glass, or plastic, with a lower refractive index.

E

Echo: The RF signal reflected back from a radar target.
ECCM: Electronic Counter-Countermeasures
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.
ECM: Electronic Counter Measures.
Effective Radiated Power (ERP): Input power to the antenna in watts times the gain ratio of the antenna. When expressed in dB, ERP is the transmitter power (PT), in dBm (or dBW) plus the antenna gain (GT) in dB. The term EIRP is used sometimes and reiterates that the gain is relative to an isotropic radiator.
Electromagnetic Coupling: The transfer of electromagnetic energy from one circuit or system to another circuit or system. An undesired transfer is termed EMI (electromagnetic interference).
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 to a target or object.

F

False Alarm: The false reporting of a target produced by a spike of noise or clutter.
Fan Beam: A wide-angle beam produced by a radar used for ground mapping or long-range surveillance.
Fast Time Constant: 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.
FFT: Fast Fourier Transform.
Field Strength: The magnitude of a magnetic or electric field at any point, usually expressed in terms of ampere turns per meter or volts per meter. Sometimes called field intensity and is expressed in volts/meter or dBFv/meter. Above 100 MHz, power density terminology is used more often.
First Harmonic: The fundamental (original) frequency.
Frequency Agility: A radar's ability to change frequency within its operating band, usually on a pulse-to-pulse basis. This is an ECCM technique employed to avoid spot jamming and to force the jammer to go into a less effective barrage mode.
Frequency Agility Radar: A radar that automatically or semi-automatically tunes through a discrete set of operating frequencies in its normal mode of operation.
Frequency Compensation Network: A circuit modification used to improve or broaden the linearity of its frequency response.
Frequency Diversity Radar: A radar system technique, employed primarily as an anti-jamming feature, where the transmitter output frequency varies randomly from pulse to pulse over a wide frequency range.
Frequency Range: A specifically designated portion of the frequency spectrum; of a device, the band of frequencies over which the device may be considered useful with various circuit and operating conditions; of a transmission system, the frequency band in which the system is able to transmit power without attenuating or distorting it more than a specified amount.
Frequency Scanning: Varying the output frequency to achieve electronic scanning.
Frequency Shift Keying (FSK): A form of FM where the carrier is shifted between two frequencies in accordance with a predetermined code. In multiple FSK, the carrier is shifted to more than two frequencies. FSK is used principally with teletype communications.
Frequency Spectrum: In a radar, the entire range of frequencies contained in an RF pulse or signal. A band of frequencies required by a transmission.
Frequency Synthesizer: A bank of oscillators in which the outputs can be mixed in various combinations to produce a wide range of frequencies.
FTC (Fast Time Constant): An anti-jam feature employed in radar systems where receiver circuits may be selected to provide a short time constant to emphasize signals of short duration to produce discrimination against the low-frequency components of clutter. Also an electrical engineering term for the rate of charge or discharge of a capacitor.
Fundamental Frequency: Depending on the application the term is used synonymously for tuned frequency, carrier frequency, center frequency, output frequency, or operating frequency.

#

"FRUIT": In a radar beacon system, a type of interference, caused by beacon replies to interrogation asynchronous with the observer's interrogator. The largest amount of this interference is received through the sidelobes of the interrogating antenna, but it can become dense enough to cause false target indications.

G

GaAs FET Amplifier: Because of their low noise, field-effect transistors are often used as the input stage of wideband amplifiers. Their high input resistance makes this device particularly useful in a variety of applications. Since the FET does not employ minority current carriers, carrier storage effects are eliminated giving the device faster operating characteristics and improved radiation-resistant qualities.
Gain: Any increase in the strength of a signal. For antennas, the value of power gain in a given direction relative to an isotropic point source radiating equally in all directions. Frequently expressed in dB (gain of an isotropic source = 0 dB).
Gate (Range): A signal used to select radar echoes corresponding to a very short-range increment. Range is computed by moving the range gate or marker to the target echo, an arrangement that permits radar signals to be received in a small, selected fraction of the time period between radar transmitter pulses.
Gated AGC: Circuit that permits automatic gain control to function only during short time intervals.
Gating: The process of selecting those portions of a wave that exist during one or more selected time intervals, the application of a square waveform of desired duration and timing to perform electronic switching the application of receiver operating voltages to one or more stages only during that portion of a cycle of operation when reception is desired. See also Gate (Range).
Ghost: A secondary image or signal resulting from echo, envelope delay distortion, or multipath reception.
GIGA: A prefix meaning 109 (times a billion). For example, gigahertz (GHz).
GLINT: The random component of target location error caused by variations in the phase front of the target signal (as contrasted with Scintillation Error). Glint may affect angle, range of Doppler measurements, and may have peak values corresponding to locations beyond the true target extent in the measured coordinate. Electronic countermeasures that uses the scintillating or flashing effect of shuttered or rotating reflectors to degrade tracking or seeking functions of an enemy weapons system.
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 Return: Reflections or echoes returned to a radar receiver due to reflections from the ground, which could produce Ground Clutter on the radar display.
Ground-Controlled Approach: Radar system used to guide aircraft to safe landings in poor visibility conditions.
Ground Station: A telemetry receiving and processing system.
GTWT: Gridded Traveling Wave Tube.
Guardband: A frequency band to which no other emitters are assigned as a precaution against interference to equipment susceptible to EMI in that band.
Guidance, Beam Rider: A missile guidance technique that is dependent on the missile's ability to determine its positions with reference to the center of scan of the guidance radar beam and thus correct its trajectory on the basis of detected errors.
Guidance Code: A technique of modulating guidance transmitter carriers with coded pulses compatible with the receiver code of the missile assigned that system, thus reducing the possibility of the missile accepting erroneous commands of other transmissions.
Guidance, Inertial: A self-contained system independent of information obtained from outside the missile, usually using Newton's second law of motion.
Gunn Diode: The Gunn diode is a transferred electron device that because of its negative resistance can be used in microwave oscillators or amplifiers. When the applied voltage exceeds a certain critical value, periodic fluctuations in current occur. The frequency of oscillation depends primarily upon the drift velocity of electrons through the effective length of the device. This frequency may be varied over a small range by means of mechanical tuning.

I

I Band: A range of frequencies between 8 and 10 GHz.
IF (Intermediate Frequency): The difference frequency resulting from mixing (beating) the received signal in a superheterodyne receiver with the signal from the local oscillator. The difference frequency product provides the advantages inherent to the processing (amplification, detection, filtering, and such) of low-frequency signals. The receiver local oscillator may operate either below or above the receiver-tuned frequency. A single receiver may incorporate multiple IF detection. IF = FLO - FO. (for a local oscillator operating above the fundamental) where: FO = Received fundamental frequency FLO = Local oscillator frequency. See also Image Frequency.
IF Amplifier: Usually a narrow-bandwidth IF amplifier that is tuned to one of the output frequencies produced by the mixer.
IFF (Identification Friend or Foe): A system using radar transmission to which equipment carried by friendly forces automatically responds by emitting a unique characteristic series of pulses thereby distinguishing themselves from enemy forces. It is the "Mode IV" for the aircraft transponder.
Image Frequency: That frequency to which a given superheterodyne receiver is inherently susceptible, thereby rendering such a receiver extremely vulnerable to EMI at that frequency. The image frequency is located at the same frequency difference to one side of the local oscillator as the tuned (desired) frequency is to the other side. An undesired signal received at the image frequency by a superheterodyne receiver not having pre-selection would, therefore, mix (beat) with the oscillator, produce the proper receiver IF, and be processed in the same manner as a signal at the desired frequency. See also receiver selectivity.
Image Jamming: Jamming at the image frequency of the radar receiver. Barrage jamming is made most effective by generating energy at both the normal operating and image frequency of the radar. Image jamming inverts the phase of the response and is thereby useful as an angle deception technique.
IMPATT Diode: The IMPATT (IMPact Avalanche and Transit Time) diode acts like a negative resistance at microwave frequencies. Because of this property, IIMPATT diodes are used in oscillators and amplifiers. Usually the frequency range is in the millimeter wave region where other solid-state devices cannot compete.
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. Also refer to an R-Scope display. The graphic shows a radar display used as a target indicator in a B52 bomber.
Ingress: Go into the target area.
Insertion Loss: The loss incurred by inserting an element, device, or apparatus in an electrical/electronic circuit. Normally expressed in decibels determined as 10 log of the ratio of power measured at the point of insertion prior to inserting the device (P1) to the power measured after inserting the device (P2). Insertion loss (dB) = 10 log P1/P2.
Integration Effect: Pulse radar usually obtain several echoes from a target. If these echoes are added to each other, they enhance the S/N ratio, making a weak target easier to detect. The noise and interference do not directly add from pulse to pulse, so the ratio of target strength to undesired signal strength improves making the target more detectable. Random noise increases by the square root of the number of integrations, whereas the signal totally correlates and increases directly by the number of integrations; therefore the S/N enhancement is equal to the square root of the number of integrations.

J

JAFF: A combination of the terms electronic jamming and chaff jamming.
Jamming: The deliberate radiation, re-radiation, or reflection of electromagnetic energy with the object of impairing the use of electronic devices, equipment, or systems by an enemy.
Jamming Margin: The level of interference (jamming) that a system is able to accept and still maintain a specified level of performance, such as maintain a specified bit-error ratio even though the signal-to-noise ratio is decreasing.
Jittered PRF: An anti-jam feature of certain radar systems which varies the PRF consecutively, and randomly, from pulse to pulse to prevent enemy ECM equipment from locking on, and synchronizing with, the transmitted PRF. PRF is synonymous with pulse repetition rate (PRR).

M

Mach Number: The ratio of the velocity of a body to the speed of sound in the medium that is being considered. In the atmosphere, the speed of sound varies with temperature and atmospheric pressure, hence, so does mach number.
Magnetron: A magnetron is a thermionic vacuum tube that is constructed with a permanent magnet forming a part of the tube and which generates microwave power. These devices are commonly used as the power output stage of radar transmitters operating in the frequency range above 1,000 MHz and are used less commonly down to about 400 MHz. A magnetron has two concentric cylindrical electrodes. On a conventional magnetron, the inner one is the cathode and the outer one is the anode. The opposite is true for a coaxial magnetron.
Magnetron Oscillator: An 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. A high-vacuum tube in which the interaction of an electronic space charge and a resonant system converts direct current power into AC power, usually at microwave frequencies. The magnetron has good efficiency, is capable of high-power outputs, and is stable.
Marine Radar: An X-band or S-band radar used on ships, boats or other marine craft.
Master Oscillator: In a transmitter, the oscillator that establishes the carrier frequency of the output.
Matched Filter: This describes the bandwidth of an IF amplifier that maximizes the signal-to-noise ratio in the receiver output. This bandwidth is a function of the pulse width of the signal.
Meaconing: A system receiving radio signals and re-broadcasting them (or just transmitting) on the same frequency to confuse navigation. The meaconing station attempts to cause aircraft to receive inaccurate range or bearing information.
Mechanical Scanning: The reflector, its feed source, or the entire antenna is moved in a desired pattern.
Medium Altitude Orbit: An orbit from 2,000 to 12,000 miles above the earth. The rotation rate of the earth and satellite are quite different, and the satellite moves quickly across the sky.
MEGA: A prefix meaning 106 (times one million). For example, megahertz (MHz).
Microvolt per Meter: A commonly used unit of field strength at a given point. The field strength is measured by locating a standard receiving antenna at that point, and the "microvolts per meter" value is then the ratio of the antenna voltage in microvolts to the effective antenna length in meters. Usually used below 100 MHz. Above 100 MHz, power density terminology is normally used.
Microwave Amplification by Stimulated Emission of Radiation (MASER): A low-noise radio frequency amplifier. The emission of energy stored in a molecular or atomic system by a microwave power supply is stimulated by the input signal.
Millimeter Band: A band of frequencies ranging from 30 to 300 GHz, also called the EHF band.
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. The receiver input power level that is just sufficient to produce a detectable/discernible signal in the receiver output.
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.
Mobile Radar: A self-propelled radar mounted on a vehicle. A towed radar mounted behind a truck might also be considered a mobile radar.
Mode Shifting: In a magnetron, shifting from one mode to another during a pulse.
Modes: Operational phases of a radar.
Mode Skipping: Rather than firing on each successive pulse as desired, the magnetron fires randomly.
Modulation: The process whereby some characteristic of one wave is varied in accordance with some characteristic of another wave. The basic types of modulation are angle modulation (including the special cases of phase and frequency modulation) and amplitude modulation. In missile radar, it is common practice to amplitude modulate the transmitted RF carrier wave of tracking and guidance transmitters by using a pulsed wave for modulating, and to frequency module the transmitted RF carrier wave of illuminator transmitters by using a sine wave.
Modulation, Amplitude: This type of modulation changes the amplitude of a carrier wave in responses to the amplitude of a modulating wave. A type of modulation used in radar only as a switch to turn on or turn off the carrier wave; i.e., pulse is a special form of amplitude modulation.
Modulation, Frequency: The frequency of the modulated carrier wave is varied in proportion to the amplitude of the modulating wave and therefore, the phase of the carrier varies with the integral of the modulating wave.
Modulation, Phase: The phase of the modulated carrier is varied in proportion to the amplitude of the modulating wave.
Modulator: Produces a high-voltage pulse that turns the transmitter on and off.
Modulator Switching Device: Controls the on (discharge) and off (charge) time of the modulator.
Moisture Lapse: Abnormal variation of moisture content at different altitudes because of high moisture located just above large bodies of water.
Monopulse: A type of tracking radar that permits the extracting of tracking error information from each received pulse and offers a reduction in tracking errors as compared to a conical-scan system of similar power and size. Multiple (commonly four) receiving antennas or feeds are placed symmetrically about the center axis and operate simultaneously to receive each RF pulse reflected from the target. A comparison of the output signal amplitude or phase among the four antennas indicates the location of the target with respect to the radar beam center line. The output of the comparison circuit controls a servo system that reduces the tracking error to zero and thereby causes the antenna to track the target.
Monopulse 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.
MOS (Minimum Operational Sensitivity): The minimum signal which can be detected and automatically digitally processed by a radar without human discrimination.
Moving Target Indicator (MTI): This radar signal process shows only targets that are in motion. Signals from stationary targets are subtracted from the return signal by a memory circuit.
Multipath: The process by which a transmitted signal arrives at the receiver by at least two different paths. These paths are usually the main direct path, and at least one reflected path. The signals combine either constructively or destructively depending upon phase, and the resultant signal may be either stronger or weaker than the value computed for free space. Related terms are located in the Dictionary of Antenna Terms, in a separate group of pages.
Multiplex: Simultaneous transmission of two or more signals on a common carrier wave. The three types of multiplex are called time division, frequency division, and phase division.
Multiband Radar: A type of radar that uses simultaneous operation on more than one frequency band through a common antenna. This technique allows for many sophisticated forms of video processing and requires any jammer to jam all channels at the same time in order to be effective.
MVS (Minimum Visible Signal): The minimum input pulse signal power level which permits visibility of the output pulse, such as on a radar A-scope display.

N

Nautical Mile: The length of a minute of arc of a great circle of the earth (6,076 ft.).
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.
Noise Figure, Receiver: A figure of merit (NF or F) of a system given by the ratio of the signal-to-noise ratio at the input, Si / Ni, divided by the signal-to-noise ratio at the output, So / No. It essentially expresses the ratio of output noise power of a given receiver to that of a theoretically perfect receiver that adds no noise. Noise figure is usually expressed in dB and given for an impedance-matched condition. Impedance mismatch will increase the noise figure by the amount of mismatch loss. NF is usually given at room temperature: 17EC or 290EK.
Noise Jamming: A continuous random signal radiated with the objective of concealing the aircraft echo from the enemy radar. In order for it to be effective, it must have an average amplitude at least as great as the average amplitude of the radar echo. There are three major categories of noise jamming, which are grouped by how jamming power is concentrated: Spot, barrage, and swept jamming.
Noncoherent: Two signals that have no set phase relationship.
Notch: The portion of the radar velocity display where a target disappears due to being notched out by the zero Doppler filter. If not filtered (notched), ground clutter would also appear on the display. A notch filter is a narrow band-reject filter.
Null Antenna Pattern: The directions of minimum transmission (or reception) of a directional antenna.
Null Fill: The nulls in an antenna pattern may be reduced (filled) by using a second ancillary (spoiler) antenna whose pattern is such that it fills in the nulls of the main antenna pattern.
Nutation: As applied to current missile system radar, this term refers to the mechanical motion of an antenna feed to produce a conical scan (fixed polarization) by the main beam of a tracking antenna, thus providing a means of developing tracking error signals. By analogy, "Nutation" also is used to denote the electrical switching of the quadrants of a seeker antenna. The effect is similar to that of a conical scan.
Nutator: A motor-driven rotating antenna feed used to produce a conical scan for a tracking radar. See also Antenna, Nutating. Also, the electrical circuits necessary to effect nonmechanical conical scans.

O

Omnidirectional Antenna: An antenna that radiates equally in all directions (nondirectional). An antenna whose radiation pattern shows equal radiation in all horizontal directions.
Operational Constraints: Limitations on operating procedures in order to prevent interference between missile systems on a ship or between missile systems in a formation of ships under operational conditions. These limitations consist of such things as limited frequency bands or channels in which the radar may be tuned, limited sectors of space into which radar beams may be pointed, limits on minimum spacing between ships, limits on what codes may be used by radar and missiles on each ship, and limits on minimum interval between firing of certain missiles.
Orange-Peel Paraboloid: A section of a complete circular paraboloid that is narrow in the horizontal plane and wide in the vertical plane. An antenna shape used in radar.
Oscillation: A periodic, repetitive motion or set of values (voltage, current, velocity).
Oscillator: Devices that generate a frequency. See also Backward Wave, Dielectrically Stabilized Oscillator, Hyperabrupt Varactor Oscillator, Magnetron Oscillator, Varactor Tuned Oscillator, and YIG tuned oscillator.
Oscillator, Local: An oscillator used as an input to a mixer.
Over-the-Horizon Radar: A radar system that makes use of the atmospheric reflection and refraction phenomena to extend its range of detection beyond line of sight.

P

Palmer Scan: Conical scan superimposed on another type of scan pattern - usually a spiral pattern.
Parabolic Antenna: An antenna consisting of a parabolic reflector and a radiating or receiving element at or near its focus. If the reflector is in the shape of a paraboloid, it is called a paraboloidal reflector.
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 T-junction.
Parameter: A quantity that may have various values, each fixed within the limits of a stated case or discussion. In the present case, some examples of parameters include radar frequency, limited by the tuning range of the radar.
Parasitic Array: An antenna array containing one or more elements not connected to the transmission line. An antenna with a driven element and one or more parasitic elements.
Parasitic Element: A directive element that is not connected to a radio transmitter or receiver either directly or via a feeder but is coupled to the driven element only by the fields.
Passive Angle Tracking: Tracking of a target using radiation from the target (such as jamming), with no radiation from the radar itself. Only angular tracking is possible under these conditions since no measurement of time of travel of radiation to the target is possible, as is required to obtain target range.
Path: In communications systems and network topologies, a route between any two points.
Pawsel Stub: A device for connecting an unbalanced coaxial feeder to a balanced antenna.
PC (Pulse Compression): The process used in search and tracking pulse radar whereby the transmitted pulse is long, so as to obtain high average transmitter output power, and the reflected pulse is processed in the radar receiver to compress it to a fraction of the duration of the transmitted pulse to obtain high definition and signal strength enhancement. Pulse compression may be accomplished by sweeping the transmitted frequency (carrier) during the pulse. The returned signal is then passed through a frequency-dependent delay line. The leading edge of the pulse is therefore delayed so that the trailing edge catches up to the leading edge to produce effectively a shorter received pulse than that transmitted. Pulse compression radar are also referred to as chirp radar.
Peak Envelope Power: The average power supplied to the antenna transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope taken under normal operating conditions.
Peak Power: Maximum power of the RF pulse from a radar transmitter.
Pencil Beam: A narrow circular radar beam from a highly directional antenna (such as a parabolic reflector), used for tracking a target.
Pentagrid Tube: An electron tube having 5 grids, with one anode and one cathode. A Pentagrid Tube differs from a Pentode tube which has five elements including the anode and cathode.
Periodic Antenna: An antenna whose impedance varies as the frequency changes.
Persistance: The length of time a phosphor dot glows on a CRT before disappearing.
Phased Array Radar: Radar using many antenna elements which are combined in a controlled phase relationship. The direction of the beam can be changed as rapidly as the phase relationships (usually less than 20 microseconds). Thus, the antenna typically remains stationary while the beam is electronically scanned. The use of many antenna elements allows for very rapid and high directivity of the beam(s) with a large peak and/or average power. There is also a potential for greater reliability over a conventional radar since the array will fail gracefully, one element at a time.
PIN Diode: A diode with a large intrinsic (I) region sandwiched between the P- and N-doped semiconducting regions. The most important property of the PIN diode is the fact that it appears as an almost pure resistance at RF. The value of this resistance can be varied over a range of approximately 1 to 10,000 ohms by direct or low-frequency current control. When the control current is varied continuously, the PIN diode is useful for attenuating, leveling and amplitude modulation of an RF signal. When the control current is switched on and off or in discrete steps, the device is useful in switching, pulse modulating, and phase shifting an RF signal.
Planar Array: An antenna in which all of the elements, both active and parasitic, are in one plane. A planar array provides a large aperture and may be used for directional beam control by varying the relative phase of each element.
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.
Point Contact Diode: A Point Contact Diode consists of a spring-loaded metal contact on a semiconducting surface, generally used as a detector or mixer. Point Contact Diodes are limited to low powers.
Polarization: The direction of the electric field (E-field) vector of an electromagnetic (EM) wave. The most general case is elliptical polarization with all others being special cases. The E-field of an EM wave radiating from a vertically mounted dipole antenna will be vertical and the wave is said to be vertically polarized. In like manner, a horizontally mounted dipole will produce a horizontal electric field and is horizontally polarized. Equal vertical and horizontal E-field components produce circular polarization.
Power Pulsed Radar: Average power for a pulse radar is the average power transmitted between the start of one pulse and the start of the next pulse (because the time between pulses is many times greater than the pulse duration time, the average power will be a small fraction of peak power). Peak power for a pulsed radar is the power radiated during the actual pulse transmission (with zero power transmitted between pulses).
Power Gain: In an antenna, the ratio of its radiated power to that of a reference.
Power Output: Power output of a transmitter or transmitting antenna is commonly expressed in dBW or dBm. One megawatt would be expressed as 60 dBW or 90 dBm.
Power for CW Radar: Since the power output of Continues Wave (CW) transmitters usually have a duty cycle of one (100%), the peak and average power are the same.
Power Density: The density of power in space expressed in Watts/meter2, dBW/m2, etc. Generally used in measurements above 100 MHz. At lower frequencies, field intensity measurements are taken.
PPI-Scope: A radar display yielding range and azimuth (bearing) information via an intensity modulated display and a circular sweep of a radial line. The radar is located at the center of the display.
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.
Preselector: A device placed ahead of the mixer in a receiver, which has bandpass characteristics such that the desired (tuned) RF signal, the target return, is allowed to pass, and other undesired signals (including the image frequency) are attenuated.
Primary Radiation: Radiation that is incident upon a material and produces secondary emission from the material.
Probe Coupler: A resonant conductor placed in a waveguide or cavity to insert or extract energy.
Propagation: The travel of waves through or along a medium. The path traveled by the wave in getting from one point to another is known as the propagation path (such as the path through the atmosphere in getting from a transmitting antenna to a receiving antenna, or the path through the waveguides and other microwave devices in getting from an antenna to a receiver).
Pulse: A rapid, transient change in the amplitude of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value.
Pulsed Doppler (PD): A type of radar that combines the features of pulsed radar and CW Doppler radar. It transmits pulses (instead of CW) which permits accurate range measurement. This is an inherent advantage of pulsed radar. Also, it detects the Doppler frequency shift produced by target range rate, which enables it to discriminate between targets of only slightly different range rate and also enables it to greatly reduce clutter from stationary targets. See also Doppler Effect.
Pulse Length: Same meaning as Pulse width.
Pulse Modulation: The modulation of a carrier by a series of pulses generally for the purpose of transmitting data. The result is a short, powerful burst of electromagnetic radiation which can be used for measuring the distance from a radar set to a target. A special case of amplitude modulation wherein the carrier wave is varied at a pulsed rate.
Pulse Repetition Frequency (PRF): The rate of occurrence of a series of pulses, such as 100 pulses per second. It is equal to the reciprocal of the pulse spacing (T) or PRT. (PRF = 1/T = 1/PRI.) Sometimes the term pulse repetition rate (PRR) is used.
Pulse Repetition Interval (PRI) or Time (PRT): Time between the beginning of one pulse and the beginning of the next.
Pulse-Repetition Rate (PRR): Average number of pulses per unit of time; pulse rate.
Pulse-Repetition Time (PRT): Interval between the start of one pulse and the start of the next pulse; reciprocal of pulse repetition frequency.
Pulse Space: The interval of time between the leading edge of one pulse and the leading edge of the next pulse in a train of regularly recurring pulses. See also Pulse Repetition Frequency. Also called the interpulse period.
Pulse Width: The interval of time between the leading edge of a pulse and the trailing edge of a pulse (measured in microseconds for the short pulses used in radar).

Q

Quadrature: The state of being separated in phase by 90 degrees (pie/2 radians). Pertaining to the phase relationship between two periodic quantities varying with the same period, that is, with the same frequency or repetition rate, when the phase difference between them is one-quarter of their period.
Quantize: The process of restricting a variable to a number of discrete values. For example, to limit varying antenna gains to three levels. To transform a time-varying analog signal into discrete digital samples or values.
Quantizing Noise: Noise caused by the error of approximation in quantization. When a digitizer or analog-to-digital converter takes a sample of an analog signal (in this case a sine wave), the sample is only as good as the resolution of the converter. The difference between the converter resolution and the actual value of the signal is the quantizing noise. That is, if the A/D converter has a resolution of a tenth of a volt, then that's the best approximation it can make to the real analog value. Using an A/D with more bits, providing a higher resolution, reduces the quantizing noise induced in the conversion process.

R

Radar: An acronym for Radio Detection and Ranging. A radio detection system that transmits short bursts (pulses) of RF energy and detects their echoes from objects (targets) such as aircraft or ships.
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 Blind Range: The range that corresponds to the situation in which a radar transmitter is on and hence the receiver must be off, so that the radar transmitted signal does not saturate, i.e., does not blind, its own receiver.
Radar Blind Speed: The magnitude of the radial component of velocity of an object, i.e., a target, relative to a radar site, that cannot be measured by the radar unit.
Radar Cross Section: A measure of the radar reflection characteristics of a target. It is equal to the power reflected back to the radar divided by power density of the wave striking the target. For most targets, the radar cross section is the area of the cross section of the sphere that would reflect the same energy back to the radar if the sphere were substituted. RCS of sphere is independent of frequency if operating in the far field region.
Radar Detector: A detector that, in its simplest form, only needs to be capable of producing an output when RF energy (reflected from a target) is present at its input.
Radar Mile: A time interval of 12.36 microseconds which equals the amount of time for RF energy to travel out from a radar to a target and back to the radar; radar nautical mile.
Radar Range Equation: The radar range equation is a basic relationship that permits the calculation of received echo signal strength, if certain parameters of the radar transmitter, antenna, propagation path, and target are known.
Radar Signature: The detailed waveform of a detected radar echo.
Radar Warning Receiver (RWR): A receiver that detects the presence of radar signals. A radar detector.
Radiation Pattern: The variation of the field intensity of an antenna as an angular function with respect to the axis.
Radome: The structure that protects a radar antenna; Radar Dome. A covering that conceals the pointing location of a radar.
Range: The length of a straight line between a radar set and a target.
Range Cell: In a radar, a range cell is the smallest range increment the radar is capable of detecting. If a radar has a range resolution of 50 yards and a total range of 30 nautical miles (60,000 yds), there are 60,000/50 = 1,200 range cells.
Range Gate: A movable gate used to select radar echoes from a very short-range interval. A gate voltage used to select radar echoes from a very short-range interval.
Range Gate Pull Off (RGPO): Deception technique used against pulse tracking radar using range gates. Jammer initially repeats the skin echo with minimum time delay at a high power to capture the AGC circuitry. The delay is progressively increased, forcing the tracking gates to be pulled away from the target echo.
Range-Height Indicator: A radar display on which slant range is shown along the X axis and height along the Y axis.
Range Rate: The rate at which a radar target is changing its range with respect to the radar (e.g., in feet per second). Note that this rate is not the same as target velocity unless the target is moving straight toward or straight away from the radar.
Range Resolution: Ability of a radar to distinguish between targets that are close together.
Range Scope: There are a number of types of Radar screens in use. See R-Scope A-Scope or PPI.
Ranging: The measurement of the distance to a remote object (target), from a known observation or reference point.
Receiver: A unit that converts RF echoes to video and/or audio signals pertaining to RADAR.
Receiver Bandwidth: The difference between the limiting frequencies within which receiver performance in respect to some characteristic falls within specified limits. (In most receivers this will be the difference between the two frequencies where the intermediate frequency [IF] amplifier gain falls off 3 dB from the gain at the center IF frequency.)
Receiver Selectivity: The degree to which a receiver is capable of differentiating between the desired signal and signals or interference at other frequencies. (The narrower the receiver bandwidth, the greater the selectivity.)
Receiver Sensitivity: The degree to which a receiver can usefully detect a weak signal; the lower limit of useful signal input to the receiver. A characteristic that determines the minimum strength of signal input capable of causing a desired value of signal output.
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.
Reflection: The turning back (or to the side) of a radio wave as a result of impinging on any conducting surface that is at least comparable in dimension to the wavelength of the radio wave.
Reflex Klystron: A microwave oscillator that is tuned by changing the repeller voltage.
Relative Bearing: Bearing of a target measured in a clockwise direction from "dead ahead" of a ship or plane.
Resolution: In radar, the minimum separation in angle or in range between two targets that the radar is capable of distinguishing.
Return: The RF signal reflected back from a radar target; echo.
RF (Radio Frequency): A term indicating high-frequency electromagnetic energy.
RFI (Radio Frequency Interference): Any induced, radiated, or conducted electrical disturbance or transient that causes undesirable responses or malfunctioning in any electrical or electronic equipment, device, or system.
Ring Around: A condition in which a repeater jammer's total gain, from receiver antenna to transmitter antenna, exceeds the antenna isolation resulting in the repeater amplifying its own internal noise. Akin to positive feedback in an amplifier that causes unwanted oscillations. The condition where radio frequency interference signals from a transmitter of one radar enter the receiving circuits of another radar.
Rope: An element of chaff consisting of a long roll of metallic foil or wire which is designed for broad, low-frequency response.
Ringing: Rf oscillations caused by shock excitation of a resonant circuit (cavity).
Ring Time: In radar, the time during which the output of an echo box remains above a specified level.
R-Scope (Range Scope): Also called an A-scope, A-Scan or PPI display. A cathode-ray oscilloscope (CRT) used in radar systems to display vertically the signal amplitude as a function of time (range) or range rate. Targets are displayed as perpendicular deflections on the time scale. Sometimes referred to as Range (R) Scope. Note: the A-Scope display is obsolete, and is shown in the right sidebar.

T

Tail Warning Radar: A radar placed in the tail of an aircraft.
Target: In radar, a specific object of radar search or detection.
Target Size: A measure of the ability of a radar target to reflect energy to the radar receiving antenna. The parameter used to describe this ability is the "radar cross section" of the target. The size (or radar cross section) of a target, such as an aircraft, will vary considerably as the target maneuvers and presents different views to the radar. A side view will normally result in a much larger radar cross section than a head-on view.
Target Resolution: The ability of a radar to distinguish between two or more targets that are close to each other.
Terminal Impedance: The equivalent impedance as seen by the transmitter/receiver.
Terrain Bounce: Term for jamming that is directed at the earth's surface where it is reflected toward the threat radar.
Thermistor: A resistor whose resistance varies with temperature in a defined manner. The word is formed from the two words "thermal" and "resistor."
Three-Dimensional Radar (3D Radar): A radar set that measures the range, bearing and altitude of a target.
Threshold ISR Threshold: The interference to signal ratio (ISR) at which the performance of a receiver starts undergoing degradation.
Thyratron: Gas tube used as a modulator switching device.
Towed Radar: A radar towed behind another vehicle.
Tower: A radar tower is erected to hold and rise the radar above the surrounding area. In the example case (right) a storm damaged radome sitting atop a radar tower.
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.
Tracking Radar: A radar whose prime function is to track a radar target and determine the target coordinates (in range and angular position) so a missile may be guided to the target, or a gun aimed at the target.
Tracking Radar Receiver: These are of two primary types: conical scan and monopulse. The conical scan system directs the radar signal in a circle around the target. The radar paints this circle 15 to 40 times per second. As the target moves out of the center of this circle, the radar develops aim error voltages and re-aims the antenna. The monopulse system directs four beams at the target simultaneously. The target is in the middle of the four beams. If the target is not in the center, the radar return develops an aim error voltage to re-aim the antenna.
Track While Scan Radar (TWS): Provides complete and accurate position information for missile guidance. In one implementation it would utilize two separate beams produced by two separate antennas on two different frequencies. The system utilizes electronic computer techniques whereby raw datum is used to track an assigned target, compute target velocity, and predict its future position, while maintaining normal sector scan. Most aircraft use only a single antenna.
Transient: A phenomenon (such as a surge of voltage or current) caused in a system by a sudden change in conditions, and which may persist for a relatively short time after the change (sometimes called ringing).
Transmit-Receive Tube: Gas-filled RF switch that is used as a duplexer.
Transmitter: Equipment that generates, amplifies, and modulates electromagnetic energy.
Transponder: A transmitter-receiver capable of accepting the electronic challenge of an interrogator and automatically transmitting an appropriate reply. There are four modes of operation currently in use for military aircraft. Mode 1 is a nonsecure low-cost method used by ships to track aircraft and other ships. Mode 2 is used by aircraft to make carrier-controlled approaches to ships during inclement weather. Mode 3 is the standard system used by commercial aircraft to relay their position to ground controllers throughout the world.
Traveling-Wave Tube Amplifier: The TWT is a microwave amplifier capable of operation over very wide bandwidths. In operation, an electron beam interacts with a microwave signal which is traveling on a slow wave helical structure. The near synchronism of the beam and RF wave velocities results in amplification. Bandwidths of 3:1 are possible. Operation at high powers or at millimeter wavelengths is possible at reduced bandwidths.
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 Pulse: A pulse used to cause a circuit to perform some function or initiate specific events.
True North: Geographic north.
Tunnel Diode: The tunnel diode is a heavily doped P-N junction diode that displays a negative resistance over a portion of its voltage-current characteristic curve. In the tunneling process, electrons from the p-side valence bands are able to cross the energy barrier into empty states in the N-side conduction band when a small reverse bias is applied. This diode is used as a microwave amplifier or oscillator.
Twinax: Connector, two side-by-side leads surrounded by a shield. A coax line with two conductor leads.
Two-Dimensional Radar: Measures the range and bearing to a target.

W

Water Line: A reference line used for vertical measurements. When used with an aircraft it is usually the ground with the landing gear extended normally. One of several aircraft location designations.
Waveguide: A transmission line consisting of a hollow conducting tube of arbitrary geometry (usually rectangular, but may be circular) within which electromagnetic waves may propagate. A hollow metal tube used as a transmission line to guide energy from one point to another.
Wavelength: The distance traveled by a wave in one period (the period is the time required to complete one cycle). Wavelength = c/f. In the atmosphere, electromagnetic waves travel at c, the speed of light (300 million meters per second or 30 cm/nsec). At 5 GHz, one wavelength = 6 cm. At 10 GHz, one wavelength = 3 cm. The distance between points of corresponding phase of two consecutive cycles of a wave.
Wavemeter: An instrument for measuring the frequency of a radio wave. The wavemeter is a mechanically tunable resonant circuit. It must be part of a reflection of transmission measurement system to measure the maximum response of a signal. Below 20 GHz, the wavemeter has been replaced by the frequency counter with much greater accuracy and ease of use.
Whip antenna: A vertical monopole, another type of antenna.
White Noise: Or random noise. White noise has random amplitude, but is equally distributed across the entire band of frequencies of interest.
Wire: Conductors in one of many different sizes with different kinds of insulation. The American Wire Gauge defines the different sizes of copper wire. [Wire and Cable Manufacturers]. Wire conductors may be insulated or uninsulated, or solid or stranded wire. Any number of wire strands could be used to produce the finished conductor.

Y

Yagi Antenna: A combination of dipoles to increase the gain. [Antenna Manufacturers], [Antenna Dictionary].
YIG Tuned Oscillator: A YIG (yttrium iron garnet) sphere, when installed in the proper magnetic environment with suitable coupling will behave like a tunable microwave cavity with Q on the order of 1,000 to 8,000. Since spectral purity is related to Q, the device has excellent AM and FM noise characteristics.

Z

Zener Diode: A diode that exhibits in the avalanche-breakdown region a large change in reverse current over a very narrow range in reverse voltage. This characteristic permits a highly stable reference voltage to be maintained across the diode despite a wide range of current.
Zener Effect: A reverse breakdown effect in diodes in which breakdown occurs at reverse voltages below 5 volts. The presence of a high energy field at the junction of a semiconductor produces the breakdown.
Zulu Time: Synonym, Coordinated Universal Time. Formerly a synonym for Greenwich Mean Time (GMT). Abbreviation: Z Time, although GMT is still widely used.