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Technical Glossary
Aviation and marine electronics terminology — free reference for all professionals.
2
2182 kHzThe international MF radiotelephone distress and calling frequency. All compulsory vessels must maintain a watch on this frequency. Silent periods are observed at :00 and :30 minutes past the hour.
A
Acceptance AngleThe maximum angle at which light can enter a fiber optic cable and still be totally internally reflected and propagated. Related to the numerical aperture of the fiber.
ADCAnalog-to-Digital Converter. A circuit that converts continuous analog signals to discrete digital values. The sampling rate must be at least twice the highest signal frequency (Nyquist theorem) to accurately represent the signal.
ADFAutomatic Direction Finder. An aircraft radio navigation instrument that determines the bearing to a ground-based NDB transmitter. Operates in the 190–1750 kHz frequency range.
ADS-BAutomatic Dependent Surveillance-Broadcast. A surveillance technology where aircraft broadcast their position, altitude, speed, and identification via transponder on 1090 MHz. Mandatory in controlled airspace in the US since 2020.
AIOAll-In-One. A flight controller board that integrates the flight controller, ESC, and sometimes OSD and VTX into a single board. Common in micro and whoop-class drones.
Alternating CurrentElectric current that periodically reverses direction. In the US, AC power alternates at 60 Hz. Aircraft use 400 Hz AC to reduce the size and weight of transformers and motors.
AltimeterAn instrument that measures aircraft altitude above sea level by sensing atmospheric pressure. The radio altimeter uses radar to measure actual height above terrain.
AMAmplitude Modulation. A modulation method where the amplitude of a carrier wave varies in proportion to the modulating signal. Susceptible to noise and interference. Used for AM broadcasting and aviation communications.
AmmeterAn instrument used to measure electric current in amperes. Connected in series with the circuit being measured. A shunt resistor is used to extend the measurement range.
AmpereThe SI unit of electric current. One ampere equals one coulomb of charge passing a point per second. Named after French physicist André-Marie Ampère.
Antenna BandwidthThe range of frequencies over which an antenna performs acceptably, typically defined as the range where SWR remains below 2:1. Narrower bandwidth indicates a more resonant, higher-Q antenna.
Antenna GainThe ability of an antenna to direct transmitted power in a specific direction compared to a reference antenna. Expressed in dBi (compared to isotropic) or dBd (compared to a dipole).
APCPAmmonium Perchlorate Composite Propellant. The solid propellant used in most commercial high-power rocket motors. Contains ammonium perchlorate oxidizer, aluminum fuel, and HTPB binder.
ApogeeThe highest point reached by a rocket during its flight. Dual-deploy recovery systems typically deploy the drogue parachute at or just after apogee.
Apparent PowerThe product of RMS voltage and RMS current in an AC circuit, measured in volt-amperes (VA). Includes both true power and reactive power. Apparent power equals true power only when power factor is 1.0.
APRSAutomatic Packet Reporting System. A real-time digital communications system using AX.25 packet radio. Used for position tracking, weather data, and messaging. Primary US frequency: 144.390 MHz.
ArmingThe process of enabling motor output on a drone flight controller. Most systems require a specific stick combination or switch to arm. Motors should not spin until intentionally armed.
ARQAutomatic Repeat Request. An error correction mode used in SITOR communications where the receiving station requests retransmission of corrupted characters. Used for two-way communications between specific stations.
ASCIIAmerican Standard Code for Information Interchange. A 7-bit character encoding standard that represents text in computers and communications equipment. Has gained widest acceptance for data exchange between computers.
ASKAmplitude Shift Keying. A digital modulation technique where the amplitude of the carrier is switched between discrete levels to represent digital data. The simplest form uses two levels (on/off keying).
ATCAir Traffic Control. The service provided by ground-based controllers to direct aircraft safely and efficiently. ATC uses radar, transponders, and VHF voice communications to manage traffic.
ATISAutomatic Terminal Information Service. A continuous broadcast of recorded non-control information at busy airports including current weather, active runways, and NOTAMs.
ATRAnti-Transmit-Receive. A device in a RADAR system that prevents the received signal from entering the transmitter, protecting the transmitter from the received echo.
AttenuationThe reduction in signal strength as it travels through a medium or component. Expressed in decibels (dB). Caused by resistive losses, impedance mismatch, and dielectric absorption.
Attenuation (Fiber)The reduction in optical signal power as light travels through a fiber, expressed in dB/km. Caused by absorption, scattering, and bending losses. Single-mode fiber has lower attenuation than multimode.
AutopilotAn automatic flight control system that maintains aircraft attitude, heading, and altitude without continuous pilot input. Modern autopilots integrate with GPS, VOR, and ILS for navigation and approach guidance.
Aviation TransponderAn airborne transmitter/receiver that responds to ground radar interrogations. Mode A provides identification, Mode C adds altitude, Mode S adds selective addressing and ADS-B capability.
B
Back EMFBack electromotive force. A voltage generated in an inductor or motor that opposes the applied voltage. In motors, back EMF limits current as speed increases.
BackscatterLight scattered back toward the source in a fiber optic cable due to impurities and irregularities in the glass. Measured by an OTDR to locate faults and characterize fiber loss.
Ballistic CoefficientA measure of a rocket's ability to overcome air resistance. Higher ballistic coefficient rockets are less affected by drag and reach higher altitudes for a given impulse.
BalunBalanced-to-Unbalanced transformer. A device used to connect a balanced antenna (such as a dipole) to an unbalanced feedline (such as coaxial cable). Prevents common-mode current on the feedline shield.
Band PlanA voluntary or regulatory allocation of frequencies within an amateur radio band for specific modes and uses. Different segments are designated for CW, phone, digital, beacons, and satellite.
Bandwidth (Fiber)The data-carrying capacity of a fiber optic cable, typically expressed in MHz·km. Higher bandwidth fiber supports faster data rates over longer distances. Single-mode fiber has essentially unlimited bandwidth.
Baud RateThe number of symbol changes per second in a digital communications system. For binary systems, baud rate equals bit rate. For multilevel modulation, bit rate exceeds baud rate.
BCDBinary Coded Decimal. A method of encoding decimal digits as 4-bit binary numbers. Each decimal digit (0–9) is represented separately. Used in display systems and counting circuits.
BeaconAn amateur radio station transmitting signals for propagation study, direction finding, or identification. Beacons transmit continuously or on a schedule on designated beacon frequencies.
BeamwidthThe angular width of an antenna's main radiation lobe, typically measured between the half-power (-3 dB) points. Narrower beamwidth indicates higher gain.
Bend RadiusThe minimum radius a fiber optic cable can be bent without causing excessive signal loss or physical damage. Exceeding the minimum bend radius causes microbending losses and can crack the fiber.
BetaflightOpen-source flight controller firmware widely used in FPV racing and freestyle drones. Provides PID tuning, motor mixing, OSD configuration, and blackbox data logging.
Bird WattmeterA directional RF wattmeter that measures forward and reflected power in a transmission line. Uses interchangeable 'slug' elements for different power and frequency ranges. Industry standard for RF power measurement.
BitThe smallest unit of digital information, representing either a 0 or a 1. Eight bits form one byte. The term comes from 'binary digit.'
BlackboxA flight data logging system in drone flight controllers that records gyro data, motor outputs, RC inputs, and other flight parameters. Used for PID tuning and crash analysis.
BNCBayonet Neill-Concelman connector. A quick-connect/disconnect RF connector commonly used on test equipment and some aviation electronics. Rated to approximately 4 GHz.
Bridge-to-BridgeA VHF radio station located on a ship's navigational bridge used only for navigational communications. Operates on Channel 13 at 1 watt. May identify by vessel name instead of call sign.
BufferThe protective coating applied directly over the fiber cladding. Provides mechanical protection and identifies fibers by color coding. Tight-buffered and loose-tube are the two main construction types.
BurnoutThe moment when a rocket motor exhausts its propellant. The rocket continues coasting upward after burnout until aerodynamic drag and gravity bring it to apogee.
ByteA unit of digital information consisting of 8 bits. Can represent values from 0 to 255. Computer memory, storage, and network speeds are commonly measured in bytes and their multiples.
C
C RatingThe continuous discharge rate of a LiPo battery expressed as a multiple of its capacity. A 1500 mAh 50C battery can deliver 75 amps continuously (50 × 1.5 Ah = 75A).
CalibrationThe process of comparing a measurement instrument to a known standard and adjusting it to ensure accuracy. Test equipment should be calibrated regularly, typically annually, to maintain measurement traceability.
CapacitanceThe ability of a component to store electrical charge. Measured in farads (F). Determined by plate area, plate separation, and dielectric constant of the insulating material between the plates.
Capacitive ReactanceThe opposition that a capacitor presents to AC current flow, measured in ohms. XC = 1/(2πfC). Decreases as frequency increases — capacitors pass high frequencies more easily.
CapacitorA passive electronic component that stores electrical energy in an electrostatic field between two conductive plates separated by a dielectric. Capacitance is measured in farads (F).
CDMACode Division Multiple Access. A digital wireless technology where multiple users share the same frequency band simultaneously, each identified by a unique pseudorandom spreading code. Used in cellular networks.
CGCenter of Gravity. The balance point of a rocket. For stable flight, the CG must be above (forward of) the Center of Pressure by at least one caliber (body diameter).
Channel 13VHF marine channel 156.650 MHz used for bridge-to-bridge navigational communications. Transmit power is limited to 1 watt except in distress situations or when rounding blind bends.
Channel 16VHF marine channel 156.800 MHz — the international distress, safety, and calling frequency. All vessels equipped with VHF radio must monitor Channel 16 while underway.
Channel 70VHF marine channel 156.525 MHz reserved exclusively for Digital Selective Calling (DSC). No voice communications are permitted on Channel 70.
Characteristic ImpedanceThe impedance of a transmission line determined by its physical construction. Standard marine and aviation coaxial cables are typically 50 ohms. Cable TV systems use 75 ohms.
ChecksumA value calculated from a data set used to detect errors in transmission or storage. The receiver recalculates the checksum and compares it to the transmitted value. Used in NMEA sentences and communications protocols.
Circuit BreakerAn automatic switching device that interrupts current flow when it exceeds a safe level. Unlike a fuse, a circuit breaker can be reset after tripping. Used to protect wiring and equipment from overcurrent.
CladdingThe outer layer of glass or plastic surrounding the fiber core with a lower refractive index. The difference in refractive index between core and cladding causes total internal reflection, keeping light in the core.
CleaveA controlled break made perpendicular to the fiber axis to create a flat end face for splicing or connector installation. A proper cleave is essential for low-loss connections.
Coast PhaseThe period of rocket flight after motor burnout and before apogee. During coast, the rocket decelerates due to drag and gravity with no thrust being produced.
Coaxial CableA transmission line consisting of a center conductor, dielectric insulator, outer conductor (shield), and protective jacket. Carries RF signals with relatively low loss and rejects external interference.
COLEMCommercial Operator License Examination Manager. FCC-authorized organizations that administer commercial radio operator examinations. Pilots, mariners, and technicians obtain FCC licenses through COLEMs.
Compulsory VesselA vessel required by law to carry radio equipment. Generally includes cargo ships over 300 gross tons and vessels carrying more than 12 passengers on international voyages.
ConductanceThe reciprocal of resistance, measured in siemens (S) or mhos. G = 1/R = I/V. Higher conductance indicates lower resistance and easier current flow.
ConductorA material with low resistance that allows electrical current to flow easily. Good conductors have many free electrons. Silver, copper, and gold are the best common conductors.
Connector (Fiber)A mechanical device that mates two fiber ends to allow signal transfer while minimizing loss. Common types include SC, LC, ST, FC, and MTP/MPO connectors. Insertion loss should be less than 0.5 dB.
CoreThe central glass or plastic region of a fiber optic cable through which light travels. Single-mode fiber has a core diameter of approximately 9 μm. Multimode fiber cores are 50 or 62.5 μm in diameter.
COSPAS-SARSATAn international satellite-based search and rescue system that detects and locates 406 MHz distress beacons from EPIRBs, ELTs, and PLBs. Uses Doppler shift to determine transmitter location.
CouplerA passive fiber optic device that splits or combines optical signals. A 3 dB coupler splits a signal equally between two output fibers, with each output receiving half the input power.
CPCenter of Pressure. The point on a rocket where aerodynamic forces act. For stability, the CP must be below (aft of) the Center of Gravity (CG) by at least one body diameter.
CrossfireTBS Crossfire. A long-range RC link system by Team BlackSheep operating on 900 MHz. Provides reliable control links at distances of many kilometers.
CTCSSContinuous Tone-Coded Squelch System. A sub-audible tone system used in FM radio communications. The receiver only opens when it detects the correct tone, filtering out unwanted transmissions.
CVRCockpit Voice Recorder. A flight recorder that captures audio from the flight deck including crew communications and ambient sounds. Required on most commercial aircraft. Retains the last 2 hours of audio.
CWContinuous Wave. Morse code transmission using on/off keying of a carrier. No longer required for amateur licensing but still widely used for DX and weak signal work due to its efficiency.
D
DACDigital-to-Analog Converter. A circuit that converts discrete digital values to a continuous analog signal. Used to convert digitally processed audio or RF signals back to analog for transmission or playback.
Dark FiberInstalled fiber optic cable that is not currently in use. Often leased to carriers or businesses who provide their own transmission equipment. The fiber is 'dark' because no light is being transmitted.
dBDecibel. A logarithmic unit expressing the ratio of two power or voltage levels. 3 dB represents a doubling of power. 10 dB represents a 10x power increase. 20 dB represents a 100x power increase.
dBiDecibels relative to an isotropic radiator. A unit for expressing antenna gain compared to a theoretical isotropic antenna that radiates equally in all directions.
dBmDecibels relative to one milliwatt. An absolute power measurement. 0 dBm = 1 mW. 30 dBm = 1 watt. 60 dBm = 1 kilowatt. Used to express transmitter power and receiver sensitivity.
DCDirect Current. Electric current that flows in one direction only. Batteries produce DC. Most aircraft systems operate on 28V DC. Electronic circuits typically require regulated DC power.
Dead ZoneIn OTDR measurements, the distance at the beginning of a fiber trace where events cannot be resolved due to the transmit pulse duration. Lengths range from a few meters to tens of meters depending on OTDR settings.
Decibel (Optical)The unit used to express optical power ratios and loss in fiber optic systems. dB = 10 × log(P1/P2). A loss of 3 dB means half the power was lost. dBm expresses absolute power relative to 1 milliwatt.
DelayIn rocket motor designations, the time in seconds between burnout and ejection charge firing. A C6-5 motor has a 5-second delay. D means no delay (staged motors).
Demodulation ProbeAn oscilloscope probe containing a diode detector and filter. Used to detect and display AM-modulated RF signals on an oscilloscope. Allows viewing of the modulation envelope without requiring a spectrum analyzer.
Deviation RatioThe ratio of the maximum carrier frequency deviation to the highest audio modulating frequency in an FM system. Used to characterize FM system design.
DielectricAn insulating material placed between capacitor plates. The dielectric constant determines how much capacitance a given physical configuration provides. Higher dielectric constant = higher capacitance.
DiodeA semiconductor device that allows current to flow in one direction only. Used for rectification (converting AC to DC), signal detection, and protection against reverse voltage.
Dip MeterAn instrument used to measure resonant frequency of tuned circuits and antennas. A variable oscillator is coupled to the circuit under test. When frequencies match, the meter dips due to energy absorbed by the resonant circuit.
DipoleA basic antenna consisting of two equal-length conductors fed at the center. A half-wave dipole is approximately 95% of a half wavelength long and has a gain of 2.15 dBi.
Direct CurrentElectric current that flows continuously in one direction. Produced by batteries and rectified power supplies. Aircraft avionics typically operate on 28V DC from generators or batteries.
Directional CouplerA passive device that samples a portion of the RF power flowing in one direction in a transmission line. Used in SWR meters and power monitoring equipment.
DispersionThe spreading of a light pulse as it travels through optical fiber, limiting bandwidth and transmission distance. Modal dispersion occurs in multimode fiber. Chromatic dispersion affects all fiber types.
DJIDa-Jiang Innovations. A leading manufacturer of consumer and professional drones, FPV systems, and camera gimbals. Known for the Mavic, Mini, and FPV product lines.
DMEDistance Measuring Equipment. An aviation navigation system that measures slant-range distance from an aircraft to a ground station. Operates in the 960–1215 MHz band. Often paired with VOR for position fixing.
DMMDigital Multimeter. An electronic measuring instrument that combines voltmeter, ammeter, and ohmmeter functions. Displays measurements numerically. Most include AC/DC voltage and current, resistance, diode test, and continuity.
Drogue ParachuteA small parachute deployed at apogee in a dual-deploy recovery system. Slows the rocket to a safe descent rate while keeping it compact until the main parachute deploys at low altitude.
DSCDigital Selective Calling. An automated digital system that allows mariners to transmit distress alerts containing vessel identity and GPS position. Operates on Channel 70 for VHF and dedicated DSC frequencies for MF/HF.
DSC ControllerThe equipment programmed to initiate DSC distress alerts and calls. When activated, it transmits a formatted digital signal containing the vessel's MMSI number and GPS position.
DShotDigital Shot. A digital communication protocol between a flight controller and ESCs. Replaces analog PWM signals with a digital signal for more precise and noise-immune motor control.
DSPDigital Signal Processing. The use of digital computation to process, filter, and analyze signals. Enables sophisticated noise reduction, filtering, and signal enhancement that would be impractical with analog circuits.
DTMFDual Tone Multi-Frequency. The system of audio tones used for telephone dialing. Each key generates two simultaneous tones from a matrix of four row frequencies and four column frequencies.
Dual DeployA recovery system using two parachutes — a drogue deployed at apogee and a main chute deployed at lower altitude. Reduces drift compared to single-chute recovery.
Dummy LoadA non-radiating resistive termination for a transmitter. Used during testing and adjustment to absorb transmitter power without causing interference. Must be rated for the transmitter's power output and frequency range.
Duty CycleThe ratio of the time a signal is active (on) to the total period, expressed as a percentage. A 50% duty cycle means the signal is on for half the period. Related to average power in pulsed systems.
DWDMDense Wavelength Division Multiplexing. A technology that combines multiple optical signals on slightly different wavelengths onto a single fiber, dramatically increasing capacity. Used in long-haul telecommunications.
DXLong distance. In amateur radio, DX refers to distant stations, especially those in rare countries or locations. DXing is the pursuit of making contacts with distant and rare stations.
DXpeditionAn organized amateur radio operation from a rare or remote location to make contacts with operators worldwide. DXpeditions activate rare DXCC entities and islands.
Dynamic RangeIn receivers, the ratio of the largest tolerable input signal to the minimum discernible signal. In spectrum analyzers, the range between the noise floor and the maximum measurable signal level.
E
E-MatchElectric match. An electrically ignited device used to ignite rocket motors and fire ejection charges in altimeter-controlled recovery systems. Ignites when a small current passes through it.
Eddy CurrentsCirculating currents induced in a conductor by a changing magnetic field. Cause energy losses in transformer cores and motors. Minimized by using laminated cores rather than solid metal.
Ejection ChargeA small pyrotechnic charge used to pressurize the rocket airframe and deploy parachutes. Either motor-ejection (built into motor) or electronic (fired by altimeter).
Electromotive ForceEMF. The voltage produced by a source such as a battery or generator. Measured in volts. Not an actual force but a potential difference that drives current through a circuit.
ElmerAn experienced amateur radio operator who mentors newcomers. The term is unique to amateur radio culture and refers to a helpful, knowledgeable guide who helps new hams get started.
ELTEmergency Locator Transmitter. An aircraft device that activates automatically upon crash impact and transmits a distress signal. Modern ELTs transmit on 406 MHz for satellite detection and 121.5 MHz for homing.
EncodingThe process of converting information into a specific format for transmission or storage. In radio communications, encoding may involve modulation, error correction coding, and protocol framing.
EPIRBEmergency Position Indicating Radio Beacon. A battery-operated device that floats free of a sinking vessel and automatically transmits a distress signal on 406 MHz to COSPAS-SARSAT satellites.
ERPEffective Radiated Power. The actual power radiated by an antenna system. ERP = transmitter power × 10^(net dB/10) where net dB = antenna gain minus all system losses.
Error CorrectionTechniques used to detect and correct errors in digital data. Forward error correction (FEC) adds redundant data allowing the receiver to correct errors. ARQ systems request retransmission of corrupted data.
ESCElectronic Speed Controller. A device that converts flight controller commands into the three-phase AC power required to drive brushless motors. Each motor typically has its own ESC.
ExpressLRSELRS. An open-source long-range RC link protocol operating on 900 MHz or 2.4 GHz. Known for very low latency and long range at low power levels.
F
F3EITU emission designator for FM telephony — frequency modulation, single channel analog, telephone. The standard emission type for VHF marine radio voice communications.
FailsafeA programmed response when a drone loses its RC link. Common failsafe actions include hovering in place, returning to home, or immediately disarming and descending.
FaradThe SI unit of capacitance. One farad stores one coulomb of charge at one volt. Practical capacitors are typically measured in microfarads (μF), nanofarads (nF), or picofarads (pF).
FCFlight Controller. The main computer board in a drone that reads sensor data (gyroscope, accelerometer, barometer, GPS) and calculates motor output commands to maintain stable flight.
FC ConnectorA fiber optic connector with a threaded coupling mechanism. Provides secure, repeatable connections resistant to vibration. Commonly used in single-mode applications and test equipment.
FDRFlight Data Recorder. Records aircraft performance parameters including altitude, airspeed, heading, and control inputs. Required on commercial aircraft. Commonly called the black box despite being bright orange.
FECForward Error Correction. A SITOR transmission mode where each character is transmitted twice for error detection. Used for broadcast communications to multiple stations simultaneously.
Feed PointThe point on an antenna where the transmission line connects. The feed point impedance determines how well the antenna matches the transmission line.
FeedlineThe transmission line (usually coaxial cable) connecting the transmitter or receiver to the antenna. Feedline losses reduce system performance and must be minimized.
Fiber Optic CableA transmission medium using thin strands of glass or plastic to carry light signals. Immune to electromagnetic interference, supports very high bandwidth, and provides galvanic isolation between endpoints.
Field Strength MeterAn instrument that measures the strength of a radio frequency field at a given location, typically in microvolts per meter or dBμV/m. Used to verify antenna coverage and measure transmitter emissions.
FinsAerodynamic surfaces at the aft end of a rocket that provide stability by moving the center of pressure below the center of gravity. Fin design affects stability and drag.
FMFrequency Modulation. A modulation method where the frequency of a carrier wave varies in proportion to the modulating signal amplitude. More resistant to noise than AM. Used for VHF/UHF voice communications.
Forward BiasVoltage applied across a diode in the direction that allows current to flow. For silicon diodes, forward bias typically requires at least 0.6–0.7 volts to overcome the junction potential.
FPVFirst Person View. Flying a drone using a live video feed from an onboard camera displayed on goggles or a monitor, giving the pilot the perspective of being inside the aircraft.
FrequencyThe number of complete cycles per second of an AC signal, measured in Hertz (Hz). Power line frequency is 60 Hz in the US. Aircraft use 400 Hz. RF frequencies range from kHz to GHz.
Frequency CounterAn electronic instrument that measures the frequency of a periodic signal by counting cycles over a precise time interval. Accuracy is determined by the stability of the internal time base (crystal oscillator).
Frequency StandardA highly stable, accurate oscillator used as a frequency reference. Rubidium standards are used in the field. Cesium beam standards provide the highest accuracy. GPS-disciplined oscillators use satellite timing.
Fresnel ReflectionLight reflected at a fiber end face or other refractive index discontinuity. Causes return loss at connectors and splices. An air gap between fiber ends causes approximately 14 dB of return loss.
FSKFrequency Shift Keying. A digital modulation technique where the carrier frequency is switched between discrete values to represent digital data. Used in AFSK, RTTY, and many wireless data systems.
FT8A weak signal digital mode developed by Joe Taylor (K1JT). Uses 15-second transmission periods and can decode signals 20 dB below the noise floor. Popular for DX on HF bands.
FuseA sacrificial overcurrent protection device containing a wire or element that melts when current exceeds a safe level. Unlike a circuit breaker, a fuse must be replaced after it blows.
Fusion SpliceA permanent connection between two fiber ends made by melting them together with an electric arc. Produces very low loss (typically 0.02–0.1 dB) and high return loss. Requires a fusion splicer.
G
Galvanic CorrosionCorrosion that occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. The more active metal corrodes preferentially. Common in marine environments with mixed metals.
GFIGround Fault Interrupter. An electrical safety device that detects current flowing through an unintended ground path and interrupts the circuit within milliseconds. Prevents electrocution in wet environments.
GlideslopeThe vertical component of the ILS approach system. Provides electronic descent guidance at approximately 3 degrees to the runway touchdown zone. Operates in the 329.15–335 MHz band.
GMDSSGlobal Maritime Distress and Safety System. An internationally agreed set of safety procedures, equipment, and communication protocols designed to automate and improve maritime distress communications.
GPSGlobal Positioning System. A satellite navigation system operated by the US Department of Defense. Uses 24+ satellites in 6 orbital planes at 20,200 km altitude. Requires 4 satellites for a complete 3D position fix.
GPS Return to HomeAn automated flight mode that returns a drone to its launch point using GPS when triggered manually or by failsafe. Requires a valid GPS fix at launch point.
GPWSGround Proximity Warning System. An aircraft safety system that warns pilots when the aircraft is in danger of controlled flight into terrain. Modern systems use GPS databases for enhanced warnings.
GroundA reference point in a circuit defined as zero volts. In vehicles and aircraft, the metal frame or fuselage serves as a common ground return path. Proper grounding is essential for RF equipment performance.
Ground PlaneA conductive surface that acts as an electrical ground reference for a vertical antenna. On a vehicle, the metal body serves as the ground plane. Affects antenna radiation pattern and feed point impedance.
Ground WaveRadio wave propagation along the surface of the Earth. Dominant at MF and lower HF frequencies. Ground wave provides reliable short to medium range coverage without relying on the ionosphere.
Guard FrequencyA frequency continuously monitored for emergency calls. In aviation, 121.5 MHz is the civilian guard frequency. In military aviation, 243.0 MHz is the guard frequency.
GyroscopeA sensor that measures angular rotation rate. In drones, the gyroscope is the primary sensor used by the flight controller PID loop to detect and correct unwanted rotation.
H
Half-Wave DipoleAn antenna element approximately half a wavelength long. Has a feed point impedance of approximately 72 ohms and a gain of 2.15 dBi over an isotropic radiator.
Ham RadioInformal term for amateur radio. The origin is unclear but may derive from 'ham-fisted' operators or from early slang for amateur communicators. Licensed by the FCC under Part 97.
HDOPHorizontal Dilution of Precision. A measure of GPS position accuracy based on satellite geometry. Low HDOP (1–2) indicates good geometry and accurate positions. High HDOP (>6) indicates poor geometry.
HDZeroA digital FPV video system from Runcam/HDZero that transmits HD video at low latency. Operates on 5.8 GHz with a traditional channel-based approach unlike DJI's proprietary system.
HenryThe SI unit of inductance. One henry produces one volt of back EMF when current changes at one ampere per second. Practical inductors are typically measured in millihenries (mH) or microhenries (μH).
HertzThe SI unit of frequency. One hertz equals one cycle per second. Named after German physicist Heinrich Hertz. Abbreviated Hz. Prefixes: kHz (thousands), MHz (millions), GHz (billions).
HexadecimalA base-16 numbering system using digits 0–9 and letters A–F. Each hexadecimal digit represents 4 binary bits. Commonly used to represent binary data in a compact, human-readable format.
HFHigh Frequency. The radio spectrum from 3 to 30 MHz. Used for long-distance communication via ionospheric propagation. Amateur HF bands include 80m, 40m, 20m, 15m, and 10m.
HPRHigh Power Rocketry. Rocketry using motors of H class or above (160 N·s and greater). Requires certification through NAR or Tripoli Rocketry Association.
HysteresisThe lag between cause and effect in magnetic materials. Energy is lost as heat each time a magnetic material is reversed. Hysteresis loss increases with frequency, limiting the use of iron cores at RF.
I
IFRInstrument Flight Rules. Flight conducted primarily by reference to flight instruments rather than visual cues. Requires specific training, certification, and equipment including navigation and communication radios.
ILSInstrument Landing System. A precision approach system providing lateral (localizer) and vertical (glideslope) guidance to a runway. The localizer operates at 108.10–111.95 MHz, glideslope at 329.15–335 MHz.
ImpedanceThe total opposition to AC current flow, combining resistance (R) and reactance (X). Z = √(R² + X²). Measured in ohms. Unlike resistance, impedance varies with frequency. Proper impedance matching between components maximizes power transfer.
Impedance MatchingThe process of making the impedance of a source equal to the impedance of its load to maximize power transfer. Accomplished with matching networks, transformers, or antenna tuners.
ImpulseTotal impulse in Newton-seconds (N·s). The product of average thrust and burn time. Determines the motor classification letter (A through O and beyond).
Index of RefractionThe ratio of the speed of light in a vacuum to the speed of light in a given material. The core has a higher index than the cladding, causing total internal reflection and light confinement.
Inductive ReactanceThe opposition that an inductor presents to AC current flow, measured in ohms. XL = 2πfL. Increases as frequency increases — inductors block high frequencies more effectively.
InductorA passive component that stores energy in a magnetic field when current flows through it. Measured in henries. Inductors oppose changes in current and are used in filters, tuning circuits, and power supplies.
Insertion LossThe reduction in signal power caused by inserting a device (connector, filter, feedline) into a transmission path. Expressed in dB. Lower insertion loss is better.
Insertion Loss (Fiber Optics)The optical power lost when a connector, splice, or other component is inserted into a fiber link. Measured in dB. Typical connector insertion loss is 0.1–0.5 dB. Cumulative losses limit link distance.
InsulatorA material with very high resistance that prevents current flow. Used to isolate conductors and prevent unwanted current paths. Common insulators include rubber, plastic, glass, and ceramic.
IonosphereThe ionized upper atmosphere (60–1,000 km altitude) that reflects radio waves back to Earth. Enables long-distance HF communication (sky wave propagation). Its behavior varies with solar activity.
IP AddressInternet Protocol address. A unique numerical identifier assigned to each device on a network. IPv4 addresses are 32 bits written as four decimal numbers. IPv6 addresses are 128 bits.
Isotropic RadiatorA theoretical antenna that radiates power equally in all directions in three dimensions. Used as a reference for expressing antenna gain in dBi. No real antenna is truly isotropic.
J
J-PoleA half-wave end-fed antenna matched to coaxial feedline using a quarter-wave matching stub, forming a J shape. Popular for VHF/UHF because it requires no ground plane.
J3EITU emission designator for single sideband telephony — SSB suppressed carrier, single channel analog, telephone. The standard emission for MF/HF marine voice communications.
JacketThe outermost protective layer of a fiber optic cable. Provides mechanical protection against abrasion, moisture, and crushing. Color often indicates fiber type or application.
JouleThe SI unit of energy. One joule equals one watt-second. The energy stored in a capacitor is E = ½CV². The energy stored in an inductor is E = ½LI².
K
Kirchhoff's Current LawThe sum of all currents entering a node equals the sum of all currents leaving that node. Conservation of charge — current cannot accumulate at a junction.
Kirchhoff's Voltage LawThe sum of all voltages around any closed loop in a circuit equals zero. The sum of voltage rises equals the sum of voltage drops around any complete path.
KV RatingMotor velocity constant expressed in RPM per volt. A 2400 KV motor on 4S (14.8V) produces approximately 35,520 RPM unloaded. Lower KV motors produce more torque for larger props.
L
L-NetworkAn impedance matching network consisting of one inductor and one capacitor arranged in an L-shape. Provides impedance transformation in one direction.
LAANCLow Altitude Authorization and Notification Capability. An FAA system that provides near-instant airspace authorization for drone pilots to fly in controlled airspace below approved altitudes.
LatencyThe delay between a signal being sent and received. In satellite communications, geostationary satellites have approximately 600 ms round-trip latency. LEO satellites have much lower latency.
LC ConnectorLucent Connector. A small form-factor fiber connector with a push-pull latch mechanism. Half the size of SC connectors. Widely used in high-density applications and SFP transceivers.
Lead-Acid BatteryA rechargeable battery using lead plates in sulfuric acid electrolyte. Fully charged cell voltage is 2.06V. Emits hydrogen gas during charging. Most common battery type for marine and automotive applications.
Lenz's LawThe induced current in a conductor always flows in a direction that opposes the change in magnetic flux that caused it. A consequence of conservation of energy — induced effects always oppose their cause.
LiPoLithium Polymer battery. The most common battery chemistry for FPV drones due to high energy density and high discharge rates. Cells are nominally 3.7V, fully charged to 4.2V.
Loading CoilAn inductor inserted in series with an antenna to electrically lengthen it below its natural resonant frequency. Used when a full-size antenna is not practical.
LocalizerThe lateral guidance component of the ILS system. Transmits two overlapping AM beams at 90 Hz and 150 Hz on either side of the runway centerline. Aircraft deviation is determined by comparing signal strengths.
Logic AnalyzerA test instrument that captures and displays multiple digital signals simultaneously. Shows timing relationships between signals and decodes serial protocols. Used for debugging digital circuits and communications.
Logic ProbeA simple test instrument that indicates logic high, low, or pulsing states at a digital circuit test point. More useful than a voltmeter for testing TTL and CMOS digital circuits.
LoiterA drone flight mode that uses GPS and barometer to hold a fixed position and altitude automatically. Also called Position Hold or PosHold in ArduPilot firmware.
M
MagnetronA vacuum tube device that generates high-power microwave pulses using crossed electric and magnetic fields. The primary RF source in pulsed radar transmitters. Generates the transmitted pulse in most marine radar systems.
Marker BeaconGround-based transmitter at 75 MHz that signals specific points along an ILS approach. Outer marker (400 Hz tone) at 4–7 miles, middle marker (1300 Hz) at decision height, inner marker (3000 Hz) at the threshold.
MAVICA line of folding consumer drones made by DJI. Known for compact form factor, obstacle avoidance, and camera stabilization. Includes Mavic Air, Mavic Pro, and Mavic 3 series.
Maximum Power TransferMaximum power is transferred from a source to a load when the load impedance equals the source impedance. At this condition, half the available power is delivered to the load.
MAYDAYThe international radiotelephony distress signal, spoken three times. Indicates grave and imminent danger requiring immediate assistance. Highest priority communication in the maritime radio service.
Mechanical SpliceA fiber optic splice that aligns and holds two fiber ends using a mechanical fixture with index-matching gel. Higher loss than fusion splicing but does not require expensive equipment.
MFMedium Frequency. The radio frequency band from 300 kHz to 3 MHz. Used for maritime communications including the international distress frequency 2182 kHz and NAVTEX on 518 kHz.
MMSIMaritime Mobile Service Identity. A unique 9-digit number assigned to a vessel's radio station. Used by DSC systems to identify vessels. Must be programmed into the VHF radio for DSC to function correctly.
Modal DispersionSignal distortion in multimode fiber caused by different light modes traveling at different speeds. Limits bandwidth and distance. Graded-index multimode fiber reduces modal dispersion compared to step-index.
Mode CA transponder mode that transmits aircraft altitude in addition to identification in response to ATC radar interrogation. Altitude is encoded in 100-foot increments from a pressure altimeter encoder.
Mode SA transponder mode that allows selective interrogation of individual aircraft. Supports ADS-B Out and TCAS collision avoidance. Provides higher data capacity than Mode A or Mode C.
ModulationThe process of varying a carrier wave to encode information. The three fundamental parameters that can be modulated are amplitude (AM), frequency (FM), and phase (PM).
Modulation IndexThe ratio of frequency deviation to modulating frequency in an FM signal. Determines the bandwidth and spectral characteristics of the transmitted signal.
Motor DesignationA standard format describing rocket motor performance. Example: H128W-14A — H=class, 128=average thrust in Newtons, W=propellant type, 14=delay in seconds, A=manufacturer code.
Multimode FiberFiber optic cable with a core large enough (50 or 62.5 μm) to support multiple light propagation paths (modes). Higher attenuation and lower bandwidth than single-mode but allows less expensive LED sources.
N
N-TypeA threaded RF connector rated for frequencies up to 11 GHz and power levels up to several kilowatts. Used in high-frequency, high-power, and outdoor applications requiring weather resistance.
NARNational Association of Rocketry. The primary US organization for sport rocketry. Provides safety codes, insurance, and High Power certification programs.
NAVTEXNavigational Telex. An automated system for receiving Maritime Safety Information broadcasts on 518 kHz (international) and 490 kHz (national). Ship stations receive only — they do not transmit on NAVTEX.
NDBNon-Directional Beacon. A ground-based radio transmitter used for aircraft navigation in conjunction with the ADF receiver. Operates in the 190–1750 kHz range.
NetA scheduled meeting on a specific frequency where amateur radio operators check in and exchange traffic. Includes emergency communications nets, traffic nets, and social nets.
Newton-SecondN·s. The unit of impulse. One Newton-second equals one Newton of force applied for one second. Determines rocket motor classification.
NiCd BatteryNickel-Cadmium rechargeable battery. Cell voltage is 1.25V. More stable discharge curve than lead-acid. Requires periodic full discharge to prevent memory effect. Contains toxic cadmium.
NMEA 0183A standard data interface for marine electronics. Uses ASCII sentences beginning with $ to transmit GPS position, speed, heading, and other navigation data between devices at 4800 baud.
NMEA 2000A successor to NMEA 0183 using a CAN bus network topology. Supports multiple devices on a single backbone with data rates up to 250 kbps. Plug-and-play device connectivity.
Noise BridgeA test instrument used to measure antenna impedance. Generates broadband noise; when the bridge is balanced against the antenna, the impedance can be read. Used with a communications receiver.
Noise FigureA measure of the noise added by a receiver's front end, expressed in dB. Lower noise figure indicates better sensitivity. The RF amplifier stage primarily determines overall receiver noise figure.
Noise Figure MeterA specialized instrument that measures the noise figure of amplifiers, receivers, and other two-port devices. Determines how much noise the device adds to the signal.
Nose ConeThe forward aerodynamic section of a rocket. Shape (ogive, conical, parabolic, Von Kármán) affects drag characteristics. Typically houses the recovery system.
NOTAMNotice to Air Missions. Official notices containing essential information about changes to aeronautical facilities, services, procedures, or hazards. Pilots must check NOTAMs before flight.
Numerical ApertureA dimensionless number that characterizes the range of angles over which a fiber can accept or emit light. NA = sin(acceptance angle). Higher NA means the fiber accepts light from a wider angle.
Nyquist TheoremA signal must be sampled at a rate at least twice the highest frequency component to be accurately reconstructed. For 3 kHz audio, a minimum sample rate of 6000 samples per second is required.
O
OhmThe SI unit of electrical resistance. One ohm allows one ampere to flow when one volt is applied. Named after German physicist Georg Simon Ohm.
Ohm's LawThe fundamental relationship between voltage, current, and resistance: V = I × R. Current is proportional to voltage and inversely proportional to resistance. Applies to DC circuits and the resistive component of AC circuits.
OmnidirectionalRadiating or receiving equally in all directions in the horizontal plane. Vertical whip antennas are omnidirectional. Contrast with directional antennas such as Yagis.
Open CircuitA break in a circuit that prevents current flow. Voltage is present but no current flows. Contrast with short circuit.
Open RocketFree open-source rocket simulation software. Allows design, simulation, and stability analysis of model and high-power rockets before building.
Optical Power MeterAn instrument that measures the power of an optical signal at a specific wavelength, expressed in dBm or milliwatts. Used to verify signal levels and calculate insertion loss in fiber links.
OscilloscopeAn electronic test instrument that displays voltage waveforms as a function of time. The vertical axis represents voltage; the horizontal axis represents time. Used to measure amplitude, frequency, phase, and waveform shape.
OSDOn-Screen Display. A system that overlays flight data (battery voltage, RSSI, altitude, speed, GPS coordinates) onto the FPV video feed for the pilot to monitor during flight.
OSI ModelOpen Systems Interconnection reference model. A seven-layer framework for network communications. Layer 1 (physical) handles actual signal transmission. Layer 7 (application) interfaces with user software.
OTDROptical Time Domain Reflectometer. A fiber optic test instrument that injects light pulses and measures backscattered and reflected light to locate faults, measure splice and connector loss, and characterize fiber length and attenuation.
P
PacketA unit of data transmitted over a network. Contains a header with addressing and control information, the data payload, and often error-checking information. Packet switching allows efficient sharing of network resources.
PAN PANThe international radiotelephony urgency signal, spoken three times before an urgency call. Indicates a serious situation that does not yet rise to the level of MAYDAY. Second-highest priority after distress.
Parabolic AntennaA dish-shaped antenna that focuses RF energy using a parabolic reflector and a feed element at the focal point. Provides very high gain and narrow beamwidth. Used for satellite and microwave links.
Parasitic ElementAn antenna element not directly connected to the feedline that is coupled electromagnetically to the driven element. Used in Yagi antennas as directors and reflectors to increase gain and directivity.
Part 107FAA regulations governing commercial drone operations in the United States. Requires a Remote Pilot Certificate, limits altitude to 400 ft AGL, and requires visual line of sight.
PCMPulse Code Modulation. A method of digitally representing analog signals by sampling the signal at regular intervals and encoding each sample as a binary number. The basis for digital audio and telephony.
Peak Envelope PowerPEP. The average power of an SSB transmitter measured during the highest peak of the modulation envelope. The standard method for rating SSB and other amplitude-modulated transmitter power.
PermeabilityThe ratio of magnetic flux density in a material to the magnetizing force applied. High-permeability materials like iron concentrate magnetic field lines and are used in transformer and inductor cores.
PhaseThe position of a point in time within a periodic cycle, measured in degrees or radians. In AC circuits, voltage and current may be out of phase due to reactive components.
Phase ModulationA modulation technique where the phase of the carrier signal varies in proportion to the modulating signal. Similar to FM in practice. The modulation index of a PM signal does not depend on carrier frequency.
Phonetic AlphabetA standardized system for spelling letters over radio to avoid confusion between similar-sounding letters. NATO standard: Alpha, Bravo, Charlie, Delta, Echo, Foxtrot, Golf, Hotel...
Pi-L NetworkAn impedance matching network consisting of two inductors and two capacitors. Provides greater harmonic suppression than a standard pi-network.
Pi-NetworkAn impedance matching network consisting of one inductor and two capacitors (or two inductors and one capacitor) in a π configuration. Provides harmonic filtering along with impedance transformation.
PIDProportional-Integral-Derivative. A control loop algorithm used by drone flight controllers to calculate motor corrections. P corrects current error, I corrects accumulated error, D dampens oscillation.
Piezoelectric EffectThe generation of an electric voltage when certain crystals are subjected to mechanical stress, or conversely the mechanical deformation of a crystal when voltage is applied. Used in crystal oscillators and microphones.
PL-259A common RF connector used on VHF and HF equipment. Also called the UHF connector. Used with RG-8, RG-213, and similar cables. Not suitable for frequencies above about 300 MHz.
PLBPersonal Locator Beacon. A portable distress beacon similar to an EPIRB but designed for personal use on land or sea. Transmits on 406 MHz to COSPAS-SARSAT satellites.
PolarizationThe orientation of the electric field of a radio wave. VHF marine and aviation antennas use vertical polarization. Most land-based HF antennas use horizontal polarization. Matching polarization reduces signal loss.
Power FactorThe ratio of true power to apparent power in an AC circuit. Power factor = cos(θ) where θ is the phase angle. Ranges from 0 (purely reactive) to 1.0 (purely resistive). Low power factor wastes energy.
Power MeterAn instrument that measures the power of an RF signal. Directional power meters measure both forward and reflected power to calculate SWR. Calibrated in watts or dBm.
Prop WashTurbulence caused by a drone flying through its own rotor downwash, particularly during rapid descents or aggressive maneuvers. Causes oscillations that require proper PID tuning to minimize.
PropagationThe way radio waves travel from transmitter to receiver. HF propagation modes include ground wave, sky wave (ionospheric), and tropospheric scatter. VHF is primarily line-of-sight.
ProtocolA set of rules governing how devices communicate. Defines message formats, timing, error handling, and procedures for establishing and terminating communications.
PSKPhase Shift Keying. A digital modulation technique where the carrier phase is switched between discrete values to represent digital data. BPSK uses 2 phases; QPSK uses 4 phases for higher data rates.
Q
QAMQuadrature Amplitude Modulation. A modulation technique that combines amplitude and phase modulation to transmit multiple bits per symbol. 64-QAM transmits 6 bits per symbol. Used in cable TV and LTE.
QRMInterference from other radio stations. In amateur radio Q-codes, QRM specifically means man-made interference, as opposed to QRN (natural static) or QSB (fading).
QRPLow power operation in amateur radio, conventionally defined as 5 watts or less on CW or 10 watts PEP or less on SSB. QRP operation is popular for portable and challenging operating.
QSL CardA written confirmation of a radio contact between two stations. Traditionally a postcard, now also electronic (eQSL). Required for many award programs including DXCC and WAS.
QSOA radio contact or conversation between two amateur radio stations. The fundamental unit of amateur radio operating. Q-codes originated in commercial telegraphy and Morse code.
Quarter-Wave VerticalA vertical antenna approximately one quarter wavelength long, fed against a ground plane. Has a feed point impedance of approximately 35 ohms and is omnidirectional in the horizontal plane.
Quiet PeriodThree-minute periods of radio silence observed on 2182 kHz at 00 and 30 minutes past each hour. During quiet periods, stations must not transmit and must listen for distress calls.
R
Radiation ResistanceAn equivalent resistance representing the power radiated by an antenna. The power dissipated in the radiation resistance equals the power radiated as electromagnetic waves.
Radio AltimeterAn aircraft instrument that measures actual height above terrain using radar. Transmits on 4200–4400 MHz and measures the time for the signal to reflect from the ground. Critical for low-visibility approaches.
Rail ButtonA guide device attached to a rocket that slides along a launch rail, keeping the rocket aligned during the low-speed phase of launch before fins become effective.
ReactanceThe opposition to AC current flow caused by inductors and capacitors. Unlike resistance, reactance does not dissipate energy. Measured in ohms. Inductive reactance is positive; capacitive reactance is negative.
Recovery HarnessThe shock cord and attachment system connecting the parachute to the rocket airframe. Must withstand the sudden jerk of parachute deployment at high descent speeds.
ReflectometerAn instrument that measures reflected power in a transmission line. Used to determine SWR and identify impedance mismatches. The ratio of reflected to forward power determines the reflection coefficient.
RelayAn electrically operated switch that uses a small control current to switch a larger load circuit. Provides electrical isolation between the control and switched circuits.
ReluctanceThe opposition that a magnetic circuit presents to magnetic flux. The magnetic equivalent of electrical resistance. High reluctance materials like air impede magnetic field lines.
Remote IDFAA-required identification broadcast from drones. Required on all UAS over 0.55 lbs (250g). Broadcasts drone ID, position, altitude, and operator location.
RepeaterAn amateur radio station that automatically receives signals on one frequency and simultaneously retransmits them on another (offset) frequency. Extends the range of low-power portable stations.
ResistanceThe opposition to current flow in a conductor, measured in ohms. Resistance converts electrical energy to heat. R = V/I. Resistance is constant and does not vary with frequency.
ResistorA passive component that provides a specific resistance value. Used to limit current, divide voltage, and set bias points. Rated in ohms and watts. Color bands indicate resistance value.
ResonanceThe condition in an antenna or circuit where inductive reactance equals capacitive reactance, resulting in minimum impedance (series) or maximum impedance (parallel). An antenna at resonance is most efficient.
Return LossThe ratio of reflected power to incident power at a discontinuity, expressed in dB. Higher return loss (more dB) indicates less reflection and better impedance matching. A perfect match has infinite return loss.
Reverse BiasVoltage applied across a diode in the direction that blocks current flow. The diode presents high impedance. Breakdown occurs if reverse voltage exceeds the PIV rating.
RF GroundAn effective earth ground at radio frequencies. Difficult to achieve because standard ground connections become inductive at RF. Essential for HF antennas and for reducing RF in the shack.
RF ProbeAn oscilloscope accessory containing a diode rectifier that converts RF voltage to DC for measurement. Allows an oscilloscope to display RF signal levels that exceed the oscilloscope's bandwidth.
RMSRoot Mean Square. The effective value of an AC voltage or current — the DC equivalent that produces the same heating effect. For a sine wave, RMS = 0.707 × peak. US household voltage is 117V RMS.
RSSIReceived Signal Strength Indicator. A measurement of RC link signal strength, typically expressed in dBm or as a percentage. Displayed in OSD to warn pilots of range limits.
RTTYRadio TeleTYpe. A digital mode using frequency shift keying to transmit text. One of the oldest digital modes, still used on HF. Uses 45 baud with 170 Hz shift for amateur radio.
S
Sacrificial AnodeA piece of metal (usually zinc) attached to a vessel or structure that corrodes preferentially, protecting the surrounding metal from galvanic corrosion. Must be inspected regularly and replaced when depleted.
SARTSearch and Rescue Transponder. A 9 GHz radar transponder that responds to X-band radar interrogation pulses, appearing as a series of 12 equally spaced dots on the rescuing vessel's radar display.
SC ConnectorSubscriber Connector or Square Connector. A push-pull fiber optic connector with a square cross-section housing. Common in telecommunications and data networking applications.
SCTSurvival Craft Transceiver. A portable VHF radio carried in survival craft for distress communications. Must operate on Channel 16, be watertight, and have a minimum power output of 1 watt.
SECURITEThe international radiotelephony safety signal, pronounced 'say-cur-ee-TAY' and spoken three times. Used to precede navigational or meteorological warnings. Third priority after distress and urgency.
SELCALSelective Calling system. Allows a ground station to alert a specific aircraft or group of aircraft without the crew monitoring the frequency continuously. Each aircraft has a unique 4-letter SELCAL code.
SemiconductorA material with conductivity between a conductor and an insulator. Silicon and germanium are common semiconductors. Conductivity can be controlled by temperature, light, or applied voltage.
Service MonitorA comprehensive radio test instrument combining a calibrated signal generator, frequency counter, power meter, deviation meter, SINAD meter, and audio analyzer. Used for complete radio transceiver testing and alignment.
SFPSmall Form-factor Pluggable transceiver. A hot-swappable module used in network equipment to interface with fiber optic or copper cables. Available in various wavelengths and data rates.
Short CircuitAn unintended low-resistance path between two points of different potential. Allows excessive current to flow, potentially damaging components or causing fire. Protected against by fuses and circuit breakers.
SidelobeSecondary radiation lobes in an antenna pattern outside the main beam. Sidelobes waste transmitter power and can cause interference or false echoes in radar applications.
SiemensThe SI unit of electrical conductance, the reciprocal of resistance. G = 1/R. Previously called mho. One siemens of conductance allows one ampere to flow per volt applied.
Signal GeneratorAn instrument that produces a calibrated RF signal at a specified frequency, amplitude, and modulation. Used for receiver alignment, sensitivity testing, and circuit troubleshooting.
SINADSignal plus Noise and Distortion ratio. A measurement used to express receiver sensitivity. Receiver sensitivity is specified as the input signal level required to achieve 12 dB SINAD at the audio output.
Single-Mode FiberFiber optic cable with a very small core (approximately 9 μm) that supports only one propagation mode. Extremely low attenuation and essentially unlimited bandwidth. Requires laser sources. Used for long-distance links.
SITORSimplex Teletype Over Radio. A digital radioteletype system used for maritime communications. Operates in ARQ mode (two-way) or FEC mode (broadcast). Uses a 7-bit code with 4 zeros and 3 ones per character.
Skin EffectThe tendency of AC current to flow near the surface of a conductor rather than through its entire cross section. The effect increases with frequency, effectively reducing the conductor's cross-sectional area at RF frequencies.
Sky WaveRadio wave propagation via reflection from the ionosphere. Enables long-distance HF communication. The ionosphere's reflecting ability varies with frequency, time of day, season, and solar activity.
SMASubMiniature version A. A small threaded RF connector rated to 18 GHz. Commonly used on portable radios, test equipment, and small RF modules.
SNRSignal-to-Noise Ratio. The ratio of desired signal power to background noise power, expressed in dB. Higher SNR indicates cleaner signal. Minimum SNR required for reliable communications depends on the modulation type.
Spectrum AnalyzerA test instrument that displays signal amplitude as a function of frequency. Shows the frequency domain representation of signals, allowing identification of harmonics, spurious emissions, and interference.
Splice LossThe optical power loss at a fiber splice point, measured in dB. Caused by core misalignment, angular misalignment, end-face quality, and core diameter mismatch. Fusion splices typically have less than 0.1 dB loss.
Sporadic EA propagation phenomenon on the 6m and 10m bands where ionized patches in the E-layer of the ionosphere reflect VHF signals over distances of 500–1500 km. Unpredictable but can be spectacular.
Spread SpectrumA transmission technique where the signal is spread over a much wider bandwidth than required. Includes frequency hopping (FHSS) and direct sequence (DSSS). Resistant to interference and difficult to intercept.
ST ConnectorStraight Tip connector. A bayonet-style fiber optic connector with a twist-lock mechanism. Commonly used in multimode applications such as building backbone and campus networks.
Stability MarginThe distance between the center of gravity and center of pressure, expressed in calibers (body diameters). Minimum stable margin is 1 caliber; 1.5–2 calibers is typical.
StubA short section of transmission line connected in parallel or series with a feedline to provide impedance matching or filtering. A quarter-wave shorted stub presents a short circuit at twice its resonant frequency.
SWRStanding Wave Ratio. A measure of impedance mismatch in a transmission line. Perfect match = 1:1. Higher ratios indicate increasing mismatch and reflected power. A ratio of 2:1 reflects approximately 11% of power.
SWR MeterStanding Wave Ratio meter. Measures the ratio of forward to reflected power in a transmission line, indicating impedance matching quality. A 1:1 reading indicates perfect match.
T
TACANTactical Air Navigation. A military navigation system providing bearing and distance information. The distance measurement component is compatible with civilian DME equipment.
TCASTraffic alert and Collision Avoidance System. An airborne system that monitors transponder replies from nearby aircraft and issues resolution advisories to prevent mid-air collisions. Requires Mode S transponder.
TDMATime Division Multiple Access. A channel access method where multiple users share the same frequency by transmitting in assigned time slots. Used in GSM cellular networks and some satellite communications.
TDRTime Domain Reflectometer. A test instrument that sends a pulse into a cable and measures reflections to locate faults, measure cable length, and identify impedance discontinuities. Uses an oscilloscope and pulse generator.
Technician LicenseThe entry-level amateur radio license in the US. Grants full privileges on all amateur bands above 50 MHz and limited HF privileges. Exam is 35 questions from a published question pool.
ThrottleThe control input that adjusts overall motor power on a drone. On a standard Mode 2 transmitter, throttle is on the left stick vertical axis.
Thrust CurveA graph of motor thrust versus time. Shows the motor's thrust profile including ignition spike, sustainer phase, and burnout. Available for certified motors at thrustcurve.org.
Time ConstantThe time required for an RC circuit capacitor to charge to 63.2% of the supply voltage, or for an RL circuit current to rise to 63.2% of maximum. τ = RC or τ = L/R. After 5 time constants the circuit is essentially at its final value.
TNCThreaded Neill-Concelman connector. Similar to BNC but with a threaded coupling mechanism. More secure than BNC in vibration-prone environments. Rated to approximately 11 GHz.
Total Internal ReflectionThe phenomenon that keeps light confined within a fiber optic core. When light strikes the core-cladding interface at an angle greater than the critical angle, it is completely reflected back into the core.
TransceiverA radio that combines transmitter and receiver functions in one unit. Modern amateur transceivers cover multiple bands and modes with a single device.
TransformerA device that transfers electrical energy between circuits through electromagnetic induction. Steps voltage up or down according to the turns ratio. Used in power supplies, impedance matching, and signal coupling.
TransistorA three-terminal semiconductor device used to amplify or switch electronic signals. Bipolar junction transistors (BJT) use base, collector, and emitter terminals. FETs use gate, drain, and source terminals.
Trap AntennaA multiband antenna that uses resonant LC circuits (traps) inserted in the antenna element. The traps present different electrical lengths at different frequencies, allowing one antenna to operate on multiple bands.
TripoliTripoli Rocketry Association. A US organization supporting high-power rocketry. Provides research motor certification, launches, and HPR certification programs.
True PowerThe actual power consumed by a circuit and converted to heat or work, measured in watts. True power = apparent power × power factor. Only the resistive component of a circuit consumes true power.
TTLTransistor-Transistor Logic. A digital logic family operating at 5 volts. Logic low is 0–0.8V; logic high is 2.0–5.5V. Open inputs float high. The most common bipolar digital logic family.
U
UNICOMUniversal Communications. A non-government air/ground radio communications facility at uncontrolled airports. Operates on designated VHF frequencies including 122.8 MHz. Provides advisory services to pilots.
V
Vector Impedance MeterA test instrument that measures both the magnitude and phase angle of impedance at a specific frequency. Provides complete impedance information needed for antenna and circuit matching.
Velocity FactorThe ratio of the speed of an electromagnetic wave in a transmission line to the speed of light in a vacuum. Solid polyethylene coax has a velocity factor of approximately 0.66. Foam dielectric is approximately 0.78–0.82.
VFRVisual Flight Rules. Flight conducted by visual reference to the ground and other aircraft in good weather conditions. VFR minimums include specific visibility and cloud clearance requirements.
VHFVery High Frequency. The radio frequency band from 30 to 300 MHz. VHF aviation communications use 118–136.975 MHz. VHF marine communications use 156–174 MHz. VHF propagation is primarily line-of-sight.
Visual Fault LocatorA fiber optic test tool that injects visible red laser light into a fiber to locate breaks, poor splices, and tight bends. Light leaks out at fault points and is visible through the cable jacket.
VoltThe SI unit of electrical potential difference (voltage). One volt drives one ampere through one ohm of resistance. Named after Italian physicist Alessandro Volta.
Voltage DividerTwo or more resistors connected in series across a voltage source to produce a fraction of the source voltage. The output voltage is proportional to the ratio of the lower resistor to the total resistance.
Voltage RegulationThe ability of a power supply to maintain constant output voltage under varying load conditions. Expressed as a percentage: VR% = (VNL - VFL) / VFL × 100. Lower percentage indicates better regulation.
VOMVolt-Ohm-Milliammeter. An analog multimeter using a D'Arsonval meter movement. Reads voltage, resistance, and current. Loading effect must be considered when measuring in high-impedance circuits.
VORVHF Omnidirectional Range. A ground-based navigation system operating at 108–117.95 MHz. Aircraft determine their bearing from the station by comparing the phase difference between two transmitted signals.
VORTACA co-located VOR and TACAN navigation station providing both civilian and military navigation services. Civilian aircraft use the VOR for bearing and the DME component of TACAN for distance.
VSWRVoltage Standing Wave Ratio. The ratio of the maximum to minimum voltage along a mismatched transmission line. Identical to SWR when measuring voltage. A perfect match gives VSWR of 1:1.
VTSVessel Traffic Service. A marine traffic monitoring service operated by the US Coast Guard in certain designated waterways to prevent collisions, groundings, and environmental harm.
VTXVideo Transmitter. The component in an FPV drone that transmits the live camera feed to the pilot's goggles or monitor. Typically operates on 5.8 GHz at power levels from 25 mW to 1 W.
W
WaiverFAA airspace authorization for rocket launches. Required for flights above 3,500 feet AGL. Obtained from the local FSDO (Flight Standards District Office) by launch organizers.
WalksnailA digital FPV video system made by Caddx/Walksnail operating on 5.8 GHz. Competes with DJI and HDZero digital FPV systems with HD low-latency video.
WattThe SI unit of electrical power. One watt equals one joule per second. P = V × I for DC circuits. Named after Scottish inventor James Watt.
WattmeterAn instrument that measures electrical power. In RF applications, a directional wattmeter measures forward and reflected power. Most accurate at full-scale deflection — the upper third of the scale is the most calibrated portion.
WaveguideA hollow metallic tube used to carry microwave RF energy with very low loss. Used at frequencies above approximately 1 GHz where coaxial cable losses become excessive. Conducts energy through electromagnetic fields in its walls.
WavelengthThe physical distance covered by one complete cycle of a radio wave. λ = 300/f(MHz) in meters. A 156 MHz VHF marine signal has a wavelength of approximately 1.92 meters.
WaypointA defined geographical position used in aviation and marine navigation. In GPS navigation, waypoints are stored coordinates that define a route. Named by latitude/longitude or by identifier codes.
WDMWavelength Division Multiplexing. A technology that transmits multiple optical signals simultaneously on different wavelengths over a single fiber. Increases fiber capacity without additional cable installation.
WhoopA class of tiny indoor FPV drones with ducted propeller guards. Named after the original Tiny Whoop. Typically 65mm to 75mm motor-to-motor, used for indoor flying.
WSPRWeak Signal Propagation Reporter. A digital protocol for propagation research. Stations transmit low-power beacon signals that are decoded and reported to a central database by receiving stations worldwide.
Y
YagiA directional antenna with a driven dipole element, a reflector, and one or more directors. Provides gain in one direction. Common for VHF/UHF amateur radio, satellite, and weak signal work.
Z
Zener DiodeA diode designed to operate in reverse breakdown at a precise voltage. Used as a voltage reference in regulators and as protection against overvoltage. Available in voltages from 2.4V to 200V and above.
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