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ADAPTIVE EXCISION FILTER A signal-processing technique that improves data transmissions. It seeks and suppresses narrowband interference in the demodulator input and reduces the effects of co-channel interference (interference on the same channel that is being used).

ADAPTIVE SYSTEM A system that automatically adjusts its parameters to improve its performance in response to changing conditions.

AGC (Automatic Gain Control) — Circuit employed to vary gain or amplifier in proportion to input signal strength so that output remains at a constant level.

ALE (Automatic Link Establishment) — A technique that permits radio stations to make contact with one another auto-matically.

AM (Amplitude Modulation) — A technique used to transmit
information in which the amplitude of the radio frequency carrier
is modulated by the audio input and the full carrier and both
sidebands are transmitted.
AME (Amplitude Modulation Equivalent) — A method of single
sideband transmission where the carrier is reinserted to permit
reception by conventional AM receivers.
AMPLITUDE The peak-to-peak magnitude of a radio wave.
ANTENNA COUPLER/TUNER A device between the trans-mitter
and antenna that modifies the characteristics of the load
presented to the transmitter so that it transfers maximum power
to the antenna.
ANTENNA DIRECTIVE GAIN The ratio of radiation intensity in
a certain direction to the average radiation intensity.
ANTENNA POWER GAIN The ratio of radiated power in a
given direction to the antenna input power.
ARQ (Automatic Repeat Request) — Data transmission tech-nique
for error-free data transfer.
ASCII (American Standard Code for Information Interchange) —
The standard code for digital data interchange. ASCII uses a
coded character set consisting of a 7-bit coded character (8 bits
including parity check).
ASYMMETRICAL KEY SYSTEM A key management system
that allows two-way secure communications among all users
that have a public key and a private key.
ASYNCHRONOUS A data communication system that adds
start-and-stop signal elements to the data for the purpose of
synchronizing individual data characters or blocks.
ATMOSPHERIC NOISE Radio noise caused by natural atmospheric
processes (primarily by lightning discharges in thunderstorms).
technique that improves data transmissions by compensating
for variations in the channel characteristics as data is received.
BANDPASS FILTER A filter that passes a limited band of frequen-cies.
It is used to remove noise and spurious signals generated in
the exciter or output frequency harmonics from the power amplifier.
BANDWIDTH The range of frequencies occupied by a given signal.
BASEBAND The frequency band occupied by a signal prior to
radio frequency carrier modulation or following demodulation.
BAUD A unit of signaling speed equal to the number of symbols,
i.e., discrete signal conditions per second.
BER (Bit Error Ratio) — The number of erroneous bits divided by
the total number of bits communicated.
BINARY Number system having base of 2, using only the
symbols 0 and 1.
BIT One binary digit (0 or 1).
BLOS (Beyond Line-of-Sight) — Communications that occur over
a great distance.
BROADBAND A term indicating the relative spectrum occu-pancy
of a signal as distinguished from a narrowband signal.
A broadband signal typically has a bandwidth in excess of twice
the highest modulating frequency. Synonym: Wideband.
CARRIER A radio frequency signal that may be modulated with
information signals.
CCIR (International Radio Consultative Committee) — An
organization of the International Telecommunications Union
(ITU) that studies technical questions related to radio communi-cations.
CHANNEL A unidirectional or bidirectional path for transmit-ting
and/or receiving radio signals.
CIPHER TEXT Encrypted data.
COLLOCATION The act or result of placing or arranging side
by side.
COMSEC (Communications Security) — Scrambling or crypto-graphic
techniques that make information unintelligible to
unauthorized persons.
COSMIC NOISE Random noise originating outside the earth’s
CRYPTOGRAPHY A COMSEC technique that translates
(encrypts) information into an apparently random message and
then interprets (deciphers) the random message by decryption.
CW (Continuous Wave) — A radio wave of constant amplitude
and constant frequency. Also, Morse code.
D LAYER First layer in the ionosphere. Reaches maximum ioniza-tion
when the sun is at zenith and dissipates quickly toward sunset.
dB ( Decibel) — The standard unit for expressing transmission
gain or loss and relative power ratios.
DE-INTERLEAVING Process used by a demodulator to reverse
interleaving and thus correct data transmission errors used in
FEC coding.
DEMODULATION The process in which the original modu-lating
signal is recovered from a modulated carrier.
DIPOLE ANTENNA A versatile antenna that is usually a wire
fed at the center of its length. Its orientation provides either
horizontal or vertical polarization.
DIRECT WAVES Travel in straight line, becoming weaker as
distance increases.
DIRECTIONAL ANTENNA An antenna that has greater gain in
one or more directions.
DSP(Digital Signal Processing) — A recently developed technology
that allows software to control digital electronic circuitry.
DTMF (Dual-Tone-Multi-Frequency) — Refers to DTMF signaling,
which is typically used in telephone systems.
E LAYER The mid-level of the ionosphere which reaches
maximum ionization at noon. It begins dissipating toward sunset
and reaches minimum activity at midnight. Irregular cloud-like
formations of ionized gases occasionally occur in the E layer.
EMI (Electromagnetic Interference) — An electromagnetic
disturbance that degrades communications performance .
Synonym: Radio Frequency Interference (RFI).
ENCRYPTION Process of translating information into an appar-ently
random message.
ERP (Effective Radiated Power) — Equivalent power transmitted
to the atmosphere, which is the product of the transmitter power
output multiplied by the gain of the antenna.
ERROR DETECTION An error correction technique that uses
binary code words to modify data messages by systematically
adding check bits to detect errors in received words.
EXCITER The part of the transmitter that generates the modu-lated
signal for a radio transmitter.
F LAYER The uppermost and most heavily ionized region of the
ionosphere. Important for long-haul communications, since this
layer remains ionized even after sunset.
FADING The variation of the amplitude and/or phase of a
received signal due to changes in the propagation path with time.
FEC (Forward Error Correction) — A system of error control for
data transmission whereby the receiver can correct any code
block that contains fewer than a fixed number of bits in error.
FM (Frequency Modulation) — A form of modulation where the
frequency of a carrier varies in proportion to an audio modulating
FOT (Frequency of Optimum Transmission) — The highest
frequency predicted to be available for sky wave transmission for
a given path and time for 85 percent of the maximum usable
frequency (MUF).
FREQUENCT The number of completed cycles per second of a
signal, measured in hertz (Hz).
FREQUENCY HOPPING The rapid switching (hopping) of radio
system frequency for both the receiver and transceiver from
frequency to frequency in apparently random patterns, using a
common timing reference.
FSK (Frequency Shift Keying) — A form of modulation in which a
digital signal shifts the output frequency between discrete
GAIN The ratio of the value of an output parameter, such as
power, to its input level. Usually expressed in decibels.
GROUND REFLECTED WAVE The portion of the propagated
wave that is reflected from the surface of the earth between the
transmitter and receiver.
GROUND WAVE A radio wave that is propagated over the
earth and ordinarily is affected by the presence of the ground.
HF (High Frequency) — Nominally, the band from 3 to 30 MHz; in
practice, the lower end of the HF band extends to 1.6 MHz.
Hz (Hertz) — Basic unit for frequency.
IF (Intermediate Frequency) — A frequency used within equip-ment
as an intermediate step in transmitting or receiving.
IMPEDANCE Opposition to current flow of a complex combina-tion
of resistance and reactance. Reactance is the opposition to
AC current flow by a capacitor or an inductor. An ideal antenna
coupler will act so as to cancel the reactive component of antenna
impedance, i.e., by providing an equal inductive reactance if the
antenna has a capacitive reactance or an equal capacitive reac-tance
if the antenna presents an inductive reactance.
INCIDENT ANGLE The angle at which sky waves enter the
INTERLEAVING A technique that increases the effectiveness of
FEC codes by randomizing the distribution of errors in communi-cation
channels characterized by error bursts.
IONCAP (Ionospheric Communications Analysis and Prediction)
— A popular and effective propagation prediction program that
predicts system performance at given times of day as a function
of frequency for a given HF path and a specified complement of
IONOSPHERE A region of electrically charged particles or gases
in the earth’s atmosphere extending from 50 to 600 kilometers
(approximately 30 to 375 miles) above the earth’s surface.
IONOSPHERIC SOUNDING An automated propagation predic-tion
ISB (Independent Sideband) — Double sideband transmission in
which the information carried by each sideband is different.
JAMMING Deliberate interference that results from transmis-sion
on operating frequencies with the intent to disrupt commu-nications.
KEK (Key Encryption Key) — Used in digital encryption.
KEY A variable that changes the mathematical algorithm in
KEY GENERATOR A device or process that generates the vari-able
for a cryptographic encoding system.
LOS (Line of Sight) — A term that refers to radio signal propaga-tion
in a straight line from the transmitter to a receiver without
refraction; generally extends to the visible horizon.
LPD (Low Probability of Detection) — Techniques for minimizing
the probability that the transmitted signal is detected by an
unauthorized party.
LPI (Low Probability of Intercept) — Techniques for minimizing
the likelihood of the intelligence on a transmitted signal being
recovered by an unauthorized party.
LQA (Link Quality Analysis) — A technique for real-time channel
evaluation in which radios measure and store values indicating
the relative quality of a radio link at different assigned frequencies.
LUF (Lowest Usable Frequency) — The lowest frequency in the
HF band at which the received field intensity is sufficient to
provide the required signal-to-noise ratio.
MAIN LOBE In an antenna radiation pattern, the lobe containing
the direction of maximum radiation intensity.
MFSK (Multi-tone Frequency Shift Keying).
MODEM(MOdulator-DEmodulator) — A device that modulates
and demodulates signals. The modem converts digital signals
into analog form for transmitting and converts the received
analog signals into digital form.
MODULATION The process, or result of the process, of varying
a characteristic of a carrier in accordance with a signal from an
information source.
MUF (Maximum Usable Frequency) — The upper limit for the
frequencies used at a specified time for radio transmission
between two points via ionospheric propagation.
MULTIPATH The propagation phenomenon that results in radio
signals reaching the receiving antenna by two or more paths.
MULTIPATH SPREAD The range of timed differences that it takes
for radio signals to reach the receiving antenna when they arrive
from several routes, which may include one or more sky wave
paths and/or a ground-wave path. The effect of multipath spread is
minimized by selecting a frequency as close as possible to the MUF.
NVIS (Near-Vertical Incidence Sky wave) — A technique for
transmitting over relatively short ranges by ionospheric refrac-tion
using very high incident angles.
OHM Unit of measurement of resistance. Its symbol is ?.
OMNIDIRECTIONAL ANTENNA An antenna whose pattern is
non-directional in azimuth.
ON-OFF KEYING Turning the carrier on or off with telegraph
key (Morse code). Same as CW.
OTAR (Over-The-Air-Rekeying) — This technique developed by
Harris eliminates the need for manual loading of encryption keys
and provides a more secure method of key management.
PARALLEL TONE MODEM Carries information on simulta-neous
audio tones, where each tone is modulated at a low-keying
PHASE In a periodic process such as a radio wave, any possible
distinguishable state of the wave.
POLARIZATION The orientation of a wave relative to a refer-ence
plane. Usually expressed as horizontal or vertical in radio
wave terminology.
POWER AMPLIFIER The part of the transmitter that boosts the
output power of the radio signal to the desired wattage before
sending it to the transmitting antenna.
PREAMBLE A known sequence of bits sent at the start of a
message which the receiver uses to synchronize to its internal clock.
PROPAGATION The movement of radio frequency energy
through the atmosphere.
PUBLIC KEY CRYPTOGRAPHY A type of key management
system used in the commercial sector. Under this standard, each
user generates two keys, a public key and a private key. The
strength of such a system lies in the difficulty of deriving the
private key from the public key.
RADIATION PATTERN Pattern determined by an antenna’s
design and strongly influenced by its location with respect to the
ground. Radiation patterns are frequency dependent.
RAU (Remote Access Unit).
REFRACTION The bending of a radio wave as it passes
obliquely from one medium to another.
RMS (Root Mean Square).
RTCE (Real-Time Channel Evaluation) — Techniques used to
select frequencies, adjust data rates, or change modulation
schemes in adaptive radio systems.
SATCOM (Satellite Communications).
SCRAMBLING A COMSEC technique that involves separating
the voice signal into a number of bands, shifting each band to a
different audio frequency range, and combining the resulting bands
into a composite audio output that modulates the transmitter.
SERIAL TONE MODEM Carries digital information on a single
audio tone.
SHORT WAVE Radio frequencies above 3 MHz.
SID (Sudden Ionospheric Disturbance) — Abnormally high
ionization densities caused by solar flares, resulting in a sudden
increase in radio wave absorption.
SIDEBAND The spectral energy, distributed above or below a
carrier, resulting from a modulation process.
SKY WAVE A radio wave that is reflected by the ionosphere.
SNR (Signal-to-Noise Ratio) — The ratio of the power in the
desired signal to that of noise in a specified bandwidth.
SOFT-DECISION DECODING An error-correction technique
where a group of detected symbols that retain their analog char-acter
are compared against the set of possible transmitted code
words. A weighing factor is applied to each symbol in the code
word before a decision is made about which code word was
SPORADIC E Layer found in the E Layer of the ionosphere.
Supports propagation of sky waves at the upper end of the HF
band and beyond.
SPREAD SPECTRUM A technique used to overcome deliberate
radio communications interference, in which the modulated
information is transmitted in a bandwidth considerably greater
than the frequency content of the original information.
SSB (Single Sideband) — A modulation technique in which the
carrier and one sideband (upper or lower) are suppressed so that
all power is concentrated in the other sideband.
STORE AND FORWARD A technique where information is
stored until a communication link is established and then sent.
SUNSPOT CYCLE Eleven-year cycle of sunspots which
generate bursts of radiation that increase levels of ionization.
SURFACE WAVES Travel along the surface of the earth and
may reach beyond the horizon.
SYMMETRIC KEY SYSTEM A key management system in
which the same key encrypts and decrypts data.
SYNCHRONOUS A form of data communications that uses a
preamble to alert the data receiver that a message is coming and
to allow it to synchronize to an internal bit clock.
TAKE-OFF ANGLE The angle between the axis of the main
lobe of an antenna pattern and the horizontal plane at the
transmitting antenna.
TEK (Traffic Encryption Key) — Used in digital encryption.
TIU (Telephone Interface Unit).
TRAFFIC The information moved over a communications
TRANSCEIVER Equipment using common circuits in order to
provide transmitting and receiving capability.
TRANSEC (Transmission Security) — Techniques that prevent
signal detection or jamming of the transmission path.
VERTICAL WHIP ANTENNA An omnidirectional antenna that
has low take-off angles and vertical polarity.
VOCODER A device that converts sounds into a data stream
that can be sent over an HF channel. Short for voice coder-decoder.
WAVELENGTH Distance between point on loop of wave to
corresponding point on adjacent wave.