(1) Absorbing light waves. Does not allow any reflection or refraction;
(2) Atmospheric absorption of rf energy with no reflection or refraction
(adversely affects long distance communications).
A conductor or set of conductors used either to radiate rf energy into
space or to collect rf energy from space.
The angle between the incident wave and the normal.
The angle between the reflected wave and the normal.
The angle between the normal and the path of a wave through the second
A horizontal, long wire antenna designed for reception and transmission
of low-frequency, vertically polarized ground waves. Also known as WAVE
An array that radiates in opposite directions along the line of maximum
An array in which the direction of maximum radiation is perpendicular
to the plane containing the elements.
Connecting the centre of an antenna to a transmission line, which is then
connected to the final (output) stage of the transmitter. Also known as
The ratio of voltage to current at any given point on a transmission line.
Represented by a value of impedance.
A type of transmission line that contains two concentric conductors.
An array with all the elements in a straight line. Maximum radiation is
perpendicular to the axis of the elements.
An array system that uses the characteristics of more than one array.
Also known as ARRAY OF ARRAYS.
A network of wire that is connected to a quarter-wave antenna at one end
and provides the equivalent of an additional ¼ wavelength.
The maximum frequency at which a radio wave can be transmitted vertically
and still be refracted back to earth.
STANDING WAVE RATIO (ISWR)
The ratio of maximum to minimum current along a transmission line.
The frequency at which the attenuation of a waveguide increases sharply
and below which a travelling wave in a given mode cannot be maintained.
A frequency with a half wavelength that is greater than the wide dimension
of a waveguide.
One complete alternation of a sine wave that has a maximum value above
and a maximum value below the reference line.
Reduction of energy by absorption.
The heating of an insulating material by placing it in a high frequency
The losses resulting from the heating effect on the dielectric material
The ratio of a given dielectric to the dielectric value of a vacuum.
The bending of the paths of waves when the waves meet some form of obstruction.
A common type of half-wave antenna made from a straight piece of wire
cut in half. Each half operates at a quarter wavelength of the output.
Radiation that varies with direction.
The parasitic element of an array that reinforces energy coming from the
driver toward itself.
The property of an array that causes more radiation to take place in certain
directions than in others.
The constants of inductance, capacitance, and resistance in a transmission
line. The constants are spread along the entire length of the line and
cannot be distinguished separately.
Another name for the dipole antenna.
An array in which all of the elements are driven. Also known as CONNECTED
An element of an antenna (transmitting or receiving) that is connected
directly to the transmission line.
A device used at the end of a transmission line or waveguide to convert
transmitted energy into heat so no energy is radiated outward or reflected
The combination of an electric (E) field and a magnetic (H) field.
Manmade or natural interference that degrades the quality of reception
of radio waves.
The radiation of radio waves into space.
A part of an antenna that can be either an active radiator or a parasitic
Connecting one end of an antenna through a capacitor to the final output
stage of a transmitter. Also known as VOLTAGE-FED.
An array in which the direction of radiation is parallel to the axis of
The angle between the line of sight to an object and the horizontal plane.
Variations in signal strength by atmospheric conditions.
A transmission line that carries energy to the antenna.
A transmission line that has no standing waves. This line requires no
special tuning device to transfer maximum power.
coaxial line made with a flexible inner conductor insulated from the outer
conductor by a solid, continuous insulating material.
An ordinary half-wave antenna (dipole) that has one or more additional
conductors connected across the ends parallel to each other.
An array with three parasitic elements and one driven element.
The loss of energy of a radio wave because of the spreading of the wavefront
as it travels from the transmitter.
The number of cycles that occur in one second. Usually expressed in Hertz.
The ratio of the energy radiated in the principal direction to the energy
radiated in the opposite direction.
The basic frequency or first harmonic frequency.
The ratio between the amount of energy propagated from an antenna that
is directional to the energy from the same antenna that would be propagated
if the antenna were not directional.
The portion of a ground plane antenna that acts as ground.
A type of antenna that uses a ground plane as a simulated ground to produce
The loss of rf energy each time a radio wave is reflected from the earth’s
A series of conductors buried below the surface of the earth and arranged
in a radial pattern. Used to reduce losses in the ground.
Radio waves that travel near the surface of the earth.
The forward progress velocity of a wave front in a waveguide.
An antenna consisting of two rods (¼ wavelength h) in a straight
line, that radiates electromagnetic energy.
A frequency that is a whole number multiple of a smaller base frequency.
A half-wave antenna installed some distance above ground and positioned
either vertically or horizontally.
On a graph, the straight line axis plotted from left to right.
The part of a radiation pattern that is radiated in all directions along
the horizontal plane.
Waves that are radiated with their E-field component parallel to the earth’s
(1) The wave that strikes the surface of a medium; (2) The wave that travels
from the sending end to the receiving end of a transmission line.
The electromagnetic field produced about an antenna when current and voltage
are present on the same antenna.
The losses that occur when the electromagnetic field around a conductor
cuts through a nearby metallic object and induces a current into that
The impedance presented to the transmitter by the transmission line and
Any disturbance that produces an undesirable response or degrades a wave.
The most important region of the atmosphere extending from 31 miles to
250 miles above the earth. Contains four cloud-like layers that affect
Disturbances in the earth’s magnetic field that make communications
practical only at lower frequencies.
The process of upsetting electrical neutrality.
The small amount of current that flows between the conductors of a
transmission line through the dielectric.
See LUMPED-IMPEDANCE TUNING.
An area of a radiation pattern plotted on a polar-coordinate graph that
represents maximum radiation.
An antenna that is a wavelength or more long at its operating frequency.
Waves in which the disturbance (back and forth motion) takes place in
the direction of propagation. Sometimes called compression waves.
(1) The curves of a standing wave or antenna that represent amplitude
of current or voltage; (2) A curved conductor that connects the ends of
a coaxial cable or other transmission line and projects into a waveguide
or resonant cavity for the purpose of injecting or extracting energy.
The minimum operating frequency that can be used for communications between
The properties of inductance, capacitance, and resistance in a transmission
The insertion of an inductor or capacitor in series with an antenna to
lengthen or shorten the antenna electrically. Also known as LOADING.
Inefficient coupling of energy from one circuit to another that is desirable
in some applications. Also called weak coupling.
The lobe in which the greatest amount of radiation occurs.
A quarter-wave antenna oriented perpendicular to the earth and operated
with one end grounded. Also known as QUARTER-WAVE ANTENNA.
Maximum frequency that can be used for communications between two locations
for a given time of day and a given angle of incidence.
The substance through which a wave travels from one point to the next.
Air, water, wood, etc., are examples of a medium.
A shorted quarter-wave section of transmission line.
The lobe in which the radiation intensity is less than a major lobe.
An array consisting of one or more arrays and classified as to directivity.
ELEMENT PARASITIC ARRAY
An array that contains two or more parasitic elements and a driven element.
The multiple paths a radio wave may follow between transmitter and receiver.
The fixed minimum points of voltage or current on a standing wave or antenna.
A transmission line that has no standing waves of current or voltage.
On a polar-coordinate graph, the area that represents minimum or 0 radiation.
Transmitting in all directions. Also known as NONDIRECTIONAL.
A transmission line that has an infinitely large terminating impedance.
The most practical operating frequency that can be used with the least
amount of problems; roughly 85 percent of the maximum usable frequency.
The impedance presented to the load by the transmission line and its source.
A circuit that acts as a high impedance at resonance.
A type of transmission line consisting of two parallel wires.
An array that has one or more parasitic elements.
The passive element of an antenna array that is connected to neither the
transmission line nor the driven element.
The plane (vertical or horizontal) with respect to the earth in which
the E-field propagates.
The ratio of the radiated power of an antenna compared to the output power
of a standard antenna. A measure of antenna efficiency usually expressed
in decibels. Also referred to as POWER RATIO.
The heat loss in current flows through it. a conductor as
STANDING WAVE RATIO (PSWR)
The ratio of the square of the maximum and minimum voltages of a transmission
Waves travelling through a medium.
See MARCONI ANTENNA.
The electromagnetic field that detaches itself from an antenna and travels
The losses that occur when magnetic lines of force about a conductor are
projected into space as radiation and are not returned to the conductor
as the cycle alternates.
A plot of the radiated energy from an antenna.
The resistance, which if inserted in place of an antenna, would consume
the same amount of power as that radiated by the antenna.
Electromagnetic frequencies that fall between 3 kilohertz and 300 gigahertz
and are used for radio communications.
The boundary beyond the natural horizon in which radio waves cannot be
propagated over the earth’s surface.
(1) A form of radiant energy that can neither be seen nor felt; (2) An
electromagnetic wave generated by a transmitter.
A longitudinal wave that has been expanded or rarefied (made less dense)
as it moves away from the source.
The object that responds to a wave or disturbance. Same as detector.
The device used to pick up an rf signal from space.
(1) The wave that reflects back from a medium; (2) Waves travelling from
the load back to the generator on a transmission line; (3) The wave moving
back to the sending end of a transmission line after reflection has occurred.
Waves that are neither transmitted nor absorbed, but are reflected from
the surface of the medium they encounter.
The parasitic element of an array that causes maximum energy radiation
in a direction toward the driven element.
The changing of direction as a wave leaves one medium and enters another
medium of a different density.
The reception and re-transmission of radio waves caused by turbulence
in the troposphere.
The condition produced when the frequency of vibrations are the same as
the natural frequency (of a cavity), The vibrations reinforce each other.
A transmission line that has standing waves of current and voltage.
A diamond-shaped antenna used widely for long-distance, high-frequency
transmission and reception.
A coaxial line consisting of a central, insulated wire (inner conductor)
mounted inside a tubular outer conductor.
The phenomenon caused by the expanding and collapsing fields of an electron
that encircles other electrons and retards the movement of the encircled
A circuit that acts as a low impedance at resonance.
A line consisting of parallel conductors separated from each other and
surrounded by a solid dielectric.
A transmission line that has a terminating impedance equal to 0.
The tendency for alternating current to concentrate in the surface layer
of a conductor. The effect increases with frequency and serves to increase
the effective resistance of the conductor.
The distance from a transmitter to the point where the sky wave is first
returned to earth.
A zone of silence between the point where the ground wave becomes too
weak for reception and the point where the sky wave is first returned
Radio waves reflected back to earth from the ionosphere.
(1) The object that produces waves or disturbance; (2) The name given
to the end of a two-wire transmission line that is connected to a source.
A radio wave that travels directly from the transmitter to the receiver
and remains in the troposphere.
(1) The entire range of electromagnetic waves; (2) VISIBLE. The range
of electromagnetic waves that stimulate the sense of sight; (3) ELECTROMAGNETIC.
The entire range of electromagnetic waves arranged in order of their frequencies.
Irregular cloud-like patches of unusually high ionization. Often forms
at heights near the normal E-layer.
Insulator used with transmission lines and antennas to keep the parallel
The distribution of voltage and current formed by the incident and reflected
waves, which have minimum and maximum points on a resultant wave that
appears to stand still.
The ratio of the maximum to the minimum amplitudes of corresponding components
of a field, voltage, or current along a transmission line or waveguide
in the direction of propagation measured at a given frequency. Measures
the perfection of the termination of the line.
Located between the troposphere and the ionosphere. Has little effect
on radio waves.
Short section of a transmission line used to match the impedance of a
transmission line to an antenna. Can also be used to produce desired phase
relationships between connected elements of an antenna.
An irregular ionospheric disturbance that can totally blank out hf radio
A radio wave that travels along the contours of the earth, thereby being
An array with two parasitic elements (reflector and director) and a driven
A device designed to guide electrical energy from one point to another.
The device used to send the transmitted signal energy into space.
The various types of lines and waveguides used as transmission lines.
The portion of the atmosphere closest to the earth’s surface, where
all weather phenomena take place.
The propagation of radio waves in the troposphere by means of scatter.
The peak of the negative alternation (maximum value below the line).
Another name for the resonant line. This line uses tuning devices to eliminate
the reactance and to transfer maximum power from the source to the line.
A type of antenna used in VHF communications that is omnidirectional and
consists of two horizontal half-wave antennas mounted at right angles
to each other in the same horizontal plane.
A line consisting of two insulated wires twisted together to form a flexible
line without the use of spacers.
A parallel line consisting of two wires that are generally spaced from
2 to 6 inches apart by insulating spacers.
RIBBON (TWIN LEAD)
A parallel line similar to a two-wire open line except that uniform spacing
is assured by embedding the two wires in a low-loss dielectric.
An array that radiates in only one general direction.
Another name for the flat or nonresonant line.
A bidirectional antenna, shaped like a V, which is widely used for communications.
The rate at which a disturbance travels through a medium.
The part of a radiation pattern that is radiated in the vertical plane.
An imaginary plane that is perpendicular to the horizontal plane.
Waves radiated with the E-field component perpendicular to the earth’s
STANDING WAVE RATIO (VSWR)
The ratio of maximum to minimum voltage of a transmission line.
See BEVERAGE ANTENNA.
A recurring disturbance advancing through space with or without the use
of a physical medium.
A continuous series of waves with the same amplitude and wavelength.
A small section of an expanding sphere of electromagnetic radiation, perpendicular
to the direction of travel of the energy.
(1) The distance in space occupied by 1 cycle of a radio wave at any given
instant; (2) The distance a disturbance travels during one period of vibration.
A multi-element parasitic array. Elements lie in the same plane as those
of the end-fire array.