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antenna ground system The earth, counterpoise,
guy wires, radials, and/or various conducting obantenna
detector • antenna pattern 33
antenna detector A circuit that warns aircraft
personnel that they are being observed by radar.
It picks up the radar pulses and actuates a warning
light or other device.
antenna diplexer A coupling device that permits
several transmitters to share one antenna without
troublesome interaction. Compare ANTENNA
DUPLEXER.
antenna directivity The directional characteristics
of a transmitting or receiving antenna, usually
expressed qualitatively (e.g., omnidirectional,
bidirectional, or unidirectional). A more precise
expression is ANTENNA BEAMWIDTH.
antenna director In a directional antenna, a PAR-
ASITIC ELEMENT situated in front of the radiator
and separated from it by an appropriate fraction
of a wavelength. Its function is to intensify radiation
in the direction of transmission. Compare
ANTENNA RADIATOR and ANTENNA REFLEC-
TOR.
antenna duplexer A circuit or device permitting
one antenna to be shared by two transmitters
without undesirable interaction.
antenna effect The tendency of wires or metallic
bodies to act as antennas (i.e., to radiate or receive
radio waves).
antenna efficiency The ratio of radio-frequency
energy supplied to a wireless transmitting antenna,
to the energy radiated into space. Electrically,
the radiation resistance of the antenna (R R )
appears in series with loss resistance (R L ). The efficiency
Eff of the antenna can be determined by
the following formula:
Eff = R R /(R R + R L )
As a percentage,
Eff % = 100 (R R /(R R + R L )
The efficiency is always less than 1 (100 percent)
because, in practice, the loss resistance can
never be reduced to zero.
antenna factor A factor (in decibels) added to an
RF voltmeter reading to find the true open-circuit
voltage induced in an antenna.
antenna field The electromagnetic field immediately
surrounding an antenna.
antennafier Low-profile antenna/amplifier device,
sometimes used with portable communications
systems. Also called an active antenna.
antenna front-to-back ratio For a directional antenna,
the ratio of field strength in front of the antenna
(i.e., directly forward in the line of
maximum directivity) to field strength in back of
the antenna (i.e., 180 degrees from the front), as
measured at a fixed distance from the radiator. It
is usually specified in decibels.
antenna gain For a given antenna, the ratio of signal
strength (received or transmitted) to that obtained
with a comparison antenna, such as a
simple dipole. Generally specified in decibels.
jects in the vicinity of an antenna which, taken
together, form the radio-frequency (RF) ground
system against which the antenna operates.
Some antennas require an extensive ground system
to function efficiently; others need no ground
system.
antenna/ground system An arrangement embodying
both an antenna and a low-resistance
connection to the earth, as opposed to an antenna
system that involves no connection to
earth.
antenna height 1. The height of an antenna above
the surface of the earth immediately beneath the
driven element(s). 2. The height of an antenna
above the effective radio-frequency (RF) ground
immediately beneath the driven element(s). 3.
The height of an antenna above average terrain,
determined against the mean altitude of a number
of points on the earth’s surface that lie within
a certain radius of the antenna structure. Also
called height above average terrain (HAAT).
antenna impedance The complex-number impedance
that an antenna presents to a transmission
line. It can vary over a tremendous range,
and depends on the antenna type, antenna size,
antenna height, operating frequency, and various
other factors.
antenna-induced potential Also called antennainduced
microvolts. The voltage across the opencircuited
terminals of an antenna.
antenna lens Also called lens antenna. A radiator
designed to focus microwave energy in much the
same manner that an optical lens focuses light
rays. Lens antennas are made from dielectric materials
and/or metals.
antenna loading 1. The insertion of inductance in
antenna elements to lower the resonant frequency
of the system without necessarily making
the system physically larger or the elements
longer. 2. The insertion of capacitance in antenna
elements to raise the resonant frequency of the
system without necessarily making the system
physically smaller or the elements shorter.
antenna lobe A well-defined region in the radiation
pattern of an antenna in which radiation is most
intense, or in which reception is strongest. Also
see ANTENNA PATTERN.
antenna matching The technique of establishing a
satisfactory relationship between the antenna
impedance and the transmission-line or transmitter-output
impedance, for maximum transfer
of power into the antenna. Also, the matching
of antenna impedance to receiver-input impedance,
for delivery of maximum energy to the
receiver.
antennamitter An antenna/oscillator combination
that serves as a low-power transmitter.
antenna pattern A polar plot of antenna performance
that shows field strength versus angle of
azimuth, with the antenna at the center. It is
usually specified in the horizontal plane.