Page 48 - The Illustrated Dictionary of Electronics

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antenna detector • antenna pattern 33

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