Page 49 - The Illustrated Dictionary of Electronics

5059F-pA_1-55 4/9/01 4:41 PM Page 34

34 antenna polarization • anthropomorphism

antenna range 1. The frequency band, communi-
cation distance characteristically covered, or
other continuum of values that specify the oper-
ating limits of an antenna. 2. The region immedi-
ately surrounding an antenna in which tests and
measurements usually are made. Sometimes
called ANTENNA FIELD.
antenna reflector In a directional antenna, a PAR-
ASITIC ELEMENT situated behind the radiator
and separated from the latter by an appropriate
fraction of a wavelength. Its function is to inten-
sify radiation in the direction of transmission.
Compare ANTENNA DIRECTOR and ANTENNA
RADIATOR.
antenna relay In a radio station, a low-loss, heavy-
duty relay that enables the antenna to be
switched between transmitter and receiver.
antenna polarization The orientation of electric antenna resistance The resistive component of
lines of flux, with respect to the surface of the ANTENNA IMPEDANCE.
earth, for which an antenna is most efficient. A antenna resonant frequency The frequency, or
vertical antenna radiates and receives vertically narrow band of frequencies, at which an an-
polarized waves. A horizontal antenna radiates tenna’s impedance appears resistive.
and receives horizontally polarized waves broad- antenna stage 1. The first RF amplifier stage of a
side to itself, and vertically polarized waves at receiver. 2. Occasionally, the final RF amplifier of
high elevation angles off its ends. In other direc- a transmitter.
tions, the polarization is slanted at various an- antenna switch In a radio station, a low-loss,
gles. heavy-duty switch that enables the antenna to be
antenna power Symbol, P ant. The RF power devel- connected to transmitter, receiver, or safety
oped in an antenna by a transmitter; P ant equals ground.
2
I R, where I is the antenna current and R is the
antenna resistance at point I is measured.
antenna power gain The ratio of the maximum ef-
fective radiated power (ERP) from a wireless
transmitting antenna to the ERP from a reference
antenna, expressed in decibels (dB). If the ERP
from an antenna under test is P T watts and the
ERP from the reference antenna is P R watts, then
the gain G dB is:
G dB = 10 log 10 (P T/P R)
Power gain is always measured in the direction in
which the test antenna performs the best. The
reference antenna, usually a dipole, is chosen antenna system Collectively, an antenna and all of
with a gain assumed to be unity, or 0 dB. Gain the auxiliary electrical and mechanical devices
relative to a dipole is expressed in dBd (decibels needed for its efficient operation, including cou-
relative to a dipole). Alternatively, the reference plers, tuners, transmission lines, supports, insu-
antenna can be an isotropic radiator, in which lators, and rotator.
case the gain is expressed in dBi (decibels relative antenna terminals 1. The points at which a trans-
to an isotropic radiator). Gain figures in dBd and mission line is attached to an antenna. 2. The sig-
dBi differ by a constant amount as follows: nal input terminals of a receiver. 3. The signal
output terminals of a transmitter.
G dBi = 2.15 + G dBd antennaverter An antenna and converter com-
antenna preamplifier A highly sensitive amplifier bined into a single circuit, intended for connec-
used to enhance the gain of a receiver. It is usu- tion to the antenna terminals of a receiver to
ally used at the very high frequencies and above. allow operation on frequencies outside the band
antenna radiation The propagation of radio waves for which the receiver has been designed.
by a transmitting antenna. antenna wire 1. The radiator element of a wire-
antenna radiator The element of an antenna that type antenna. 2. A strong solid or stranded wire
receives RF energy from the transmitter and radi- (e.g., hard-drawn copper, copper-clad steel, or
ates waves into space. Also known as the driven phosphor-bronze) used for antennas.
element. Compare ANTENNA DIRECTOR and AN- anthropomorphism The perception, by people, of
TENNA REFLECTOR. machines as having human qualities. This can