Antenna terminology - For reference only.
The
purpose of this technical brief is to provide introductory insights into
wireless antennas and their characteristics. The definitions in quotation
marks are taken from IEEE Standard Definitions of Terms for Antennas,
IEEE Std 145-1983.
Antenna:
"That part of a transmitting or receiving system which is designed
to radiate or to receive electromagnetic waves". An antenna can also
be viewed as a transitional structure (transducer) between free-space
and a transmission line (such as a coaxial line). An important property
of an antenna is the ability to focus and shape the radiated power in
space e.g.: it enhances the power in some wanted directions and suppresses
the power in other directions.
Frequency
bandwidth: "The range of frequencies within which the performance
of the antenna, with respect to some characteristics, conforms to a specified
standard". VSWR of an antenna is the main bandwidth limiting factor.
Input impedance: " The impedance presented by an antenna at
its terminals". The input impedance is a complex function of frequency
with real and imaginary parts. The input impedance is graphically displayed
using a Smith chart.
Reflection
coefficient: The ratio of the voltages corresponding to the reflected
and incident waves at the antenna's input terminal (normalized to some
impedance Z0). The return loss is related to the input impedance Zin and
the characteristic impedance Z0 of the connecting feed line by: Gin =
(Zin - Z0) / (Zin+Z0).
Voltage
standing wave ratio (VSWR): The ratio of the maximum/minimum values
of standing wave pattern along a transmission line to which a load is
connected. VSWR value ranges from 1 (matched load) to infinity for a short
or an open load. For most base station antennas the maximum acceptable
value of VSWR is 1.5. VSWR is related to the reflection coefficient Gin
by: VSWR= (1+|Gin|)/(1-| Gin |).
Isolation:
"A measure of power transfer from one antenna to another". This
is also the ratio of the power input to one antenna to the power received
by the other antenna, expressed in decibel (dB). The same definition is
applicable to two-port antennas such as dual-polarization antennas.
Far-field
region: "That region of the field of an antenna where the angular
field distribution is essentially independent of the distance from a specified
point in the antenna region". The radiation pattern is measured in
the far field.
Antenna
polarization: "In a specified direction from an antenna and at
a point in its far field, is the polarization of the (locally) plane wave
which is used to represent the radiated wave at that point". "At
any point in the far-field of an antenna the radiated wave can be represented
by a plane wave whose electric field strength is the same as that of the
wave and whose direction of propagation is in the radial direction from
the antenna. As the radial distance approaches infinity, the radius of
curvature of the radiated wave's phase front also approaches infinity
and thus in any specified direction the wave appears locally a plane wave".
In practice, polarization of the radiated energy varies with the direction
from the center of the antenna so that different parts of the pattern
and different side lobes sometimes have different polarization. The polarization
of a radiated wave can be linear or elliptical (with circular being a
special case).
Co-polarization:
"That polarization which the antenna is intended to radiate".
Cross-polarization:
"In a specified plane containing the reference polarization ellipse,
the polarization orthogonal to a specified reference polarization".
The reference polarization is usually the co-polarization.
Antenna
pattern: The antenna pattern is a graphical representation in three
dimensions of the radiation of the antenna as a function of angular direction.
Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes (such as E-Plane and H-plane or vertical and
horizontal planes). The pattern is usually plotted either in polar or
rectangular coordinates. The pattern of most base station antennas contains
a main lobe and several minor lobes, termed side lobes. A side lobe occurring
in space in the direction opposite to the main lobe is called back lobe.
Normalized
pattern: Normalizing the power/field with respect to its maximum value
yields a normalized power/field pattern with a maximum value of unity
(or 0 dB).
Gain
pattern: Normalizing the power/field to that of a reference antenna
yields a gain pattern. When the reference is an isotropic antenna, the
gain is expressed in dBi. When the reference is a half-wave dipole in
free space, the gain is expressed in dBd.
Radiation
efficiency: "The ratio of the total power radiated by an antenna
to the net power accepted by the antenna from the connected transmitter".
E-plane:
"For a linearly polarized antenna, the plane containing the electric
field vector and the direction of maximum radiation". For base station
antenna, the E-plane usually coincides with the vertical plane.
H-plane:
"For a linearly polarized antenna, the plane containing the magnetic
field vector and the direction of maximum radiation". For base station
antenna, the H-plane usually coincides with the horizontal plane.
Front-to-back
ratio: "The ratio of the maximum directivity of an antenna to
its directivity in a specified rearward direction". Sometimes the
directivity in the rearward direction is taken as the average over an
angular region.
Major/main
lobe: "The radiation lobe containing the direction of maximum
radiation". For most practical antenna there is only one main beam.
Side
lobe level: Is the ratio, in decibels (dB), of the amplitude at the
peak of the main lobe to the amplitude at the peak of a side lobe.
Half-power
beamwidth: " In a radiation pattern cut containing the direction
of the maximum of a lobe, the angle between the two directions in which
the radiation intensity is one-half the maximum value". The Half-power
beamwidth is also commonly referred to as the 3-dB beamwidth.
Antenna
directivity: The directivity of an antenna is given by the ratio of
the maximum radiation intensity (power per unit solid angle) to the average
radiation intensity (averaged over a sphere). The directivity of any source,
other than isotropic, is always greater than unity.
Antenna
gain: The maximum gain of an antenna is simply defined as the product
of the directivity by efficiency. If the efficiency is not 100 percent,
the gain is less than the directivity. When the reference is a loss less
isotropic antenna, the gain is expressed in dBi. When the reference is
a half wave dipole antenna, the gain is expressed in dBd (1 dBd = 2.15
dBi ).
Antenna
efficiency: The total antenna efficiency accounts for the following
losses: (1) reflection because of mismatch between the feeding transmission
line and the antenna and (2) the conductor and dielectric losses.
Effective radiated power (ERP): "In a given direction, the
relative gain of a transmitting antenna with respect to the maximum directivity
of a half-wave dipole multiplied by the net power accepted by the antenna
from the connected transmitter".
Power
handling: Is the ability of an antenna to handle high power without
failure. High power in antenna can cause voltage breakdown and excessive
heat (due to conductor and dielectric antenna losses) which would results
in an antenna failure.
Passive
intermodulation (PIM): As in active devices, passive intermodulation
occurs when signals at two or more frequencies mix with each other in
a non-linear manner to produce spurious signals. PIM is caused by a multitude
of factors present in the RF signal path. These include poor mechanical
contact, presence of ferrous contents in connectors and metals, and contact
between two galvanically unmatched metals. PIM spurious signal, which
falls in the up link band, can degrade call quality and reduce the capacity
of a wireless system.
Side
lobe suppression: "Any process, action or adjustment to reduce
the level of the side lobes or to reduce the degradation of the intended
antenna system performance resulting from the presence of side lobes".
For base station antenna, the first side lobe above the horizon is preferred
to be low in order to reduce interference to adjacent cell sites. At the
other hand, the side lobes below the horizon are preferred to be high
for better coverage.
Null
filling: Is the process to fill the null in the antenna radiation
pattern to avoid blind spots in a cell site coverage.
Isotropic
radiator: "A hypothetical, loss less antenna having equal radiation
intensity in all direction". For based station antenna, the gain
in dBi is referenced to that of an isotropic antenna (which is 0 dB).
Omnidirectional
antenna: "An antenna having an essentially non-directional pattern
in a given plane of the antenna and a directional pattern in any orthogonal
plane". For base station antennas, the omnidirectional plane is the
horizontal plane.
Directional
antenna: "An antenna having the property of radiating or receiving
electromagnetic waves more effectively in some directions than others".
Half-wave
dipole: "A wire antenna consisting of two straight collinear
conductors of equal length, separated by a small feeding gap, with each
conductor approximately a quarter-wave length long".
Log-periodic
antenna: "Any one of a class of antennas having a structural
geometry such that its impedance and radiation characteristics repeat
periodically as the logarithm of frequency".
Microstrip
antenna: "An antenna which consists of a thin metallic conductor
bonded to a thin grounded dielectric substrate". An example of such
antennas is the microstrip patch.
Linear
array: A set of radiating elements (e.g. dipole or patch) arranged
along a line. Radiating elements such as dipole and patch have dimensions
comparable to a wavelength. A linear array has a higher gain, than a single
radiator, and its radiation pattern can be synthesized to meet various
antenna performance requirements such as upper side lobe suppression and
null fill. It should be noted that the gain of any antenna is proportional
to its size.
Coaxial
antenna: "An antenna comprised of a extension to the inner conductor
of a coaxial line and a radiating sleeve which in effect is formed by
folding back the outer conductor of the coaxial line".
Collinear
array antenna: "A linear array of radiating elements, usually
dipoles, with their axes lying in a straight line".
Adaptive
(smart) antenna: "An antenna system having circuit elements associated
with its radiating elements such that one or more of the antenna properties
are controlled by the received signal".
