| Antenna: An antenna is a
device that transmits and/or receives electromagnetic waves.
Electromagnetic waves are often referred to as radio waves. Most
antennas are resonant devices, which operate efficiently over a
relatively narrow frequency band. An antenna must be tuned to
the same frequency band that the radio system to which it is
connected operates in, otherwise reception and/or transmission
will be impaired. |
| Antenna Factor: The Antenna
Factor (AF) is defined as the ratio of the incident Electromagnetic
Field to the output voltage from the antenna. |
| Beamwidth: Depending on the
radio system in which an antenna is being employed there can be
many definitions of beamwidth. A common definition is the half
power beamwidth. The peak radiation intensity is found and then
the points on either side of the peak represent half the power of
the peak intensity are located. The angular distance between the
half power points traveling through the peak is the beamwidth.
Half the power is —3dB, so the half power beamwidth is sometimes
referred to as the 3dB beamwidth. |
| Directivity: It is a measure
of how focused an antenna coverage pattern is in a given direction.
A theoretical loss-less antenna element, referred to as a isotropic
element, has 0 dBi directive gain equally distributed in all 3
dimensions. In order to achieve higher directive gain, antennas
are normally designed to focus or concentrate the antenna pattern
only in the direction of the radio link, thereby maximizing energy
usage. |
| Electric Field (E-Field): An
electric field or E-field is an effect produced by an electric charge
that exerts a force on charged objects in its vicinity. |
| Far-Field: The far field
corresponds to an RF source-to-measurement antenna distance great
enough that energy radiates from the source only in a radial direction.
The E and H fields are mutually perpendicular to that direction and
each other, and their ratio is 377 W,
the impedance of free space. |
| Gain: The ratio of the signal,
usually expressed in dB, received or transmitted by a given antenna
as compared to an isotropic or dipole antenna. Antenna gain can only
be achieved by making an antenna directional, that is, with better
performance in one direction than in others. |
| Impedance: As the electric wave
travels through the different parts of the antenna system (radio, feed
line, antenna, free space) it may encounter differences in impedance.
At each interface, some fraction of the wave's energy will reflect back
to the source, forming a standing wave in the feed line. The ratio of
maximum power to minimum power in the wave can be measured and is called
the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is
considered to be marginally acceptable in low power applications where
power loss is more critical, although an SWR as high as 6:1 may still be
usable with the right equipment. Minimizing impedance differences at each
interface will reduce SWR and maximize power transfer through each part of
the antenna system. |
| Magnetic Field (H-Field): The
measured intensity of a magnetic field at a specific point. Usually
expressed in amperes/meter. |
| Near-Field: The close-in region
of an antenna where the angular field distribution is dependent upon
the distance from the antenna. |
Polarization: Polarization is
defined as the orientation of the electric field of an electromagnetic
wave. Polarization is in general described by an ellipse. Two often
used special cases of elliptical polarization are linear polarization
and circular polarization. The initial polarization of a radio wave is
determined by the antenna that launches the waves into space. The
environment through which the radio wave passes on its way from the
transmit antenna to the receive antenna may cause a change in
polarization.
With linear polarization the electric field vector stays in the same plane.
In circular polarization the electric field vector appears to be rotating
with circular motion about the direction of propagation, making one full
turn for each RF cycle. The rotation may be right-hand or left-hand.
Choice of polarization is one of the design choices available to the RF
system designer. For example, low frequency (< 1 MHz) vertically polarized
radio waves propagate much more successfully near the earth than horizontally
polarized radio waves, because horizontally polarized waves will be cancelled
out by reflections from the earth. Mobile radio systems waves generally are
vertically polarized. TV broadcasting has adopted horizontal polarization as
a standard. This choice was made to maximize signal-to-noise ratios. At
frequencies above 1 GHz, there is little basis for a choice of horizontal or
vertical polarization, although in specific applications, there may be some
possible advantage in one or the other. Circular polarization has also been
found to be of advantage in some microwave radar applications to minimize the
"clutter" echoes received from raindrops, in relation to the echoes from
larger targets such as aircraft. Circular polarization can also be used to
reduce multipath. |
| Voltage Standing Wave Ratio (VSWR): It
is a ratio of the maximum to minimum amplitude (or the voltage or current) of
the corresponding field components appearing on a line that feeds an antenna.
|
| Wavelength: The wavelength is the
distance between repeating units of a wave pattern. It is commonly designated
by the Greek letter lambda (λ). |
