An antenna gives the wireless system three fundamental properties. They are
gain, direction, and polarization. Gain is a measure of increase in power.
Direction is the shape of the transmission pattern. A good analogy for an
antenna is the reflector in a flashlight. The reflector concentrates and
intensifies the light beam in a particular direction similar to what a
parabolic dish antenna would to a RF source in a radio system.
Antennas
are rated in comparison to isotropic or dipole antennas. An isotropic antenna
is a theoretical antenna with a uniform three-dimensional radiation pattern
that is similar to a light bulb with no reflector. Unlike isotropic antennas,
dipole antennas are real antennas. Dipole antennas have a different radiation
pattern compared to isotropic antennas. The dipole radiation pattern is 360
degrees in the horizontal plane, and 75 degrees in the vertical plane assuming
the dipole antenna is standing vertically, and resembles a donut in shape.
Antennas generally fall into two categories. They are directional and
omnidirectional. Figure
provides
examples of the two types. A good antenna transfers power efficiently.
Efficient power transfer depends on correct antenna alignment or polarization,
and proper impedance match. Achieving an impedance match involves electrically
matching the transmission line to the antenna. This means that the transmission
line transfers all of the power to the antenna and does not radiate energy
itself. Figure
shows
the directional versus omnidirectional antennas and some of the typical
applications.
The antennas used for WLANs have two functions:
- Receive: This is the sink or terminator of a signal on a transmission
medium. In communications, it is a device that receives Information, control,
or other signals from a source.
- Transmit: This is the source or generator of a signal on a transmission
medium.
Some knowledge of antennas is essential in order to understand how to
set up and optimize wireless networks for best performance. Antennas are
available with different gain and range capabilities, beam widths, and form
factors. Coupling the right antenna with the right AP or bridge allows for
efficient coverage in any facility, as well as better reliability, at higher
data rates. The Cisco antennas for APs are shown in Figure
. A variety of
antennas are available for bridges, depending upon the required distance and
mounting possibilities. All Antennas sold with a spread spectrum product must
comply with t he U.S. FCC Antenna Regulations shown in Figure
. Onmidirectional
antennas are generally used for point-to-multipoint implementations. Figures
and
show the Cisco
antennas for wireless bridges.
This module will cover some of the basics
of antennas and antenna operation. These fundamentals are needed when choosing
antennas for a WLAN installation.