A wireless antenna focuses a radio signal in a specific direction and in a narrow beam.
The increase in the signal power (compared to an omnidirectional antenna) when
it is focused in the desired direction is called gain.
Antennas are tuned to operate on a specific group of frequencies. Other
specific attributes such as beamwidth and gain are also fixed by the
manufacturer. Antennas should be selected and placed according to your site and
your application.
In general, the larger the antenna, the higher the gain and the larger the
mast required. It is best to use the smallest antenna that will provide
sufficient protection from interference and enough signal at the far end of the
link to provide good reception even with fading.
Other considerations include antenna beamwidth, front-to-side ratios,
front-to-back ratios, and cross-polarization rejection. Where interference from
other licensees on the same channel or adjacent channels is an issue, narrow
beamwidths, high front-to-back and front-to-side ratios, and high
cross-polarization rejection are likely to be required. Even when other
licensees are not an issue, if you are using a network deployment using the
"cell" approach, all these considerations are still important to reduce
interference between your own adjacent installations.
Antennas are an important part of a wireless system
because it directs where a wireless signal is transmitted and it determines the
direction that signals and noise are received from.
A short list of different types of antennas
- Corner Reflector
Antenna - A directional antenna that is made up of a dipole driven element
mounted in front of a 60-degree or 90-degree corner-shaped reflecting element.
- Dipole Antenna - A
two-piece (di = two; pole=“pole” or “piece”) antenna that is the basic “building
block” antenna element. A dipole is normally used as the “driven element” in
most antenna systems. A dipole is made up of two ¼ wavelength-long antenna
pieces arranged in a straight line. A coax transmission line feeds power to the
middle of the dipole.
- Directional Antennas
- An antenna with a radiation pattern that concentrates both the transmitting
and receiving signal power into one favored direction. The power gain (the
increase in signal power in the favored direction) is measured in dbi or dbd.
- Isotropic Antenna - An
isotropic antenna is a theoretical antenna. If it existed in the real world, it
would radiate a wireless signal equally in all directions (front, back, left,
right, up, and down). The signal strength from a theoretical isotropic antenna
is used as a reference level to measure the gain (focusing power) of real-world
antennas.
- Omnidirectional Antenna - An
antenna with a radiation pattern that, when viewed from above, is equally strong
in all directions.
- Panel Antenna - A
directional antenna made up of several phased driven elements mounted in front
of a flat reflecting element. Panel antennas usually have a plastic or
fiberglass cover that gives the antenna a panel-like appearance.
- Parabolic Antenna - A
directional antenna made up of a dipole driven element mounted in front of a
parabolic-shaped reflector. The reflector may be either a solid metal “dish” or
a dish-shaped screen made of metallic rods or mesh.
- Patch Antenna - A smaller
version of a directional panel antenna often used indoors.
- Yagi Antenna - A directional
antenna made up of one “driven element” that is connected to the transmission
line and one or more “reflectors” (signal reflecting elements) and/or
“directors” (signal directing elements).
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Sources for
Wireless Antennas Product Information:
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