Asha Babani-Maghirang
Nov. 22, 2011
Antennas are most often used to increase the range of a wireless LAN system
Omnidirectional antenna – radiate RF in a fashion similar to the way a table or floor lamp radiates light. They are designed to provide general coverage in all directions.
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Antennas can focus or direct the signal that they are transmitting, the higher the dBi or dBd value of an antenna, the more focused the signal.
With higher gain omni-directional antennas, the vertical signal is decreased and the horizon power is increased.
With higher gain omni-directional antennas, the vertical signal is decreased and the horizon power is increased.
Horizontal beamwidth of omni-directional antennas is always 360 degrees, and the vertical beamwidth ranges from 7 to 80 degrees.
Because of the narrow vertical coverage of the higher-gain omni-directional antennas, it is important to carefully plan how they are used. Placing one of these highergain antennas on the first floor of a building may provide good coverage to the first floor, but because of the narrow vertical coverage, the second and third floors may
receive minimal signal.
Omni-directional antennas are typically used in point-to-multipoint environments. The omni-directional antenna is connected to a device (such as an access point) that
is placed at the center of a group of client devices, providing central communications capabilities to the surrounding clients.
Semi-directional Antennas- designed to direct a signal in a specific direction. Semi-directional antennas are used for short – to medium-distance communications, with long
distance communications being served by highly-directional antennas.
Three types of antennas:
Flat patch
Panel
Can be used outdoor point-to-point communications up to about a mile but are more commonly used as a central device for
indoor point-to-multipoint communications.
It is common for patch or panel antennas to be connected to access points to provide directional coverage within a building.
Planar antennas can be used effectively in libraries, warehouses, and retail stores with long aisles of shelves.
Planar antennas have a horizontal beamwidth of 180 degrees or less.
Yagi (pronounced “YAH-gee”)
The traditional television antenna that is attached to the roof of a house or apartment is a yagi antenna. Yagi antenna that is used for 802.11 communications is
designed to support a vary narrow range of frequencies, so the elements are all about the same length. Commonly used for short-to medium-distance point- to- point
communications of up to about 2 miles, high-gain antennas can be used for longer distances. Has a coverage of 30 to 78 degrees.
Advantages:
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Can be installed high on a wall and tilted downward toward the area to be covered. This cannot be done with an omni-directional antenna without causing the signal on
the other side to be tilted upward.
Can have a very long range of signal transmission, depending on its size.
Disadvantages:
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Potential for signal interference. Trees, walls, or any objects located within the line of sight of the antenna signal can be a potential source of interference.
Limited mounting options – must face the signal source in order to be effective. This can severely limit the mounting options for the antenna, specially when the
antenna must be mounted in a congested urban areas or indoors, and even more so when the antenna is large. Yagi antennas, which protrude from the mounting
location, can be a particular challenge to mount in a constricted space.
Highly-directional Antennas – are strictly used for point-to-point communications, typically to provide network bridging between two buildings. They provide the most focused,
narrow beamwidth of any of the antenna types.
Two types:
Parabolic dish – similar in appearance to the small digital satellite TV antennas that can be seen on the roof of many houses.
Grid antennas- resembles the rectangular grill of a barbecue, with the edges slightly curved inward. The spacing of the wires on a grid antenna is determined by the wavelength
of the frequencies that the antenna is designed for.
Advantages:
 High gain or highly-directional antennas, ideal for long distance communications as far as 35 miles (58km).
Disadvantages:
 Due to long distances and narrow beamwidth, highly- directional antennas are affected more by antenna wind loading, which is antenna movement or shifting caused
by wind. Even slight movement of a highly directional antenna can cause the RF bean to be aimed away from the receiving antenna, interrupting the communications.
In high-wind environments, grid antennas, due to spacing between the wires, are less susceptible to wind load and may be a better choice.
Another option in high-wind environments is to choose an antenna with a wider beamwidth. In this situation, if the antenna were to shift slightly, due to its wider coverage area,
the signal would still be received.
No matter which type of antenna is installed, the quality of the mount and antenna will have a huge effect in reducing the wind load.