domingo, 21 de marzo de 2010

Wireless Antenas TypesThere are three main categories of antennas

Wireless Antenas TypesThere are three main categories of antennas:
Omni-directional - Omni-directional antennas 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.

Semi-directional - Semi-directional antennas radiate RF in a fashion similar to the way a wall sconce is designed to radiate light away from the wall or the way a street lamp is designed to shine light down on a street or a parking lot, providing a directional light across a large area.

Highly-directional - Highly-directional antennas radiate RF in a fashion similar to the way a spotlight is designed to focus light on a flag or a sign. Each type of antenna is designed with a different objective in mind.

In addition to antennas acting as radiators and focusing signals that are being transmitted, it is often overlooked that they also focus signals that are received. If you were to walk outside and look up at a star, it would appear fairly dim.
If you were to look at that same star through binoculars, it would appear brighter. If you were to use a telescope, it would appear even brighter. Antennas function in a similar way.
Not only do they amplify signal that is being transmitted, they also amplify signal that is being received. High gain microphones work in the same way, allowing us to not only watch the action of our favorite sport on television, but to also hear the action.
Omni-directional

Antennas Omni-directional antennas radiate RF signal in all directions. The small rubber dipole antenna , often referred to as a “rubber duck” antenna, is the classic example of an omni-directional antenna and is the default antenna of most access points. A perfect omni-directional antenna would radiate RF signal.
The closest thing to an isotropic radiator is the omni-directional dipole antenna. An easy way to explain the radiation pattern of a typical omni-directional antenna is to hold your index finger straight up (this represents the antenna) and place a bagel on it as if it were a ring (this represents the RF signal).
If you were to slice the bagel in half horizontally, as if you were planning to spread butter on it, the cut surface of the bagel would represent the azimuth chart, or H-plane, of the omni-directional antenna.
If you took another bagel and sliced it vertically instead, essentially cutting the hole that you are looking through in half, the cut surface of the bagel would now represent the elevation, or E-plane, of the omni-directional antenna.
In previous article we learned that antennas can focus or direct the signal that they are transmitting. It is important to know that the higher the dBi or dBd value of an antenna, the more focused the signal.
When discussing omni-directional antennas, it is not uncommon to initially question how it is possible to focus a signal that is radiated in all directions. With higher-gain omni-directional antennas, the vertical signal is decreased and the horizontal power is increased.
Figure 1 shows the elevation view of three theoretical antennas. Notice that the signal of the higher-gain antennas is elongated, or more focused horizontally.

The horizontal beamwidth of omni-directional antennas is always 360 degrees, and the vertical beamwidth ranges from 7 to 80 degrees, depending upon the particular antenna. Because of the narrower vertical coverage of the higher-gain omni-directional antennas, it is important to carefully plan how they are used.
Placing one of these higher-gain 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.
In some installations you may want this; in others you may not. Indoor installations typically use low-gain omni-directional antennas with gain of about 2.14 dBi. Antennas are most effective when the length of the element is an even fraction (such as 1/4 or 1/2 ) or a multiple of the wavelength ( λ ).
A 2.4 GHz half-wave dipole antenna (see Figure 2) consists of two elements, each 1/4λ in length (about 1 inch), running in the opposite direction from each other.
Although this drawing of a dipole is placed horizontally, the antenna is always placed in a vertical orientation. Higher-gain omni-directional antennas are typically constructed by stacking multiple dipole antennas on top of each other and are known as collinear antennas.
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.
High-gain omni-directional antennas can also be used outdoors to connect multiple buildings together in a point-to-multipoint configuration. A central building would have an omni-directional antenna on its roof, and the surrounding buildings would have directional antennas aimed at the central building.
In this configuration, it is important to make sure that the gain of the omni-directional antenna is high enough to provide the coverage necessary but not so high that the vertical beamwidth is too narrow to provide an adequate signal to the surrounding buildings.
Figure 3 shows an installation where the gain is too high.

Semi-directional Antennas

Unlike omni-directional antennas that radiate RF signals in all directions, semi-directional antennas are 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.
It is common to use semi-directional antennas to provide a network bridge between two buildings in a campus environment or down the street from each other. Longer distances would be served by highly-directional antennas.
There are three types of antennas that fit into the semi-directional category:
Patch
Panel
Yagi (pronounced “YAH-gee”)

Unfortunately, it has become common practice to use the terms patch and panel interchangeably. If you are unsure of the antenna’s specific design, it is better to refer to it as a planar antenna. Patch refers to a particular way of designing the radiating elements inside the antenna.
Highly-directional Antennas

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.
There are two types of highly-directional antennas: parabolic dish and grid antennas. The parabolic dish antenna is similar in appearance to the small digital satellite TV antennas that can be seen on the roofs of many houses.
The grid antenna 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.
Because of the high gain of highly-directional antennas, they are ideal for long-distance communications as far as 35 miles (58 km). Due to the 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 beam to be aimed away from the receiving antenna, interrupting the communications. In high-wind environments, grid antennas, due to the 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 wind load.
These antennas can be used for 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.
Due to the tall, long shelves, omni-directional antennas often have difficulty providing RF coverage effectively. In contrast, planar antennas can be placed high on the side walls of the building, aiming through the rows of shelves.
The antennas can be alternated between rows with every other antenna being placed on the opposite wall. Since planar antennas have a horizontal beamwidth of 180 degrees or less, a minimal amount of signal will radiate outside of the building.
With the antenna placement alternated and aimed from opposite sides of the building, the RF signal is more likely to radiate down the rows, providing the necessary coverage. Planar antennas are also often used to provide coverage for long hallways with offices on each side or hospital corridors with patient rooms on each side.
A planar antenna can be placed at the end of the hall and aimed down the corridor. A single planar antenna can provide RF signal to some or all of the corridor and the rooms on each side and some coverage to the floors above and below.
How much coverage will depend upon the power of the transmitter, the gain and beamwidth (both horizontal and vertical) of the antenna, and the attenuation properties of the building.
Maria Gabriela Medina Maldonado
C.I. 16779553
CRF

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