There 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.

The building to the left will be able to communicate, but the building on the right is likely to have problems.

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”)

Patch and panel antennas, as seen in Figure 4, are more accurately classified or referred to as planar antennas. Patch refers to a particular way of designing the radiating elements inside the antenna.

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.

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.

Using semi-directional antennas indoors often reduces reflections, thus minimizing some of the negative effects of multipath such as data corruption. Yagi antennas, as seen in Figure 5, are not as unusual as they sound.

The traditional television antenna that is attached to the roof of a house or apartment is a yagi antenna. The television antenna looks quite different because it is designed to receive signals of many different frequencies (different channels) and the length of the elements vary according to the wavelength of the different frequencies.

A yagi antenna that is used for 802.11 communications is designed to support a very narrow range of frequencies, so the elements are all about the same length. Yagi antennas are commonly used for short-to medium-distance point-to-point communications of up to about 2 miles, although high-gain yagi antennas can be used for longer distances.

Another benefit of semi-directional antennas is that they 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 of the antenna to be tilted upward.

Since the only RF signal that radiates from the back of a semi-directional antenna is incidental, the ability to aim it vertically is an additional benefit. Figure 6 shows the radiation patterns of a typical semi-directional panel antenna that we discussed.

Remember that these are actual azimuth and elevation charts from a specific antenna and that every manufacturer and model of antenna will have a slightly different radiation pattern.

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.

Phased Array

A phased array antenna is actually an antenna system and is made up of multiple antennas that are connected to a signal processor. The processor feeds the individual antennas with signals of different relative phases, creating a directed beam of RF signal aimed at the client device.

Because it is capable of creating narrow beams, it is also able to transmit multiple beams to multiple users simultaneously. Phased array antennas do not behave like other antennas since they can transmit multiple signals at the same time.

Because of this unique capability, they are often regulated differently by the local RF regulatory agency. Phased array antennas are extremely specialized, expensive, and not commonly used in the 802.11 market.

In fact, the leading manufacturer of 802.11 phase array antenna systems recently went out of business. It is an interesting and very capable technology; however, time will tell whether it has a future in the 802.11 market.

Sector Antennas

Sector antennas are a special type of high-gain, semi-directional antennas that provide a pieshaped coverage pattern. These antennas are typically installed in the middle of the area where RF coverage is desired and placed back to back with other sector antennas.

Individually, each antenna services its own piece of the pie, but as a group, all of the pie pieces fit together and provide omni-directional coverage for the entire area.

Unlike other semi-directional antennas, a sector antenna generates very little RF signal behind the antenna (back lobe) and therefore does not interfere with the other sector antennas that it is working with.

The horizontal beamwidth of a sector antenna is from 60 to 180 degrees, with a narrow vertical beamwidth of from 7 to 17 degrees. Sector antennas typically have a gain of at least 10 dBi.

Installing a group of sector antennas to provide omni-directional coverage for an area provides many benefits over installing a single omni-directional antenna.

To begin with, sector antennas can be mounted high over the terrain and tilted slightly downward, with the tilt of each antenna at an angle appropriate for the terrain it is covering.

Omni-directional antennas can also be mounted high over the terrain; however, if an omni-directional antenna is tilted downward on one side, the other side will be tilted upward.

Since each antenna covers a separate area, each antenna can be connected to a separate transceiver and can transmit and receive independently of the other antennas.

This would provide the capability for all of the antennas to be transmitting at the same time, providing much greater throughput. A single omni-directional antenna would be capable of transmitting to only one device at a time.

The last benefit of the sector antennas over a single omni-directional antenna is that the gain of the sector antennas is much greater than the gain of the omni-directional antenna, providing a much larger coverage area. Sector antennas are used extensively for cellular telephone communications and are starting to be used for 802.11 networking.

Visual Line of Sight

When light travels from one point to another, it travels across what is perceived to be an unobstructed straight line, known as visual line of sight (LOS). For all intents and purposes, it is a straight line, but due to the possibility of light refraction, diffraction, and reflection, there is a slight chance that it is not.

If you have been outside on a summer day and looked across a hot parking lot at a stationary object, you may have noticed that because of the heat rising from the pavement, the object that you were looking at seemed to be moving. This is an example of how visual LOS is sometimes altered slightly. When it comes to RF communications, visual LOS has no bearing on whether the RF transmission is successful or not.

RF Line of Sight

Point-to-point RF communication also needs to have an unobstructed line of sight between the two antennas. So the first step for installing a point-to-point system is to make sure that from the installation point of one of the antennas, you can see the other antenna.

Unfortunately, for RF communications to work properly, this is not sufficient. An additional area around the visual LOS needs to remain clear of obstacles and obstructions. This area around the visual LOS is known as the Fresnel zone and is often referred to as RF line of sight.

Antenna Polarization

Another consideration when installing antennas is antenna polarization. Although it is a lesserknown concern, it is extremely important for successful communications.

Proper polarization alignment is vital when installing any type of antennas. Whether the antennas are installed with horizontal or vertical polarization is irrelevant, as long as both antennas are aligned with the same polarization.

Polarization is not as important for indoor communications because the polarization of the RF signal often changes when it is reflected, which is a common occurrence indoors. Most access points use low-gain omni-directional antennas and they should be polarized vertically when mounted from the ceiling.

Laptop manufacturers build diversity antennas into the sides of the monitor. When the laptop monitor is in the upright position, the internal antennas are vertically polarized as well.

Antenna Diversity

Wireless networks, especially indoor networks, are prone to multipath signals. To help compensate for the effects of multipath, antenna diversity, also called space diversity, is commonly implemented in wireless networking equipment such as access points (APs).

Antenna diversity is when an access point has two antennas and receivers functioning together to minimize the negative effects of multipath. Figure 7 shows a picture of an access point that uses antenna diversity.

Since the wavelengths of 802.11 wireless networks are less than 5 inches long, the antennas can be placed very near each other and be effective.

When the access point senses an RF signal, it compares the signal that it is receiving on both antennas and uses whichever antenna has the higher signal strength to receive the frame of data. This sampling is performed on a frame-by-frame basis, choosing whichever antenna has the higher signal strength.

Multiple Input Multiple Output (MIMO)

Multiple input multiple output (MIMO, pronounced “MY-moh”) is another, more sophisticated form of antenna diversity. Unlike conventional antenna systems, where multipath propagation is an impairment, MIMO systems take advantage of multipath.

There is much research and development currently happening with this technology and thus much disagreement about MIMO. There currently are no official or de facto standards for the technology.

MIMO can safely be described as any RF communications system that has multiple antennas at both ends of the communications link being used concurrently. How the antennas are to be used has not yet been standardized.

There are multiple vendors providing different current and proposed solutions. Complex signal processing techniques known as Space Time Coding (STC) are often associated with MIMO.

These techniques send data using multiple simultaneous RF signals and the receiver then reconstructs the data from those signals. The proposed 802.11n standard will include MIMO technology.