WiFoo..The.Secrets.of.Wireless.Hacking [Electronic resources] نسخه متنی

Antennas

The proper use of antennas can improve the performance of a WLAN dramatically. In fact, antennas are probably the single easiest way to refine the performance of a WLAN. But it is important to have an understanding of the basics of antenna theory, as well as the various types that are available for use in WLANs.

All antennas have the three fundamental properties:

Gain A measure of increase in power

Direction The shape of the transmission pattern

Polarization The angle at which the energy is emitted into the air

All three of these properties are discussed in detail in the following sections.

Gain

isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern (similar to a light bulb with no reflector). The dBi rating is used to compare the power level of a given antenna to the theoretical isotropic antenna (hence the use of the i in dBi). The FCC, as well as many other regulatory bodies, use dBi for defining power levels in the rules and regulations covering WLAN antennas. Most mathematical calculations that include antennas and path loss also use the dBi rating. An isotropic antenna is said to have a power rating of 0 dBi (that is, zero gain/loss when compared to itself).

Unlike isotropic antennas, dipole antennas are physical antennas that are standard on many WLAN products. Dipole antennas have a different radiation pattern when compared to an isotropic antenna. The dipole radiation pattern is 360 degrees in the horizontal plane and usually about 75 degrees in the vertical plane (assuming the dipole antenna is standing vertically) and resembles a bowtie in shape (see Figure 2-11). Because the beam is slightly concentrated, dipole antennas have a gain over isotropic antennas in the horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi (in comparison to an isotropic antenna).

Figure 2-11. Dipole Radiation Pattern

Some antennas are rated in comparison to dipole antennas. This is denoted by the suffix dBd. Hence, dipole antennas have a gain of 0 dBd (0 dBd = 2.14 dBi).

dBd, instead of dBi, as the reference point. To convert any number from dBd to dBi, just add 2.14 to the dBd number. For instance a 3-dBd antenna would have a rating of 5.14 dBi (or rounded up to 5.2 dBi).

Directional Properties

Any antenna, except for an isotropic antenna (theoretical perfect antenna that radiates equally in all directions), has some sort of radiation pattern. That means that it radiates energy in certain directions more than others. A good analogy for antenna directionality is that of a reflector in a flashlight. The reflector concentrates and intensifies the light beam in a particular direction. This is very similar to what a dish antenna does to an RF signal.

In RF, you usually have to give up one thing to gain something else. In antenna gain, this comes in the form of coverage area or what is known as beamwidth. As the gain of an antenna goes up, the beamwidth (usually) goes down.

An isotropic antenna's coverage can be thought of as a perfect round balloon. It extends in all directions equally. The size of the balloon represents the amount of RF energy that the transmitter is sending to the antennas, and the antenna is converting the energy to radiated RF energy. As you learn about other antenna types, you will see that the overall energy radiated from the antenna is not increased, it is just redirected. As was the case with the dipole antenna discussed earlier in this chapter, this perfect round balloon of energy that an isotropic antenna provides becomes something totally different in shape.

Omni-Directional Antennas

An omni antenna is designed to provide a 360-degree radiation pattern (on one plane, usually the horizontal plane). This type of antenna is used when coverage in all directions surrounding the antennas on that one plane is required. The standard 2.14-dBi Rubber Duck is one of the most common omni antennas. When an omni antenna is designed to have higher gain, it results in loss of coverage in certain areas.

Imagine again, the balloon of energy for an isotropic antenna, which extends from the antenna equally in all directions. Now imagine pressing in on the top and bottom of the balloon. This causes the balloon to expand in an outward direction, covering more area in the horizontal pattern, but reducing the coverage area above and below the antenna. This yields a higher gain, as the antenna appears to extend to a larger coverage area. The higher the gain on an antenna means the smaller the vertical beamwidth.

If you continue to push in on the ends of the balloon, it results in a pancake effect with very narrow vertical beamwidth, but very large horizontal coverage (see Figure 2-12). This type of antenna design can deliver very long communications distances, but has one drawback: poor coverage below the antenna.

Figure 2-12. High Gain Omni-Directional Radiation Pattern

In some cases, the gain of an antenna can be high enough and the radiation patterns so small, that even small motions of the antenna (from wind, for instance) can cause the signal to move away from the intended target and lose communication. For this reason, extremely high-gain antennas are typically mounted to a very strong and permanent structure and almost never used in a mobile or portable environment.

With high-gain omni antennas, this problem can be partially solved by designing in something called downtilt. An antenna that uses downtilt is designed to radiate at a slight angle rather that at 90 degrees from the vertical element. Downtilt helps for local coverage, but reduces effectiveness of the long-range capability (see Figure 2-13). Cellular antennas use downtilt.

Figure 2-13. Antenna Downtilt

Directional Antennas

directional antenna increases, the overall coverage area usually decreases. Common form factors for WLAN directional antennas include dish antennas, patch antennas, and Yagi antennas.

Consider the common Mag-Lite flashlight (one of the adjustable-beam-focus flashlights). There are only two batteries, and the one light bulb, but the intensity and width of the light beam can be changed. Moving the back reflector and directing the light in tighter or wider angles accomplishes this. As the beam gets wider, the intensity in the center decreases, and it travels a shorter distance. The same is true of a directional antenna. The same power is reaching the antenna, but by building it in certain ways, the RF energy can be directed in tighter and stronger waves, or wider and less-intense waves, just as with the flashlight.

Polarization

Two planes are used in RF radiations: the E and the H plane. The E plane (electric field) defines the orientation of the radio waves as they are radiated from the antenna. If the E is perpendicular to the Earth's surface, it is referred to as vertically polarized. In WLAN systems, for instance, an omni-directional antenna is usually a vertically polarized antenna.

Horizontally polarized (linear) antennas have their electric field parallel to the Earth's surface. WLANs seldom use horizontally polarized antennas, except in certain outdoor, point-to-point systems.

Antenna Examples

You can choose from a wide variety of antennas for use with WLAN equipment. The use of different antennas can simplify the installation of a WLAN system, and in some cases reduce the overall cost of the system. A thorough understanding of different antenna types available will enable the survey engineer and installer to provide a WLAN that not only provides adequate coverage but also helps to stay within budgetary constraints.

Appendix B, "Antenna Radiation Patterns," provides an assortment of WLAN antennas and the associated polar plots. The polar plot is the common method to define an antenna's beamwidth, or radiation pattern, and gain factors.

Patch Antenna

A patch antenna is typically small and somewhat flat and is usually designed to mount against a wall or on a small bracket. It has a beamwidth that is less than 180 degrees, and is sometimes referred to as a hemispherical antenna.

Panel Antenna

A panel antenna (sometimes also referred to as a sectorized antenna) is similar to a patch antenna, but is generally a higher gain and physically larger. Many times a panel antenna has an adjustable back reflector that can be used to change the beamwidth as well as mounting brackets that can be adjusted for downtilt.

Panel antennas are usually used outdoors and can have gains ranging from as little as 5 dBi to more than 20 dBi. They can be used as a single antenna or in multiples to cover a larger area.

Yagi Antenna

A Yagi antenna has a series of small elements, referred to as reflectors or directors, and an active element. These are placed in a straight line and direct the energy in a given direction. Generally Yagi antennas have fairly high gain. The more reflectors and directors a Yagi has, the higher the gain. Due to the short wavelength for frequencies used in WLAN systems, the elements are fairly small, and most Yagis used for 2.4-GHz or 5-GHz contain some type of cover to protect the antenna's components from the weather and to provide more structural strength. Yagi antennas can range in gain from as low as 5 dBi to as high as 17 dBi or more.

Dish Antennas

There are really two main types of dish antennas: the parabolic and the grid dish. The parabolic dish contains a reflector that is solid in construction, and a driven or active element supported in the center of the reflector. These are similar to what you would find for a standard satellite TV dish antenna, except the placement of the active element is typically centralized on the WLAN antenna.

The grid dish antenna is very similar to the parabolic antenna, except the reflector is not solid. It is made of a grid-type structure to permit wind and rain to flow through it. This provides less wind resistance and therefore requires a smaller mounting structure. Chapter 14, "Outdoor Bridge Deployments," provides more information about outdoor mounting.

Diversity

Diversity antenna systems are used to overcoming a phenomenon known as multipath distortion or multipath fading. It uses two identical antennas, located a small distance apart, to provide coverage to the same physical area.

To understand diversity, it is important to give you an overview of multipath distortion as well as an understanding of how this can occur. Multipath distortion is a form of RF interference that can occur when a radio signal has more then one path between the transmitting antenna and the receiving antennas. Environments with a high probability of multipath interference include such places as airport hangars, steel mills, manufacturing areas, distribution centers, and other locations where the antenna is exposed to metal walls, ceilings, racks, shelving, or other metallic items that reflect radio signals and create this multipath condition (see Figure 2-14).

Figure 2-14. Multiple Signal Paths

access point (AP) can't physically move its antenna, many have been designed with two antenna ports. The radio performs an assessment of each antenna port and selects to use the antenna with the best reception.

Another example of multipath interference occurs while listening to the radio when driving an automobile. As you pull up to a stop sign, the radio station might appear distorted, or you may even lose the signal altogether as a result of a radio null, which is also referred to as a dead spot. As you move the car forward a few inches or feet, the radio reception starts to come in clearer. As you move the vehicle, you are actually moving the antenna slightly, out of the point where the multiple signals converge. In all probability, the radio signal was reflecting off another vehicle or metal object nearby.

In some cases, if signals are received in equal strength, yet delayed in such a manner that they are opposite in polarity, they will actually cancel each other out completely, creating a total absence of received signal by the receiver. This is known as a multipath null.

Many do not understand the method of how a diversity antenna system works, and this lack of understanding often leads to confusion and improper installation. The diversity antenna system includes two antennas that are connected to an RF switch, which in turn connects to the receiver (see Figure 2-15). The receiver actually switches between antennas on a regular basis as it listens for a valid signal.

Figure 2-15. Diversity Antenna Switch

Note that this switching occurs extremely fast. The AP samples part of the radio header and determines and utilizes the best antenna to receive the client's data and then uses that very same antenna when transmitting back to the client. If the client doesn't respond, the AP will then try sending the data out the other antenna port.

Improper Diversity Deployment Example

A golf course with an electronic scoring application used an AP with an outdoor antenna to cover the front nine holes of the golf course. Originally the AP was placed in the clubhouse, and one outdoor antenna was used to cover the front nine of the course. Because there was little multipath interference (few things outside to reflect the radio signal), one antenna was sufficient and communication seemed to be fine. In this case, the customer had used a directional Yagi antenna. This antenna was chosen for its distance characteristics and ease of installation.

Later it was determined that coverage was needed on the back nine of the golf course as well. Instead of adding another AP, the customer decided just to connect another directional Yagi antenna to the other antenna port and point it off in another direction (the back nine), as shown in Figure 2-16. While driving around in the golf cart, performing a survey, the customer had no issues with coverage.

Figure 2-16. Improper Diversity Installation

[View full size image]

But as the tournament started and many users were added, they encountered difficulty. When the first users (clients on the front nine of the course) registered to the AP, the AP sampled both antennas (one at a time) and selected the antenna pointing to the front section.

When users started migrating to the back nine, and more users entered the front nine, problems started popping up. As the AP was communicating to the users on the front section of the course users on the back section could not hear that RF traffic because the back-nine antenna was being used at that instant. Therefore, the back users tried to send their own traffic, which was not heard by the AP.

In the case of the golf course, two methods could resolve this problem. One method is to replace the directional Yagi antenna with a similar-gain omni antenna. The AP's radio would then be able to work in all directions rather than the limited directional pattern of the Yagi.

Another method is to add an AP to cover the other radio cell. This way both APs could properly handle the RF traffic, and each AP could use the higher-gain Yagi antenna to cover each area.