Chapter 7

Wireless LAN Antennas and Accessories

The following CWTS exam objectives are covered in this chapter:

• 2.3 Identify the purpose, features, and proper implementation of the following types of antennas
  • Omnidirectional/dipole
  • Semidirectional
  • Highly directional
• 2.4 Describe the proper locations and methods for installing RF antennas
  • Internal and external (to the AP) antennas
  • Pole/mast mount
  • Ceiling mount
  • Wall mount
• 3.6 Understand and apply basic RF antenna concepts
  • Passive gain
  • Beamwidth
  • Simple diversity
  • Polarization
• 3.7 Identify the use of the following WLAN accessories and explain how to select and install them for optimal performance and regulatory domain compliance
  • RF cables
  • RF connectors
  • Lightning arrestors and grounding rods

Antennas are an essential part of a successful wireless LAN deployment. From the transmitter perspective, an antenna takes the energy from the transmission system, transforms it into radio waves, and propagates it through the free air. From the receiver perspective, an antenna performs the opposite task—it receives the radio waves, transforms them back to AC signals, and finally sends the information to a computer or other wireless device.

Many factors are involved in determining the proper antenna to be used in an application or deployment of a wireless LAN. These factors include:

• Indoor or outdoor installation
• Distance between transmitter and receiver
• Frequency to be used
• Horizontal or vertical orientation/polarization
• Aesthetics
• Cost
• Manufacturer
• Intended use
• Mounting brackets
• Electrical characteristics
• Height
• Location
• Local ordinances

Basic Radio Frequency Antenna Concepts

It is important to understand some of the basic theory, characteristics, and terminology associated with antennas prior to learning how they operate. Becoming familiar with this will help in making decisions when it comes to sales and support of antennas and wireless LAN systems. Some of the terminology for characteristics of antennas is listed here:

Radio Frequency Lobes

The term lobe has many meanings, depending on the context in which it is used. Typically it is used to define the projecting part of an object. In anatomical terms, an example would be part of the human ear known as the ear lobe. In botanical terms, a lobe is the divided part of a leaf. As a radio frequency technology term, lobe refers to the shape of the RF energy emitted from an antenna element. RF lobes are determined by the physical design of the antenna. The antenna design also determines how the lobes project from an antenna element.

The effect of antenna design, particularly the shape of the RF lobes, is one reason why choosing the correct antenna is a critical part of a wireless LAN design. Antennas may project many lobes of RF signal, some of which are not intended to be usable areas of coverage. The RF lobes that are not part of the main or intended lobe coverage—that is, the rear and side lobes—contain usable RF but are not intended to be used to provide coverage for the wireless LAN cell. They are for the most part unintentional coverage areas and are not part of a good wireless LAN design and planned coverage area. The type of antenna utilized—omnidirectional, semidirectional, or highly directional—will determine the usable lobes. These antennas as well as the RF radiation patterns they project will be discussed in more detail later in this chapter. Figure 7.1 shows an example of RF lobes emitted from an antenna element. The “main signal” is the lobe intended to be used.

Figure 7.1 Radio frequency lobes' shape and coverage area are affected by the type and design of an antenna.

Antenna Beamwidth

The design of an antenna will determine how radio frequencies propagate and the specific patterns in which the energy propagates from an antenna element. As mentioned earlier, the patterns of energy emitted from an antenna are known as lobes. For antennas, the beamwidth is the angle of measurement of the main RF lobe measured at what is called the half-power point, or −3 dB point. Beamwidth is measured both horizontally and vertically, in degrees. It is important to understand that antennas shape the RF coverage or isotropic energy that radiates from the antenna element. Changing types or remaining with the same type of antenna but changing the gain will also change the coverage area provided by the wireless LAN system.

Documents or antenna specifications are available to illustrate the horizontal and vertical beamwidths. Azimuth and elevation charts available from the antenna manufacturer will show the beamwidth angles.

The azimuth refers to the horizontal RF coverage pattern, and the elevation is the vertical RF coverage pattern. The azimuth is the view from above or the “bird's-eye view” of the RF pattern; in some cases it will be 360°. Think of the elevation as a side view. If you were to look at a mountain from the side view, it would have a certain height or elevation measured in feet or meters. For example, Pikes Peak, a mountain in the front range of the Rocky Mountains, has an elevation of 14,115’ (4,302 meters). Figure 7.2 shows a representation of horizontal and vertical beamwidths. Some predictive modeling site survey software programs will allow the wireless LAN designer to adjust the azimuth and elevation of the antennas used in the predictive modeling design to more closely depict the real world coverage of the wireless LAN system. Wireless LAN site surveys and predictive modeling will be discussed in more detail in Chapter 11, “Performing an RF Wireless LAN Site Survey.”

Figure 7.2 Horizontal (azimuth) and vertical (elevation) beamwidths measured at the half power, or −3 dB point

Reading Azimuth and Elevation Charts

Understanding how to read an azimuth and elevation chart is useful from a technical sales, design, or integration perspective. Knowing these patterns will help when making hardware recommendations for customers based on needed coverage and device use. These charts show the angles of radio frequency propagation from both the azimuth (horizontal or looking down, top view) and the elevation (vertical or side view). They give a general idea of the shape of the RF propagation lobe based on antenna design.

Antenna manufacturers test antenna designs in a laboratory. Using the correct instruments, an engineer is able to create the azimuth and elevation charts. These charts show only approximate coverage area based on the readings taken during laboratory testing and do not take into consideration any environmental conditions such as obstacles or interference. The following image shows an example of an azimuth and elevation chart for a semidirectional antenna.

Image provided by www.L-com.com

Understanding how to read one of these charts is not complicated. Notice that the chart is a circular pattern with readings from 0° to 360°, and there are many rings within these charts. The outermost ring shows the strongest signal from the testing process of this antenna. The inner rings show measurements and dB ratings less than the strongest measured signal from the outside ring. A good chart will show the most accurate readings from the testing process. A sales or technical support professional can use these charts to get an idea of how the radiation pattern would look based on a specific antenna type and model.

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Antenna Gain

The gain of an antenna provides a change in coverage that is a result of the antenna focusing the area of radio frequency propagation. This gain is produced from the physical design of the antenna element. In Chapter 6, “Radio Frequency Fundamentals for Wireless LAN Technology,” we looked at various characteristics of radio frequency. One of these characteristics is amplitude, which was defined as the height (voltage level) or the amount of power of a sine wave. The amplitude is created by varying voltage over a period of time and is measured at the peaks of the signal from top to bottom. Amplification of an RF signal will result in gain. An antenna is a device that can change the coverage area, thus propagating an RF signal further. Antenna gain is measured in decibels isotropic (dBi), which is a change in power as a result of increasing the isotropic energy. Isotropic energy is defined as energy emitted equally in all directions. The sun is a good example of isotropic energy, emitting energy in a spherical fashion equally in all directions. Figure 7.3 illustrates energy being emitted from an isotropic radiator.

Figure 7.3 A perfect isotropic radiator emits energy equally in all directions.

Passive Gain

It's quite intriguing how an antenna can provide passive gain, a change in coverage without the use of an external power source. Because of how antennas are designed, they focus isotropic energy into a specific radiation pattern. Focusing this energy increases coverage in a particular direction. A common example used to describe passive gain is a magnifying glass. If a person is standing outside on a beautiful sunny day, the sun's energy is not intense because it is being diffused across the entire earth's hemisphere. Thus, there is not enough concentrated energy to cause any harm or damage in a short period of time. However, if this person were to take a magnifying glass and point one side of it toward the sun and the other side toward a piece of paper, more than likely the paper would start to heat quickly. This is because the convex shape of the magnifying glass focuses or concentrates the sun's energy into one specific area, thus increasing the amount of heat to that area.

Antennas are designed to function in the same way by focusing the energy they receive from a signal source into a specific RF radiation pattern. Depending on the design of the antenna element, as the gain of an antenna increases, both the horizontal and vertical radiation patterns (beamwidths) will decrease or create narrower beamwidths. Conversely, as the gain of an antenna decreases, the beamwidths will increase, making a larger radiation pattern. One exception to this behavior is the omnidirectional antenna. This type of antenna has a horizontal beamwidth of 360°. When the gain is increased or decreased, the beamwidth will remain 360° but the size of this coverage area will increase or decrease depending on the change in the gain. Omnidirectional antennas are discussed in more detail later in this chapter. Figure 7.4 shows a drawing of a wireless LAN system with 100 mW of RF power at the antenna. Because of passive gain, the antenna has the effect of emitting 200 mW of RF power.

Figure 7.4 An access point supplying 100 mW of RF power and an antenna with a gain of 3 dBi for an output at the antenna of effectively 200 mW of RF power

Exercise 7.1 is a simple way to demonstrate passive gain.

Active Gain

Active gain will also provide an increase in signal strength. In a wireless LAN system, active gain is accomplished by providing an external power source to an installed device. An example of such a device is an amplifier. An amplifier is placed in series in the wireless LAN system and will increase the signal strength based on how much gain it provides.

If an amplifier is used in a wireless LAN system, certain regulatory domains require that the amplifier be certified as part of the system. It is best to carefully consider whether an amplifier is necessary before using such a device in an IEEE 802.11 wireless LAN system. Using an amplifier may nullify the system's certification and potentially exceed the allowed RF limit.

Antenna Polarization

Antenna polarization describes how a wave is emitted from an antenna and the orientation of the electrical component or electric field of the waveform. To maximize radio frequency signals, the transmitting and receiving antennas should be polarized in the same direction or as closely as possible. Antennas polarized the same way ensure the best possible RF signal.

If the polarization of the transmitter and receiver are different, the power of the RF signal will decrease depending how different the polarization is. Figure 7.5 shows an example of horizontal and vertical polarized antennas.

With the large number of wireless LAN devices available, it is a challenging task to accomplish the same polarization for all devices on the network. Performing a wireless LAN site survey will show signal strength based on several factors, including polarization of access point antennas. This survey will help determine the received signal strength of the wireless LAN devices. Site surveys and antenna polarization are discussed in more detail in Chapter 11.

Figure 7.5 Horizontally and vertically polarized antennas

Wireless LAN Antenna Types

The type of antenna that is best for a particular installation or application will depend on the desired radio frequency coverage pattern. Making the correct choice is part of a good wireless LAN design. Using the wrong type of antenna can cause undesirable results, such as interference to neighboring systems, poor signal strength, or incorrect coverage pattern for your design.

Three common types of antennas for use with wireless LANs are:

• Omnidirectional/dipole antennas
• Semidirectional antennas
• Highly directional antennas

This section describes each type of antenna in more detail and provides specifications and installation or configuration information about these antennas.

Omnidirectional Antennas

Omnidirectional antennas are common on most access points of either SOHO or enterprise grade. An omnidirectional antenna has a horizontal beamwidth (azimuth) of 360°. This means that when the antenna is vertically polarized (perpendicular to the earth's surface) the horizontal radiation pattern is 360° and will propagate RF energy in every direction horizontally. The vertical beamwidth (elevation) will vary depending on the antenna's gain. As the gain of the antenna increases, the horizontal radiation pattern will increase, providing more horizontal coverage. Keep in mind the beamwidth is still 360°, but it will be a larger 360° area that is covered because of the higher gain of the antenna. However, the vertical radiation pattern will decrease, thus providing less vertical coverage.

The shape of the radiation pattern from an omnidirectional antenna looks like a donut and is known as a torus. Figure 7.6 shows an example of the toroidal radiation pattern of an omnidirectional antenna.

Figure 7.6 The omnidirectional radiation pattern has a toroidal shape.

Omnidirectional antennas are one of the most common types of antenna for indoor wireless LAN deployments. Most access points use omnidirectional antennas. Access points come with fixed, removable, or integrated antennas. If the antenna is removable, the installer can replace it with one of different gain. Enterprise-grade access points typically have removable antennas that are sold separately.

Some regulatory domains require the use of proprietary connectors with respect to antennas. These connectors limit access points to the specific antennas tested with the system. Therefore, it is best to consult with the manufacturer of the access point or other wireless LAN transmitting device to determine which antennas may be used with the system.

The most common type of omnidirectional antenna used indoors is known as the “rubber duck antenna.” This type of antenna typically has a low gain of 2 dBi to 3 dBi and connects directly to an access point. Rubber duck antennas usually have a pivot point so the polarization can be adjusted vertically or horizontally regardless of how the access point is mounted.

Some antennas will operate in both the 2.4 GHz ISM band and the 5 GHz UNII band and can thus work with a multiband wireless device.

Figure 7.7 shows a rubber duck omnidirectional antenna.

Figure 7.7 2.4 GHz rubber duck omnidirectional antenna

Image provided by www.L-com.com.

Omnidirectional Antenna Specifications

In addition to the beamwidth and gain, omnidirectional antennas have various other specifications to be considered, including:

• Frequency range
• Voltage standing wave ratio (VSWR)
• Polarization
• Attached cable length
• Dimensions
• Mounting requirements

Table 7.1 is an example of a specification sheet for a rubber duck omnidirectional antenna.

Table 7.1 Omnidirectional antenna specifications

A physical representation of the antenna is also helpful for sales and integration professionals. Figure 7.8 shows the physical specifications diagram for a rubber duck omnidirectional antenna.

Figure 7.8 Rubber-duck omnidirectional antenna physical specifications

Azimuth and elevation charts are usually available to allow visualization of the radio frequency radiation pattern emitted from the antenna. This is useful for a wireless LAN professional to determine the approximate RF propagation pattern. The purpose of these charts, and how to read them, were explained in the Case Study “Reading Azimuth and Elevation Charts” earlier in this chapter. Figure 7.9 shows the charts for a rubber duck omnidirectional antenna.

Figure 7.9 Vertical (elevation) and horizontal (azimuth) charts for omnidirectional antenna

Image provided by www.L-com.com.

Semidirectional Antennas

Semidirectional antennas take radio frequency power from the transmitting system and focus it into a more specific pattern than an omnidirectional antenna offers. Semidirectional antennas are available in various types, including patch, panel, sector, and Yagi. These antennas are manufactured for either indoor or outdoor use and are designed to provide more specific coverage by focusing the horizontal radiation pattern to a value of less than 360°. A semidirectional antenna will allow the wireless LAN designer to provide RF coverage to a specific area within a deployment. This coverage area may consist of rooms or areas in which an omnidirectional antenna may not be the perfect solution. For indoor installations, such areas include rectangular rooms or offices, hallways, and long corridors. For outdoor deployments, they include point-to-point and point-to-multipoint bridging installations.

Patch/Panel Antennas

In the wireless LAN world, the terms patch and panel are commonly used to describe the same type of antenna. The intended use will affect the choice of patch/panel antenna to be used in a specific application. Choosing the correct patch/panel antenna will require knowing the dimensions of the physical area to be covered as well as the amount of gain required. A patch/panel antenna can have a horizontal beamwidth of as high as 180°, but usually the horizontal beamwidth is between 35° and 60°. The vertical beamwidth usually ranges between 30° and 80°. Figure 7.10 shows a 2.4 GHz flat patch antenna. Sector antennas are a type of semidirectional antenna that can be configured in an array to provide omnidirectional coverage. Sector antennas are covered in more detail later in this chapter.

Figure 7.10 2.4 GHz 8 dBi flat patch antenna

Image provided by www.L-com.com.

Appropriate Use of a Semidirectional Antenna

A small business consultant is tasked with providing wireless LAN access to several offices in a multi-tenant building. The client wants to provide adequate coverage for the offices they lease but would like to minimize the number of access points. The client wishes to use access points and antennas that are aesthetically pleasing, since these offices allow public access. The areas to be covered are rectangular, as shown here:

One solution would be to provide several access points using low-gain omnidirectional antennas. The following image illustrates how several access points could be used to provide coverage to this area.

However, the consultant believes that if low-gain rubber duck omnidirectional antennas are used, an access point with significant output power would be required to cover the length of the rooms. In addition, the client wants to minimize the number of access points and make the installation aesthetically pleasing.

An alternate solution is to use a patch antenna on both sides of the office, thereby providing adequate coverage and minimizing the use of access points. The following image shows patch antennas mounted at both ends of the office area as well as the projected coverage area of both antennas.

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Patch/Panel Antenna Specifications

The specifications for semidirectional antennas such as patch or panel vary based on the design of the antenna. Semidirectional antennas are available in single- or dual-band capability. Semidirectional antennas may be used indoors or outdoors depending on the application. Table 7.2 is an example of a specification sheet for a 2.4 GHz 8 dBi flat patch antenna.

Table 7.2 Flat patch antenna specifications

A radome cover will protect an antenna from outdoor elements and certain weather conditions. Attenuation from the materials that the radome covers are constructed of will be minimal. They mainly protect the antenna from the collection of elements such as snow and hail.

Azimuth and elevation charts are also available for patch/panel antennas. Figure 7.11 shows the charts for the 2.4 GHz 8 dBi flat patch antenna.

Figure 7.11 Vertical (elevation) and horizontal (azimuth) charts for 2.4 GHz 8 dBi patch antenna

Image provided by www.L-com.com.

Sector Antennas

Sector antennas can be used to create omnidirectional radiation patterns using semidirectional antennas. These antennas are often used for base station connectivity for point-to-multipoint connectivity. Sector antennas have an azimuth that varies from 90° to 180°. These are typically configured to offer a total azimuth of 360°. For example, using sector antennas with an azimuth of 120° each would require three antennas in order to get omnidirectional or 360° coverage. This is a common configuration used with cellular phone technology. Figure 7.12 shows a sector panel antenna.

Figure 7.12 2.4 GHz 14 dBi 90° sector panel antenna

Image provided by www.L-com.com.

Sector Antenna Specifications

As mentioned earlier, sector antennas are commonly configured in an array to allow semidirectional antennas to provide omnidirectional coverage. This is useful in a campus environment or community arrangement to provide wireless LAN access such as Internet access. Sector antennas will usually have wide horizontal beamwidth (azimuth) and a narrow vertical beamwidth (elevation). Table 7.3 is an example of a specification sheet for a 2.4 GHz 14 dBi 90° sector panel WLAN antenna.

Table 7.3 90° sector panel WLAN antenna specifications

Figure 7.13 shows the charts for the 2.4 GHz 14 dBi 90° sector antenna.

Figure 7.13 Vertical (elevation) and horizontal (azimuth) charts for 2.4 GHz 14 dBi 90° sector panel antenna

Image provided by www.L-com.com.

Yagi Antennas

Yagi antennas are designed to be used indoors in long hallways and corridors, or outdoors for short-range bridging (typically less than two miles). Yagi antennas have vertical and horizontal beamwidths ranging from 25° to 65°. The radiation pattern may look like a funnel or a cone. As the signal propagates away from the antenna, the RF coverage naturally widens (diffusion). The aperture of the receiving antenna is much narrower than the signal at that point. This is a result of diffusion, which is the biggest form of loss in an RF link. Figure 7.14 shows a Yagi antenna.

Figure 7.14 2.4 GHz 15 dBi Yagi antenna

Image provided by www.L-com.com.

Yagi Antenna Specifications

Table 7.4 is an example of a specification sheet for a 2.4 GHz 15 dBi Yagi WLAN antenna.

Table 7.4 15 dBi Yagi antenna specifications

Figure 7.15 shows the charts for the 2.4 GHz 14 dBi Yagi antenna.

Figure 7.15 Vertical (elevation) and horizontal (azimuth) charts for 2.4 GHz 14 dBi Yagi antenna

Image provided by www.L-com.com.

Highly Directional Antennas

Highly directional antennas are typically parabolic dish antennas used for long-range point-to-point bridge connections. These antennas are available with a solid reflector or a grid. Some manufacturers of parabolic dish antennas advertise ranges of 25 miles or more depending on the gain and the environmental conditions. Parabolic dish antennas have very narrow horizontal and vertical beamwidths. This beamwidth can range from 3° to 15° and has a radiation pattern similar to that of a Yagi, with the appearance of a funnel. The beamwidth starts very narrow at the antenna element and naturally widens because of diffusion. Because these antennas are designed for outdoor use, they will need to be manufactured to withstand certain environmental conditions, including a wind rating and appropriate mounting. Grid antennas can provide similar coverage and are less susceptible to wind loading. Figure 7.16 shows a parabolic dish antenna.

Figure 7.16 5 GHz 28.5 dBi parabolic dish antenna

Image provided by www.L-com.com.

Highly Directional Antenna Specifications

Table 7.5 is an example of a specification sheet for a 2.4 GHz 30 dBi grid parabolic dish antenna. Notice that the vertical and horizontal beamwidths of this antenna are 5.3°, very narrow compared to other antenna types.

Table 7.5 2.4 GHz 30 dBi grid parabolic dish antenna specifications

Figure 7.17 shows the charts for the 15 2.4 GHz 30 dBi grid parabolic dish antenna.

Figure 7.17 Vertical (elevation) and horizontal (azimuth) charts for 2.4 GHz 30 dBi grid antenna

Image provided by www.L-com.com.

Radio Frequency Cables and Connectors

Radio frequency cables play a role in various wireless LAN deployment situations. For example, cables may be used to connect access points and client devices to antennas or to connect other devices that may used in wireless networking. Several factors need to be taken into consideration when using cables in a wireless LAN system, including these:

• Type of cable
• Length of cable
• Cost of cable
• Impedance rating

Choosing the correct cable for use in wireless LAN systems is an important part of a successful wireless LAN deployment. The right cable for the right job will help ensure that signal loss—a decrease in signal strength—is minimized and performance is maximized.

Radio Frequency Cable Types

The type of cable used will depend on the application. Many systems use cables to extend from the wireless device such as an access point to an antenna located outside of a building. It is important to choose the correct type of cable in order to optimize the performance of the wireless LAN system. Cables vary in diameter, and the application will determine the type of cable to use. For example, connecting a wireless LAN adapter on a notebook computer to an external antenna requires a specific type of cable that should be short and flexible. Thick, rigid cables are best used for longer runs. The radio frequency range in which the cable will be used also is important to consider. Where the cable is used will determine the radio frequency rating of the cable. For example, wireless LANs use 50 ohm cable, whereas television (such as satellite and cable) will use 75 ohm cable. Using cable with the correct rating will minimize voltage standing wave ratio (VSWR), a phenomenon discussed in the section “Impedance and VSWR.” Figure 7.18 shows a spool of high-quality 50 ohm cable.

Figure 7.18 L-com spool of low-loss 400-series coaxial bulk cable

Image provided by www.L-com.com.

Radio Frequency Cable Length

The length of a cable used in a wireless LAN system is another factor to consider. A cable of even a very short length will have some level of attenuation or loss. As a reminder, loss is a decrease in signal strength. This decrease in signal strength means less overall performance and throughput for users of the wireless LAN. Professionally manufactured cables typically are available in many standard common lengths. Best practices recommend using the correct length and minimizing connections. For example, if a run from an access point to an external antenna is 27', it would be best to use a single cable as close to that length as possible. Connecting two or more pieces of cable together will increase the loss to the system. One might be tempted to use a longer piece, such as 50', but this is not recommended since the extra length will add loss to the system.

Figure 7.19 shows a short length of cable known as a pigtail used to connect a standard cable to a proprietary cable. If an RF cable is used or extended, the attenuation that is introduced can be offset with the use of an amplifier or with a higher-gain antenna. An amplifier will provide active gain and an antenna will provide passive gain. Keep in mind that using an incorrect amplifier may void the system certification and that using a higher-gain antenna may exceed the rules set by the local RF regulatory agency.

Figure 7.19 Short pigtail adapter cable

Image provided by www.L-com.com.

Radio Frequency Cable Cost

Cable cost may also play a role in the type of cable to be used. The old saying “you get what you pay for” is true with cables as well. I recommend using high-quality name-brand RF cables to optimize the performance of your system. Premium cables may come at a higher price, but the benefit of better quality signal is the main advantage.

Impedance and VSWR

Impedance is the measurement of alternating current (AC) resistance. It is normal to have some level of impedance mismatch in a wireless LAN system, but the impedances of all components should be matched as closely as possible in order to optimize performance of the system. Impedance mismatches can result in what is called voltage standing wave ratio (VSWR). A large impedance mismatch can cause high level of VSWR and will have an impact on the wireless LAN system and transmitted or received signal.

Electrical resistance is measured in ohms. IEEE 802.11 wireless LAN devices have an impedance of 50 ohms.

Radio Frequency Connectors

In a wireless LAN system, radio frequency connectors are used to join devices together, allowing the RF signal to transfer between the devices. These devices may connect access point to antenna, antenna to cable, cable to cable, or various other components to each other. RF connectors also cause an impedance mismatch to some degree and increase the level of VSWR. To minimize the effects of VSWR, best practices suggest keeping the use of connectors to a minimum. Using connectors can also result in insertion loss. Insertion loss is usually minor by itself, but it can contribute to overall loss in a system, thereby resulting in less RF signal and less throughput.

Standard RF connectors may be used in wireless LAN systems to connect devices that are not part of the point connecting to the antenna. For example, an access point connecting to a length of cable that is then connected to an amplifier could use a standard RF connector. The cable connecting the amplifier to the antenna would require a proprietary connector. Figure 7.20 shows examples of common RF connectors.

Figure 7.20 Several common RF connectors used with wireless LANs

Image provided by www.L-com.com.

Factors in Antenna Installation

Several factors are important to consider when you are planning to install a wireless network. These include earth curvature, multipath, and radio frequency line of sight. This section includes information about how to take these factors into account when planning a wireless installation.

Addressing the Effects of Earth Curvature

Beyond seven miles, the curvature of the earth will have an impact on point-to-point or point-to-multipoint wireless LAN connections. Therefore, it is important to add height to the antenna in order to compensate for the earth curvature, sometimes referred to as earth bulge. A formula is used to calculate the additional height of antennas when a link exceeds seven miles. However, this is beyond the scope of the CWTS exam objectives and is not discussed in this book.

Antenna Placement

The installation location and placement of antennas depend on the type of antenna and application in which it will be used. When installing antennas, consider the placement based on the design of the wireless LAN and the intended use of the antenna. When antennas are used outdoors, lightning arrestors, grounding, and adherence to local codes, laws, and government regulations must be followed as well as good RF design. Increasingly, local ordinances dictate how or if outdoor antennas can be mounted for looks as well as safety. Lightning arrestors and grounding methods are discussed later in this section.

Omnidirectional Antenna Placement

Placement of an omnidirectional antenna will depend on the intended use. Some omnidirectional antennas can be connected directly to an access point or may be integrated within the access point. In this configuration, the installation is fairly straightforward; it involves simply attaching the antenna to the access point or using the integrated antenna. Omnidirectional antennas are usually placed in the center of the intended coverage area. High-gain omnidirectional antennas are typically used in outdoor installations for point-to-multipoint configurations. This configuration is more complex because more than likely it requires mast or tower mounting. The exact placement depends on the intended coverage area as well as the gain of the antenna.

Semidirectional Antenna Placement

Semidirectional antennas may be used for either outdoor or indoor installations. When mounted indoors, a patch/panel antenna typically will be mounted flat on a wall with the connector upward for connections to a cable or directly to an access point. A template with the hole placement may be included for ease of installation. These antennas usually will use four mounting holes (one in each corner) to securely fasten the antenna to the wall. Yagi antennas can also be mounted either indoors or outdoors. The most common installation is outdoors for short-range point-to-point or point-to-multipoint bridging solutions. This will require a mounting bracket such as a tilt and swivel for wall mounting or U-bolts and plate for mast or pole mounting.

Highly Directional Antenna Placement

Highly directional antennas such as a parabolic dish are almost always used exclusively in outdoor installations. This type of antenna is used mostly for long-range point-to-point bridging links and will require installation on building rooftops or antenna towers. Alignment for long-range links is critical for reliable communications. Software and hardware tools are available for the installer to use for accurate alignment. As with other outdoor installations, secure mounting is essential in order to maintain safety and link reliability.

Minimizing the Effects of Multipath Using Antenna Diversity

In Chapter 6, we discussed some of the behaviors of radio frequency, including reflection, refraction, scattering, and diffraction. To review, reflection is caused by an RF signal bouncing off a smooth, nonabsorptive surface and changing direction. Indoor environments are areas that are prone to reflections. Reflections are caused by the RF signal bouncing off walls, ceilings, floors, and furniture; thus some installations will suffer from reflection more than others. The effect of reflection will be a decrease in signal strength due to a phenomenon called multipath. Multipath is the result of several wavefronts of the same transmission signal received out of phase at slightly different times. This can cause the receiver to be confused about the received signals. The result is corrupted signal and less overall throughput. Figure 7.21 illustrates multipath.

Figure 7.21 Effects of multipath

Antenna diversity is one way to help reduce the effects of multipath. Antenna diversity is a technology used in wireless LANs where a station (access point or client device) will utilize two antennas combined with one radio to decrease the effects of multipath. Using multiple antennas and some additional electronic intelligence, the receiver will be able to determine which antenna will receive and send the best signal. In diversity systems, two antennas are spaced at least one wavelength apart. This allows the receiver to use the antenna with the best signal to transmit and receive. With respect to radio frequency diversity, the antennas are required to be of the same design, frequency, gain, and so on.

Combating Effects of Wind and Lightning in WLAN Installations

Weather conditions such as rain, snow, and sleet typically do not affect wireless LAN communications unless the conditions are extreme or snow and sleet collect on antenna elements. However, some weather conditions that can affect wireless communication are wind and lightning.

Most outdoor antennas that can be affected by wind will have wind-loading data in the specification sheet. Wind loading is the result of wind blowing at high speeds and causing the antenna to move.

Lightning can destroy components connected to a network if the antenna takes either a direct or an indirect lightning strike. A properly grounded lightning arrestor will help protect wireless LAN and other networking equipment from indirect lightning strikes.

Lightning Arrestors

Transient or induced electrical currents are the result of an indirect lightning strike in the area of a wireless LAN antenna system. Lightning arrestors are an in-series device installed after the antenna and prior to the transmitter/receiver. Although this device will not provide protection from a direct lightning strike, it will help protect against an indirect lightning strike, which can damage electronics at distances away from the source of the strike. When the induced electrical currents from a lightning strike travel to the antenna, a lightning arrestor will shunt this excess current to ground, protecting the system from damage. Figure 7.22 shows a lightning arrestor.

Figure 7.22 L-com AL6 series 0-6 GHz coaxial lightning and surge protector

Image provided by www.L-com.com.

Grounding Rods

A grounding rod is a metal shaft used for grounding a device such as an antenna used in wireless networking. The rod should be driven into the ground at least 8’ deep. Grounding rods are available in various types of steel, including stainless, galvanized, and copper clad. They are also available in a variety of diameters and lengths. Depending on the local electrical code, the grounding system should measure resistance between 5 and 25 ohms. Local code should also be consulted regarding material, diameter, and length of grounding rods. You should not share grounding rods with other equipment because interference or damage may occur.

Installation Safety

Professional contractors should be considered in the event you are not comfortable with performing the installation of a wireless LAN antenna yourself. Installing antennas may require bonded or certified technicians. Be sure to check local building codes prior to performing any installation of a wireless LAN antenna. Never underestimate safety when installing or mounting antennas. All safety precautions must be adhered to while performing an installation. The following are some general guidelines and precautions to be considered for a wireless LAN antenna installation:

• Read the installation manual from the manufacturer.
• Always avoid power lines. Contact with power lines can result in death.
• Always use the correct safety equipment when working at heights.
• Correctly install and use grounding rods when appropriate.
• Comply with regulations for use in the area and for use of towers as well.

Antenna Mounting

In addition to choosing the correct antenna to be used with a wireless LAN system, you must take into account the antenna mounting. The required antenna mounting fixture will depend on the antenna type, whether it will be used indoors or outdoors, and whether it will be used for device/client access or bridging solutions such as point-to-point or point-to-multipoint. It is best to consult with the antenna or device manufacturer to determine which mounting fixture is appropriate for use based on the intended deployment scenario. The following are several mounting types that may be used for a wireless LAN antenna solution:

• Internal and external (to the AP) antennas
• Pole/mast mount
• Ceiling mount
• Wall mount

Internal and External (to the Access Point) Antennas

Some wireless access points allow the use of either integrated or external antennas. Most modern controller-based and cooperative access points provide integrated antennas, and some have connectors to allow for the use of external antennas. Integrated antennas may be a better solution, mostly to match the aesthetics requirements of the organization where the access point will be used. Integrated antennas make an access point less noticeable if it is hanging from a standard 8'−10’ high ceiling. Integrating them will also prevent individuals from tampering with antennas that if external might be within a person's reach. Tampering could be an issue in areas that are accessible to the general public, such as installations in educational, library, and medical environments.

It is important to take the mounting orientation into consideration when using access points with integrated antennas. The access point may be designed to be mounted on a ceiling. If you were to mount this type of access point on a wall, the radiation patterns would change, and this might create a coverage problem, especially with omnidirectional antennas.

One disadvantage of access points with integrated antennas is that the antennas cannot be changed to any other type. Basically you would be committed to using the antennas that are part of the access point and would be unable to add an antenna with higher gain or a different radiation pattern. Access points that have external antenna connectors allow the user to add different antennas that may be better suited to the environment or installation location. The external antennas will allow for a higher gain and possibly a different radiation pattern. Using an external antenna usually requires a software configuration that will disable the integrated antenna when external antennas are installed. You should not be able to use both the integrated and external antennas simultaneously. Many indoor access point models that use internal antennas offer the same coverage as their external antenna model counterparts This, however, will vary based on the manufacturer.

Pole/Mast Mount

Pole/mast mounts typically consist of a mounting bracket and U-bolt mounting hardware. The mounting bracket is commonly L-shaped. One side of the bracket has a hole to mount an omnidirectional or similar antenna. The other side of the bracket has predrilled holes for fastening the bracket to a pole using U-bolts. Figure 7.23 shows an example of a heavy-duty mast mount.

Figure 7.23 Heavy-duty mast mount with U-bolts

Image provided by www.L-com.com.

Exercise 7.2 describes the basic steps for installing an omnidirectional antenna using a mast mount adapter.

Ceiling Mount

It may be necessary to mount certain antennas or access points with attached or integrated antennas from a ceiling. Many antennas can be mounted directly to a hard ceiling made from concrete, drywall, or similar material. Another possibility is a drop ceiling with acoustic tiles. Regardless of the type of ceiling in question, follow the manufacturer's instructions on the appropriate fixture to be used for mounting and detailed instructions for ceiling mounts. Figure 7.24 shows an example of a ceiling mount antenna.

Figure 7.24 L-com 2.3 GHz to 6 GHz 3 dBi omnidirectional ceiling mount antenna

Image provided by www.L-com.com.

Wall Mount

Antennas or access points with attached antennas may need to be mounted to a wall based on the use or site survey results. Just as with a ceiling mount, follow the manufacturer's instructions on the appropriate fixture for wall mounting. When mounting an antenna to the wall, consider the polarization of the antenna. Keep in mind that some antennas are designed to be mounted on the ceiling; these types should not be mounted on a wall. This is especially true for access points with integrated antennas. It is best to try to match the polarization of the access points and the wireless client devices. In other words, if the access point's antennas are vertically polarized, the wireless client devices should be polarized in the same manner to promote better connectivity. However, with the wide variety of newer wireless client devices available this is getting harder to achieve. Choosing the correct antenna and mounting position is typically part of a wireless LAN site survey. Site surveys will be discussed in more detail in Chapter 11.

Maintaining Clear Communications

Several factors affect whether two wireless devices can communicate with each other. These factors include line of sight (both visual and RF) and Fresnel zone. Indoor wireless LAN installations use a low amount of radio frequency transmit power, usually around 30 mW to 50 mW, and will be able to communicate effectively even if the client device does not have a line of sight with an access point. This is because the RF will be able to penetrate obstacles such as walls, windows, and doors. Outdoor installations usually use a much higher output transmit power and will require an RF line of sight for effective communication.

Visual Line of Sight

Visual line of sight (LoS) is defined as the capability of two points to have an unobstructed view of one another. A visual LoS is usually not necessary for communications using IEEE 802.11 wireless LAN systems; it is implied with RF LoS. If a wireless LAN engineer was planning to connect two buildings together using wireless LAN technology, one of the first things the engineer would do is to verify that there is a clear, unobstructed view between the planned locations in order to provide an RF LoS.

Radio Frequency LoS

For two devices to successfully communicate at a distance via radio frequency, including a point-to-point or point-to-multipoint connection, a clear path for the RF energy to travel between the two points is necessary. This clear path is called RF LoS. This RF LoS is the premise of the Fresnel zone.

Fresnel Zone

The Fresnel zone for an RF signal is the area of radio frequency coverage surrounding the visual LoS. The width or area of the Fresnel zone will depend on the specific radio frequency used as well as the length or distance of the signal path. There is a formula used to calculate the width of the Fresnel zone at the widest point. However, that is beyond the scope of the CWTS exam objectives and is not shown in this book.

In an outdoor point-to-point or point-to-multipoint installation, it is important for the Fresnel zone to be clear of obstructions for successful communications to take place between a radio frequency transmitter and receiver. Best practices recommend maintaining an obstruction-free clearance of the least 60 percent for the Fresnel zone in order to have acceptable RF LoS. Maintaining a clear RF LoS becomes more difficult as the distance between two points increases. Obstructions can cause the Fresnel zone to be blocked enough for communications to suffer between a transmitter and receiver. Such obstructions include:

• Trees
• Buildings or other structures
• Earth curvature
• Natural elements such as hills and mountains

Figure 7.25 illustrates the Fresnel zone between two highly directional antennas.

Figure 7.25 Visualization of Fresnel zone

In order to stay clear of obstructions, carefully plan your antenna placement and antenna height. Keep in mind that a wireless LAN link may cross public areas in which an integrator or installer will have no control over the environment. There is a possibility, depending on the environmental conditions, that an IEEE 802.11 wireless LAN link may not be a feasible solution due to the inability to maintain an RF LoS. You should perform an outdoor site survey prior to the procurement and installation of wireless LAN hardware to ensure the installation and operation of the wireless LAN will be successful.

Summary

Antennas are a critical component in a successful operation of a wireless LAN. In this chapter, you learned about radio frequency signal characteristics and basic RF antenna concepts, including these:

• Radio frequency lobes
• Beamwidth (horizontal and vertical measurements)
• Passive and active gain
• Horizontal and vertical polarization of antennas

By understanding these characteristics and concepts, a sales engineer, integrator, or other wireless LAN professional can help choose the best antenna to be used for a specific use.

Understanding the radio frequency propagation patterns of various antenna types as well as the recommended use of an antenna will assist in deciding which antenna is best suited for the desired application. As discussed in this chapter, antennas are available in various types:

• Omnidirectional
• Semidirectional
  • Patch/panel
  • Sector
  • Yagi
• Highly directional
  • Parabolic dish

Omnidirectional antennas are one of the most common types of antenna used for indoor applications of wireless networking. Omnidirectional antennas provide a horizontal radiation pattern of 360°. Other antennas such as patch/panel, Yagi, or parabolic dish can be used if justified by the intended use. You learned about the radiation patterns of each of these types of antennas as well as how each may be used.

A proper mounting fixture is required to ensure safety and correct operation of the antenna and wireless network. In this chapter we looked at various methods for mounting antennas, including integrated and external (to the access point), pole/mast mount, ceiling mount, and wall mount.

Finally, you learned about other factors to be considered when choosing and installing an antenna for use with wireless LANs. The other areas of concerns are:

• Visual line of sight
• Radio frequency line of sight
• Fresnel zone

Understanding these concepts will help you achieve more successful deployment, operation, and use of antennas in wireless LANs.

Exam Essentials

Review Questions

1. Omnidirectional antennas have a horizontal beamwidth of degrees.

A. 90

B. 180

C. 270

D. 360

2. Antennas provide an increase in RF coverage by using gain.

A. Active

B. Passive

C. Positive

D. Maximum

3. Horizontal beamwidth is to the earth's surface.

A. Parallel

B. Perpendicular

C. Positive

D. Negative

4. An access point requires antennas for diversity functionality.

A. One

B. Two

C. Three

D. Six

5. What device is used to shunt transient current to ground in the event of an indirect lightning strike?

A. Lightning striker

B. Lightning arrestor

C. Lightning prevention

D. Lightning breaker

6. Amplifiers provide an increase in signal strength by using gain.

A. Active

B. Passive

C. Positive

D. Maximum

7. Highly directional antennas are typically used for connectivity.

A. Short-range

B. Omnidirectional

C. Long-range

D. Dipole

8. You are a network engineer. While moving a handheld wireless LAN device, you notice that the signal strength increases when the device is moved from a horizontal to a vertical position. This is because the is changing.

A. Polarization

B. Wavelength

C. Frequency

D. Diffusion

9. RF line of sight is required for what type of IEEE 802.11 WLAN installation? (Choose two.)

A. Point-to-point

B. Scattered

C. Point-to-multipoint

D. Reflected

E. Refracted

10. Which can cause a loss of signal strength? (Choose two.)

A. Antenna

B. Amplifier

C. Cable

D. Connector

E. Transmitter

11. An IEEE 802.11g access point requires a minimum of how many antennas to move data?

A. One

B. Two

C. Four

D. Six

12. 802.11a access points support which antenna technology to help reduce the effects of multipath?

A. Adjustable gain

B. Antenna diversity

C. Adjustable polarization

D. Antenna multiplexing

13. The following graphic shows what type of antenna?

Image provided by www.L-com.com

A. Omnidirectional

B. Yagi

C. Patch/panel

D. Parabolic dish

14. Which weather element would commonly have an effect on a wireless LAN system?

A. Rain

B. Snow

C. Wind

D. Hail

15. Wireless network cables and devices have impedance (AC resistance) of ohms.

A. 10

B. 25

C. 50

D. 75

16. The curvature of the earth will have an impact on the wireless LAN signal after how many miles?

A. 2

B. 7

C. 10

D. 25

17. A patch antenna is an example of what type of antenna?

A. Semidirectional

B. Omnidirectional

C. Highly directional

D. Dipole-directional

18. An azimuth chart shows which RF radiation pattern?

A. Vertical

B. Horizontal

C. Positive

D. Negative

19. A point-to-point wireless link requires what percent of the Fresnel zone to be clear in order to be considered to have an acceptable RF line of sight?

A. 0

B. 20

C. 40

D. 60

20. The image below shows what type of antenna?

Image provided by www.L-com.com

A. Highly directional

B. Dipole-directional

C. Omnidirectional

D. Semidirectional