4 Signal and Antenna
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✓ Multiple-input multiple-output (MIMO)
■ MIMO antennas
■ Indoor MIMO antennas
■ Outdoor MIMO antennas
✓ Antenna connection and installation
■ Voltage standing wave ratio (VSWR)
■ Signal loss
■ Antenna mounting
■ Placement
■ Outdoor mounting considerations
■ Indoor mounting considerations
■ Appropriate use and environment
■ Ingress Protection Rating
■ NEMA Enclosure Rating
■ ATEX directives
■ National Electrical Code (NEC)
■ Orientation and alignment
■ Safety
■ Maintenance
✓ Antenna accessories
■ Cables
■ Connectors
■ Splitters
■ Amplifiers
■ Attenuators
■ Lightning arrestors
■ Grounding rods and wires
✓ Regulatory Compliance
received and understood by the receiver. The installation of antennas has the greatest abil- ity to affect whether or not the communication is successful. Antenna installation can be as simple as placing an access point in the middle of a small offi ce to provide full coverage for your company, or it can be as complex as installing an assortment of directional antennas, kind of like piecing together a jigsaw puzzle. Do not fear this process; with proper under- standing of antennas and how they function, you may fi nd successfully planning for and installing antennas in a wireless network to be a skillful and rewarding task.
This chapter focuses on the categories and types of antennas and the different ways that they can direct an RF signal. Choosing and installing antennas is like choosing and installing lighting in a home. When installing home lighting, you have many choices: table lamps, ceiling lighting, narrow- or wide-beam directional spotlights. In Chapter 3, “Radio Frequency Components, Measurements, and Mathematics,” you were introduced to the concept of antennas focusing RF signal. In this chapter, you will learn about the various types of antennas, their radiation patterns, and how to use the different antennas in dif- ferent environments. You will also learn that the installation and alignment of omnidi- rectional antennas should vary depending on whether the access point supports 802.11n, 802.11ac, or legacy physical layer technologies.
You will also learn that even though we often use light to explain RF radiation, dif- ferences exist between the way the two behave. You will learn about aiming and aligning antennas, and you will learn that what you see is not necessarily what you will get.
In addition to learning about antennas, you will learn about the accessories that may be needed for proper antenna installation. In offi ce environments, you may simply need to connect the antenna to the access point. In outdoor installations, you will need special cable and connectors, lightning arrestors, and special mounting brackets. In this chapter, we will introduce you to the components necessary for successfully installing an antenna.
To summarize, in this chapter you will gain the knowledge that will enable you to prop- erly select, install, and align antennas. These skills will help you successfully implement a wireless network, whether it is a point-to-point network between two buildings or a net- work providing wireless coverage throughout an offi ce building.
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Azimuth and Elevation Charts (Antenna Radiation Envelopes)
There are many types of antennas designed for many different purposes, just as there are many types of lights designed for many different purposes. When purchasing lighting for your home, it is easy to compare two lamps by turning them on and looking at the way each disperses the light. Unfortunately, it is not possible to compare antennas in the same way.
Actual side-by-side comparison of antennas requires you to walk around the antenna with an RF meter, take numerous signal measurements, and then plot the measurements either on the ground or on a piece of paper that represents the environment. Besides the fact that this is a time-consuming task, the results could be skewed by outside infl uences on the RF signal, such as furniture or other RF signals in the area. To assist potential buyers with their purchasing decision, antenna manufacturers create azimuth charts and elevation charts, commonly known as radiation patterns, for their antennas. These radiation patterns are created in controlled environments where the results cannot be skewed by outside infl u- ences and represent the signal pattern that is radiated by a particular model of antenna.
These charts are commonly known as polar charts or antenna radiation envelopes.
Figure 4.1 shows the azimuth and elevation charts of an omnidirectional antenna. The azimuth chart, labeled H-plane, shows the top-down view of the radiation pattern of the antenna. Since this is an omnidirectional antenna, as you can see from the azimuth chart, its radiation pattern is almost perfectly circular. The elevation chart, labeled E-plane, shows the side view of the radiation pattern of the antenna. There is no standard that requires the antenna manufacturers to align the degree marks of the chart with the direc- tion that the antenna is facing, so unfortunately it is up to the reader of the chart to under- stand and interpret it.
Here are a few statements that will help you interpret the radiation charts:
■ In either chart, the antenna is placed at the center of the chart.
■ Azimuth chart = H-plane = top-down view
■ Elevation chart = E-plane = side view
The outer ring of the chart usually represents the strongest signal of the antenna. The chart does not represent distance or any level of power or strength. It represents only the relationship of power between different points on the chart.
One way to think of the chart is to consider the way a shadow behaves. If you were to move a fl ashlight closer or farther from your hand, the shadow of your hand would grow larger or smaller. The shadow does not represent the size of the hand. The shadow repre- sents the relative shape of the hand. Whether the shadow is large or small, the shape and pattern of the shadow of the hand is identical. With an antenna, the radiation pattern will grow larger or smaller depending on how much power the antenna receives, but the shape and the relationships represented by the patterns will always stay the same.
F I G U R E 4 .1 Azimuth and elevation charts 0°
30°
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10 20
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S2403BPX 2450 MHz Azimuth/H-plane
0°
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Interpreting Polar Charts
As we stated, the antenna azimuth (H-plane) and elevation (E-plane) charts are commonly referred to as polar charts. These charts are often misinterpreted and misread. One of the biggest reasons these charts are misinterpreted is that they represent the decibel (dB) map- ping of the antenna coverage. This dB mapping represents the radiation pattern of the antenna; however, it does this using a logarithmic scale instead of a linear scale. Remember that the logarithmic scale is a variable scale, based on exponential values, so the polar chart is actually a visual representation using a variable scale.
Take a look at Figure 4.2. The numbers inside the four boxes in the upper-left corner tell you how long and wide each box is. So, even though visually in our drawing we represented the boxes as the same size, in reality each one is twice as long and wide as the previous one. It is easier to draw the four boxes as the same physical size and just put the number in the box to represent the actual size of the box. In the middle drawing, we drew the boxes showing the relative size of the four boxes.
F I G U R E 4 . 2 Logarithmic/linear comparison 1
1 2 4 8
2 4 8
1 2 4 8 16 32 64 128 256 512
What if we had more boxes, say 10? By representing each box using the same-sized drawing, it is easier to illustrate the boxes, as shown with the boxes in the lower-left corner.
In this example, if we tried to show the actual differences in size, as we did in the middle of the drawing, we could not fi t this drawing on the page in the book. In fact, the room that you are in may not have enough space for you to even draw this. Because the scale changes so drastically, it is necessary to not draw the boxes to scale so that we can still represent the information.
In Chapter 3, you learned about RF math. In that chapter, one of the rules that you learned was the rule of 6 dB, which indicates that a 6 dB decrease of power decreases the distance the signal travels by half. A 10 dB decrease of power decreases the distance the signal travels by approximately 70 percent. In Figure 4.3, the left polar chart displays the logarithmic representation of the elevation chart of an omnidirectional antenna. This is what you are typically looking at on an antenna brochure or specifi cation sheet. Someone who is untrained in reading these charts would look at the chart and be impressed with how much vertical coverage the antenna provides but would likely be disappointed with the actual coverage. When reading the logarithmic chart, you must remember that for every 10 dB decrease from the peak signal, the actual distance decreases by 70 percent. Each con- centric circle on this logarithmic chart represents a change of 5 dB. Figure 4.3 shows the logarithmic pattern of an elevation chart of an omnidirectional antenna along with a linear representation of it’s coverage. Notice that the fi rst side lobe is about 10 dB weaker than the main lobe. Remember to compare where the lobes are relative to the concentric circles. This 10 dB decrease on the logarithmic chart is equal to a 70 percent decrease in range on the linear chart. Comparing both charts, you see that the side lobes on the logarithmic chart are essentially insignifi cant when adjusted to the linear chart. As you can see, this omnidi- rectional antenna has very little vertical coverage.
F I G U R E 4 . 3 Omnidirectional polar chart (E-plane)
Δ = −10 dB = 0.3 numeric dBi, logarithmic
−180
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−15 −30 15 0
30 45 60 75 90 105
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Normalized to max gain, linear 1−180
0.90.8 0.7 0.60.5 0.40.3 0.2 0
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30 45 60 75 90 105
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Δ
Image © Aruba Networks, Inc. All rights reserved. Used with permission.
To give you another comparison, Figure 4.4 shows the logarithmic pattern of the elevation chart of a directional antenna along with a linear representation of the vertical
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F I G U R E 4 . 4 Directional polar chart (E-plane)
0345 330 315 300 285 270 255 240 225 210 195180165150
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3015
0345 330 315 300 285 270 255 240 225 210 195180165150
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20 15 10 5 0 –5 –10 –15 –20 –25 –30 1
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
dBi, logarithmic Normalized to max gain, linear
Image © Aruba Networks, Inc. All rights reserved. Used with permission.
Beamwidth
Many fl ashlights have adjustable lenses, enabling the user to widen or tighten the concen- tration of light. RF antennas are capable of focusing the power that is radiating from them, but unlike fl ashlights, antennas are not adjustable. The user must decide how much focus is desired prior to the purchase of the antenna.
Beamwidth is the measurement of how broad or narrow the focus of an antenna is—and is measured both horizontally and vertically. It is the measurement from the center, or stron- gest point, of the antenna signal to each of the points along the horizontal and vertical axes where the signal decreases by half power (–3 dB), as seen in Figure 4.5. These –3 dB points are often referred to as half-power points. The distance between the two half-power points on the horizontal axis is measured in degrees, giving the horizontal beamwidth mea- surement. The distance between the two half-power points on the vertical axis is also measured in degrees, giving the vertical beamwidth measurement.
Most of the time when you are deciding which antenna will address your communica- tions needs, you will look at the manufacturer’s spec sheets to determine the technical specifi cations of the antenna. In these brochures, the manufacturer typically includes the numerical values for the horizontal and vertical beamwidths of the antenna. It is important for you to understand how these numbers are calculated. Figure 4.6 illustrates the process.
F I G U R E 4 . 5 Antenna beamwidth
–3 dB
–3 dB
–3 dB –3 dB
F I G U R E 4 . 6 Beamwidth calculation 1
3 2 2 3
–10 –20 –30
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1. First determine the scale of the polar chart.
On this chart, you can see that the solid circles represent the –10 dB, –20 dB, and –30 dB lines and the dotted circles therefore represent the –5 dB, –15 dB, and –25 dB lines. These represent the dB decrease from the peak signal.
2. To determine the beamwidth of this antenna, first locate the point on the chart where the antenna signal is the strongest.
In this example, the signal is strongest where the number 1 arrow is pointing.
3. Move along the antenna pattern away from the peak signal (as shown by the two num- ber 2 arrows) until you reach the point where the antenna pattern is 3 dB closer to the center of the diagram (as shown by the two number 3 arrows).
This is why you needed to know the scale of the chart fi rst.
4. Draw a line from each of these points to the middle of the polar chart (as shown by the dark dotted lines).
5. Measure the distance in degrees between these lines to calculate the beamwidth of the antenna.
In this example, the beamwidth of this antenna is about 28 degrees.
It is important to realize that even though the majority of the RF signal that is gener- ated is focused within the beamwidth of the antenna, a signifi cant amount of signal can still radiate from outside the beamwidth, from what is known as the antenna’s side or rear lobes. As you look at the azimuth charts of different antennas, you will notice that some of these side and rear lobes are fairly signifi cant. Although the signal of these lobes is drasti- cally less than the signal of the main beamwidth, they are dependable, and in certain imple- mentations very functional. It is important when aligning point-to-point antennas that you make sure they are actually aligned to the main lobe and not a side lobe.
Table 4.1 shows the types of antennas that are used in 802.11 communications.
Table 4.1 provides reference information that will be useful as you learn about various types of antennas in this chapter.
TA B L E 4 .1 Antenna beamwidth
Antenna types Horizontal beamwidth (in degrees) Vertical beamwidth (in degrees)
Omnidirectional 360 7 to 80
Patch/panel 30 to 180 6 to 90
Antenna types Horizontal beamwidth (in degrees) Vertical beamwidth (in degrees)
Yagi 30 to 78 14 to 64
Sector 60 to 180 7 to 17
Parabolic dish 4 to 25 4 to 21
Antenna Types
There are three main categories of antennas:
Omnidirectional:Omnidirectional antennas radiate RF in a fashion similar to the way a table or fl oor lamp radiates light. They are designed to provide general coverage in all directions.
Semidirectional: Semidirectional antennas radiate RF in a fashion similar to the way a wall sconce radiates light away from the wall or the way a street lamp shines 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 focuses light on a fl ag or a sign.
Each type of antenna is designed with a different objective in mind.
It is important to keep in mind that this section is discussing types of antennas and not lighting. Although it is useful to refer to lighting to pro- vide analogies to antennas, it is critical to remember that unlike lighting, RF signals can travel through solid objects such as walls and floors.
In addition to antennas acting as radiators and focusing signals that are being transmit- ted, they 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 operate in the same way, enabling you to not only watch the action of your favorite sport on television but to also hear the action.
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Although it is not a matter of critical importance, many are often curious whether the plural of antenna is antennas or antennae. The simple answer is both, but the complete answer is it depends. When antenna is used as a biological term, the plural is antennae, such as the antennae of a bug. When it is used as an electronics term, the plural is anten- nas, such as the antennas on an access point.