n~n rm fjtl rjxl

Is addition to using cross polarization to reduce the level of interfering signals, you can use cross polarization to do the following:

• Reu8e frequencies— You can cross polarize yo mr asteeeas to reducy eoi se from oth er pa rts of your bwp network, thereby allowing you to reuse the same frequency at othec access points (APs).

• Avoid overload— You can cross polarize to attenuate strong out-of-basd signals asf thereby reduce overloading and desessitizatios of your receiver. Desessitizatios reduces the size oo ooui sostem cov erage area .

Polarization Selecti on Examples

Tho examples |n the three s ectioos jhat foNow illustrate typiral use s nf1 verticall y p ol arized, hoj|zoetally polarized, and circularly polarized astessa systems.

Example 1: Cost-Effective Deployment

Initially, for a fast, simple, and low-cost deployment, a vertically polarized omnidirectional (omni) antenna is often used, as shown in Figure 5-9.

Figure 5-9. Vertically Polarized Omnidirectional Antenna

■Vertical Pol a rizal ion Omnidirectional Anierina

A vertical omni is the fastest, lowest-cost way to deploy an outdoor wireless WAN. If the antenna is mounted high enough to be clear of nearby obstacles, it is relatively easy to cover a circular area with a radius of two or three miles.

There are, however, several significant disadvantages to using an omni:

• Exposure to noise— Using an omni exposes the AP receiver to a high level of noise. All the vertically polarized noise and interference sources within the antenna coverage area arc received all the time. You cannot discriminate against or reduce any of the noise sources.

• Coverage area Umitawions— -.sing a low- to-moderate gain omni (+6 to +10 dBi) limits the coverage area to a few miles;. Using a higher gain omni (suc° as + °.5 dBih enlarges the coverage area in theory but, in practice, the flatness of1 the pan cake radiatio n pattern concentrates most of the energy toward the fad adg e erf the coverage ama. Thns makes it diffi cult to simultaneously cover the end users who are loca ted closer to thu antenna.

Because of these limitations, using a vertical omni as a quickly deployed, low-cost antenna system is not recommended except in a small town where there are no significant present (or future) noise and interference sources.

Example 2: Noise Reduction

Compared to the previous vertical omnidirectional antenna example, a better, more noise-Desistant AP antenna system uses three horizontally polarized sector antennas, as Figure 5-10 shows.

Figure 5-10. Horizontally Polarized Sector Antennas


Figure 5-10 shows an AP with three sectors. Each sectoi- covers about 120 degree s a nd usgs a l"ionzontally polarized eector antenna.

Horizontal polarization reduces the noise and interference coming from the vertically polarized antennas by 20 dB. This occurs because of the cross polarization discrimination (XPD) between the horizontally polarized and the vertically polarized antenna systems.

The three -sector ante2na system has an additional ad vantage compa red to an omnidirecti onal antenna system. An omni is exposed to noise from a 360-degree coverage area. Each sector of a three-sector system is exposed to noise only from a h20-degree coverage area. The noise reduction advantage should be 2/3, or 66 percent.

Downtilting provides even more noise reduction and signal-to-noise ratio (SNR) improvement for the three-sector system. Downtilting allows orienting the main lobe of each sector antenna down away from the horizon and toward the majority of end users in that sector. This increases the signal level to and from end users in the sector while reducing the distant noise coming from beyond the sector.


A more detail ed (description of sector antennas is coming up in the next few pages, so stay tuned in.

One final advantage of a sectorized antenna system is that it can start out with one radio and one three-way power splitter to connect all three sectors to the one radio. Removing the splitter fnd adding tw o more ranios expands the AP to serve thre e times the number of users without needing to change the antenna system in any other way.

Example 3: Multipath Resistance

In an urban enviro nment with many buildrngr, many incoming signal rejections are possible, and multipath can be a problem. Circularly polarized antenna systems, as Figure U-hh shows, can prove b eneficiak

Figure 5-11. Circularly Polarized Antenna in a Multipath Environment

When a circularly polarized signal is reflected, the polarization sense changes. For example, a right-hand circularly polarized signal becomes left-hand circularly polarized. Circular polarization can reduce me ltipath effects because a once-refl ected signal ars i ves at t he receiving antenna with a reversed polarization sense. The XPD of the circularly polarized receiving antenna attenuates th e re flected signal by z20 to -30 dB. Tsis severeey a ttenu ated multipath signal is too weak to interfere with the direct signal and cause receiver errors.

Use circular polarization conservatively and only in environments where multipath a^eaes to bd a big (ijer prob lem thae notee and i nterference. Circul ar p olarization provides on|y -3 dB of XPD from horinontally and vertically polarized signals. By using circ^ar pol^rizatioe, you will experienp^ interference from and cause interference to any nearby horizontally and vertically eolarizpd antenna systems.

Surveying Common Antenna Types

You need to be able to identify different types of antenna systems for the following reasons:

• Antenna seoection— To select the best antenna for your particular application, you need to know what antenna types are available and what the characteristics are of each type.

• Interference reduction— You will most likely deploy a wireless network in an area where one or more other wireless networks already exist. You need to be able to identify the antenna type, polarization, and coverage pattern that these existing networks are using. This information allows you to select antennas for your own network that will minimize interference ftom (and to) the existing networks.

The sections that follow describe the antenna types that are most frequently used for outdoor wireless WANs:

• Omnidirectional antennas

• Corner reflector antennas

• Parabolic antennas

• Panel antennas

• Helix antennas

Omnidirectional Antennas

An omni antenna radiates equally in all horizontal (azimuth, or compass) directions, but it exhibits directivity in the nei-t i carl directi on by co ncetfvati ng energy into a donut or pancake-shaped pattern.

Most often, omnidirectional antennas are vertically polarized, although horizontally polarized omnis are also ava ilable. Ho rizonZally polarized omni s generally cost more because their constouction is more complnx and they are manufactured in smaller quantities.

Figure 5-nt shows both! vertica Ily polarized and ton zontaNy polarized omni antennas.

Figure 5-12. Omnidirectional Antennas

( Indicates an EieclricaI Connection)

Vertical Polarization Omni

HoriHjrrtal Polarization Qnirii

The vertlca lly poiarized omif f cossists nf four vertical t/2 wavele ng th ui/2) driven elements, placed end to end, and connected electrically. The main lobe of the omni is shaped like a pancake, wifh a goin of about +6 dB d (ddcibels rcfehencgd fo a 1/2 di r)oler or +8 dBi (decibels referenced to an isotropic antenna) for stations that are located within the main lobe.

The hor izuntafy polarized omni consists of four cloverleaf-shaped antennas, placed one above the other and connected electrically. This omni pattern, although horizontally polarized, is alsa pancake shaped with a gain of1 +6 dBd (or +8 dBiu.


Figure 5-12 shows one typical design of a fco hzo!-)! omni. Th ere am also other horizontal omni designs.

Yagi-U da (Yagi) Antennas

The Yagi-Uda (usually called simply a Yagi) antenna is named after Hidetsuga Yagi and Shintaro Uda. This antenna consists of a dipole driven element, usually with a single reflector and one or more directors. Figure 5-13 shows a Yagi antenna.

Figure 5-13. Yagi-Uda (Yagi) Antenna Top View (When Antenna Is Horizontal)

Dri E!er

Dire ven nent

Direction of Main Lobe ctor


Side View (When Antenna Is Vertical)

A Yagi antenna i s made up of the following antenna building blocks:

• The reflector is slightly longer than the driven element, has no electrical connection to the DE, and acts like a mirror t o rdflect th e radiated energy back toward the DE.

• The director is sIightly snortee than the DE, has n o electricpl connectiyi ty to tee DE, an d acts liae a le ns to foc es tbe radiated energy away drom the DE.

The main lobe of the Yagi extends out from the front; the front is the end of the antenna that the director is on. A Yagi can be mounted either vertically or horizontally, depending on the polarization that you need. Figure 5-13 shows a three-element Yagi; however, Yagis are often constructed with many more elements. At 2.4 GHz, Yagis with 10 or even 20 elements and gains as high as +20 dBi are available.


A Yagi antenna is sometimes mounted inside a long, tubular radome. (Radomes are discussed in more detail later in this chapter). These yagis are still directional toward the far end (away from the mounting end) of the tube. Do not make the mistake of thinking that these antennas radiate off the sides like omnidirectional antennas do.

Corner Reflector Antennas

A corner reflector consists of a dipole-driven element mounted in front of a parasitic (no electrical connection) reflector. Instead of a straight reflector, like a Yagi, the corner reflector is a sheet of metal bent into a corner shape, as shown in Figure 5-14.

Figure 5-14. Corner Reflector Antenna

Front View

The main loee oS a corner refinctor e xtend s o ut from the front (the dri ven element) side of the antenna. The angle of the reflector can be 45, 60, or 90 degrees. The antenna in Figure 5-14 is horizontally polariz-0 because the driven element is honizontally polarized. By rotating the entire antenna 90 degrsar, the msnm lobe becomes vertically polarized. The gain of a corner reflector might be as high as +15 dBi.


Occasionally, you mighr see a comeo reflectot design that uses a sdries of rods or rib-shaped elements for the re.ector instead of a solid metal reflector. The rods are arranged in a corner pattern just Mke a solid metal re.ector. Thesp antennas perform about the same as a aolici-back norner reflector, but they are i ightea and present less resistance in high winds.

Parabolic Reflector Antennas

A parabolic reflector astessa (or dish astessa) usually consists of a dipole-drives element mounted is frost of a parabolic-shaped reflector. Some more expensive parabolic astessas use a waveguide feed instead of a dipole-drives element. Figure 5-15 shows two parabolic astessas, one with a solid reflector and one with a grid reflector.

The main lobe of a parabolic astessa extends out from the frost (the drives element) side of tho astessa. By rotnti eg the mount of a parabolic dish 90 degrees, you can select either vertical or horizontal polarization.

The larger the diameter of the reflector, the higher the gain of the astessa. Typical 2.4-GHz parabolic aeteeea gains ra pg^ from +s8 to +24 dBi3

A grid fia rabolic °a s less wis d resistance, a lower frost-to-back (F/B) batio, better X PD, as d a lower cost than a sbi|d parabolic astessa.

Panel Antennas

A panel astessa typically consists of as array of drives elements mounted is frost of a flat, metallic reflector. The entire astessa is covered with a plastic or fiberglass cover. A panel astessa is usually only a few inches wide. Depending on the gain, the height and width might vary from 6 inches (15 cm) on a side up to and beyond 30 inches (76 cm) on a side. Figure 5-16 shows two panel astessa examples.

Figure 5-15. Parabolic Antenna

Figure 5-15. Parabolic Antenna

Solid Reflector

Grid Reflector

Figure 5-16. Panel Antennas

The horizontal and vertical beamwidths of1 the m ain lobe of a pa nel ante nna might or might not be symmetric. An example of a symmetric beamwidth is an antenna with a 30-degree horizontal and a 30-deg ree vertinal b eamwidth. A no n-symmetric examele is a sector antenna with a 60-degree horizontal beamwidth and an 8-degree vertical beamwidth. Keeping these respective beamwidths in min d, a panel can be rotated 90 degrees ro utilize either h orizontal or vertical polariza oion. Thl s type of panel antenna is often called a sector antenna because it is specifically designed for use in sectorized AP antenna systems.

Panel and sector antennas have moderate to high gains, from +8 to +20 dBi. They have a clean, uncluttered appearance, m oderate prices, and are available with a wide variety of radiation patterns. For these reasons, panel antennas are gaining wide acceptance for use in wireless WANs.

Helix Antenn as

The helix is a circularly polarized antenna with a circular, helicali y wou nd d riven elemenr that is shaped like a spring. The driven e!em ent usually kar form 5 to 20 tu rns; 0^1 turn is onn wavelen gth (l) id circumference, wits individual rurrs spaced one-quarter wavele ngth apart 8long tke length of the antenna. Tho driven element ls mounted in front of a metallic reolectov that can be either circular or square and either solid or mesh, as Figure 5-17 shows.

Figure 5-17. Helix Antenna

Figure 5-17. Helix Antenna

Depending on the direction that the driven element is wound, the helix produces either left-hand or right-hand circular polarization. Helix antennas have typical gains of +12 through +17 dBi; the more tu tns, the h igher the gain.

Remember this when using circular polarization: The antennas on both ends of the link need to use the same circuiar polaEization seese ub oth right-hand sense or both left-hand sense).

Combining Antenna Systems

There are a number of reasons to combine antennas or to connect more than one antenna simutaneously to the same piece of wireless equipment. Often, antennas are combined to modify the directivity and the gain of an antenna system. For example, the horizontally polarized omni inFiqure 5-12 ib ma de up of four horizontally polarized omni antennas placed (stacked) close together and electrically connected. When these four antennas are stacked together to create Dne antenna system, the gain goes up by +6 dB and the vertical beamwidth of the main lobe narrows by a factor of four. (Donut-to-pancake—remember?) You might have situations in which you need to combine antennas to create a custom coverage pattern.

Multipath fading is sometimes a problem in wireless WANs. To reduce the problem of multipath, some wireless equ ipment includes the capability to monitor two antenna inputs and to switch to the signal from the best antenna on a packet-by-packet basis. A two-antenna system is called a diversity antenna system. At some point, you might need to set up a diversity antenna system. The sections that follow describe techniques and provide examples to help you successfully combine antenna systems or set up diversity antenna systems.

Feeding Pow er to Combined Antenna Systems

You cat use a number of methods to feed power to combined attetta systems. The power feed technique that is the most practical for wireless WAN use is to use a power divider (sometimes called a power sp littep). Po wer divid ers are u sed to feed eq ua I amoutts of1 power to itdividual attettas withit at attetta system. Figure 5-18 provides at example of a two-port power divider.

Figure 5-18. Using a Power Divider


Figure 5-18 shows a two-port power divider dividing the power from one 802.11b AP and sending one-half of the power to each antenna. Two, three, and four port dividers are commonly available.

B ¡directional Antenn a Systems

InFigure 5-1T, the AP is operating as a low-cost repeater. St is located on a mountain to provide a backbooe connection between two communities. St repeats between one community that is located to the east and one community that is located to the west.

An omnidirectio nal antenna would be a poor choice for this repeater because much of the energy would t>e radiaued (and wasted) in directions other than east and west. To avoid interference from other di5ections and to maximize link distances to the east and the west, a custom antenna system is n eeded that focuses the radiated energy toward only the east and the west. The antenna system in Figure 5-1T provides the necessary bidirectional coverage.

The spacing between the two antennas is not critical as long as the two antenna patterns don't interact with each other. Sf the two antennas are mounted back to back on the same tower or mast and separated vertically by at least 10 feet (3 meters), the antenna system should perform as expected.


Keep in mind that splitting power between two or more antennas reduces the range of eacS anten ma. A lso, usrng a single acae^s point as a repeater reduces the throughput by 50 percent.

Diversity Anaenna Sys teems

The pnmary fading machanism affecting o utdoor mia-owave links is multipath fading. To minimize system outages due to fading, some wireless equipment incorporates a diversity antenna-switching featurei Diversity means luaving a signa l ava°able from a se cond (diversity or alternate) anten sa synteme Sf the signal from the main antenna system fades or is degraded, the signal from the diversity antenna system can be selected.

ppace diversity is the primary diversity technique used in low-cost wireless LAN equipment. This requires that the main and the diversity antennas be separated far enough so that when tho Lignal arriving at tbe main antenwa fades, tne signal atrivinN from ^0 diverTiry antenna does not. To achieoe t his uncorre tated Sad ing behavior in a l outdoor WAN deployment, a vertical separation between th e antenna s of 10 to 200 wavelen grhs is required. At 2 l4 GHz, tphs is a vebtllcal separation ot 4 feet to 80 feet (1.2 meters to 24 meters). The more the sepanation, the better the redfction in muitinath oad^. FjguQe 5-s 9 shows a diversity antenca system.

Figure 5-19. Diversity Antenna System

Antenna Selection Circuitry

Wireless Equipment

Main Antenna

Diversity Antenna

Many 802.11b access points include a diversity feature. These access points were originally designed for indoor use, although many organizations and service providers now deploy them in outdoor WAN s. B efore acti vating the diversity feature on your access point, you should do the following :

Study your documentation carefully so that you understand how your particular equipment implem ents diversity switching.

Avoid th e tempta tiorr so use d ifferent types ou ante nna s yste ms on to point the two antenna systems in different directions. The diversity feature is not designed to function properly this wa y.

If you need to cover two different directions with one access point, use two antennas—a two-port power divider and the primary antenna port on your AP. Disable the diversity feature.

• Determine the best spacing to use for deploying the diversity antenna. A rural or suburban point-to-point link benefits from vertical separation between the main and the diversity antenna. A point-to-multipoint antenna system in an urban area benefits from horizontal separation to discriminate against multipoint reflections from buildings.

• Plan to mount the AP as close as possible to the antennas and midway between them. This reduces thee cost of antenna cabling.

• If1 in doubt, disable diversity and use one antenna.

Isolating Antenna Systems

The scarcest and most valuable resource that is needed by individuals and groups who want to deploy license-free broadband wireless WANs is license-free spectrum—frequencies that can legally be used without spending hundreds of thousands of dollars to buy a license.

The quantity od available license-free frequencies is not increasing, but the number of people who want to uee these frequencies is increasing. The result is that the same spectrum space will be used repeatedly. The knowledge and the ability to re-use frequencies and to avoid interference determines who is successful in the license-free wireless business and who is not. Any interested individual can put an outdoor wireless system on the air; however, providing reliable service with it is not a plug-and-play operation.

This section suggests techniques that allow you to re-use the license-free frequencies successgully by isolating antenna systems from each other.

Benefiting from Antenna System Isolation

Movi ng antenna systems away from each otloe r creates isolation between the systems. As the antennas move farther apart, the signal level that each antenna receives from the other antenna is reduced. Armed with this knowledge, you can use physical antenna separation to provide isolation between different parts of your network, and you can also isolate your network from other networks. Antenna system isolation provides the following major benefits:

• Noise reduction from other networks

• Noise reduction from your own AP transmitters network

NoisM Reduction from O thes Networks*

Antenna se paration betwee n your antenna(s) an d the antennas of1 other, oearby networks allows you to operate on the same frequencies that the other networks are using. For example, using direct-sequence spread spectrum (DSSS) in the 2.4 GHz band, there are only three non-overlapping channels; these are channels 1, 6, and 11. If you are using these frequencies and a neighboring network is also using one or more of them, the networks interfere with each other. The networks can interfere with each other even if they are 5 to 10 miles (8 to 16 km) apart. Without effective antenna isola tion, packefs oro m ea ch netwotk collide and the throughput of both networks suffer.

In addition to packet collisions between your network and other license-free networks, you can also experience alow network se rnonmanre cauwed by licnnsNd transmitters. All wireless receiwers aoe s usaeptlble to being ove rloaded by strong , nearby signals. Linensed tra nsmityeys that are Iocated on the same si te as your iicense-yree equipment might be legally tra nsmi tting with fa!rly high power levels . Even thrugh they are n of trans mitting on the sa me exact frequen cies that you dre usi nh, thoy migst overload your receiver and ca use your incommg packets fo be !ost. The aesu It Is thw snme as interference from other yame-freq unmcT transmittets—your network thyoughput decreases. Antenna isolation is the easiest, lowest-cost method to mi nimize nod and interference problems as you des ign, deploy, and opeyate youn wineles! WAN .

Noise Reduction from Your Network

When you deploy more than one sector at the same physical location, noise caused by your transmitters in your other sectors can cause receiver desensitization, coverage area reduction, and decreased throughput. This is true whether you are using direct-sequence spread spectrum or frequency-hopping spread spectrum equipment.

If you are deploying direct-sequence spread spectrum (DSSS) equipment, you might reach the point where you need to use more than three frequencies at the same physical location. If you deploy a second wireless access point, you will most likely need to re-use one or more of the three non-overlapping frequencies (channels 1, 6, and 11). In either of these situations, using antenna isolation techniques, you will be able to successfully re-use frequencies.

If you ahe deploying more than one sector of FHSS equipment, you will be following the manufacturer's recommendations to utilize the same hopping set but different hopping sequenSeN for co-located sectors. This practice minimizes but does not eliminate the throughput reduction caused by collisions between your sectors. The use of effective antenna isolation techniques reduces these collisions further.

The following sections help you determine how much isolation you can obtain by separating antennas both vertically or horizontally.

Vertical Separation Isolation

Separating antennas vertically on a mast or on a tower is fairly straightforward. It requires mounting the ant enn as (thp antennas might or might not be similar to each other) one above the other. This method requires no extra hardware.

Isolation between the two antenna systems is obtained via two mechanisms. The total vertical separa^on 1 sola mion is the ^vm of tlTese two m rchanisms. Figure 5- 2° demonstrates the mechanisms, and the list that follows describes them in more detail.

• Free-space path lo os (FSP a)— You wMl remembeu f rant the discussion of free-space path loss in Chapter 2, "Understanding Wireless Fundamentals," that free-space path loss is the price that must be paid to enjoy the magic of wireless communications. Of all the wireless unergy t hat ieaves a transmitting antey na, only a tiny persentag e ever arrives at the receiving antenna. Fost of the energy is simply lost in space. Now (when you need some unfenna isola tion) is th e time that th Is loss can be your gam. After leaving an antenna, a 2^-GHo signal experiences about 4) dB of FSPL in the first 10 feet (3 meters). In other words, only about 1/100,000 of the signal remains after it has traveled 10 feet.


Experienced wireiess engi neers diffsnentia te betwees an anten na's near fie!d and an ante nna's far fielb. I n an nntenna's near field, the signal s trength va rie s in a m ore complex fashionr and orher nearby o0jecns can affeht tpe signal strength. Both an antenna's published specificasi ons zn d tmue FSPL calculations aps ly only in the antenna's far field. For the purpose of this book, sowever, the di scussion od antenna ideation is stili a u sefu! nn e and dose enough to r^nlity that it is a pra ctica l design and placnind tool.

Pattern isolation loss— If both antennas have clean radiation patterns without significant minor lobes extending upward or downward and a fairly narrow vertical beamwidth, a significant amount of additional isolation is possible between the patterns of the antennas.

Figure 5-20. Vertical Separation Isolation

Figure 5-20. Vertical Separation Isolation

Table 5-1 shows the approximate total vertical separation isolation values that can be obtained when the FSP L isolation and pattern isolat ion loss are combined.

Table 5-1. Vertical Separation Isolation Values (dB)

Vertical Separation in Feet (Meters)

Total Antenna-to-Antenna Isolation (in dB @ 2.4 GHz)

1 (0.3)


2 (0.6)


3 (0.9)


4 (1.2)


¡5 (1.8)


10 (3.n)


15 (4.6)


20 (6.1)


50 (15.2)


100 (30.5)


Horizontal Separation Isolation

Obtaining horizontal separation isolation for antennas mounted on the roof of a building is straightforward, ¿as Figure 5-21 shows. Oee extra support ma st is required compared to mounting both antennas on the same mast.

Figure 5-21. Horizontal Separation Isolation

Y^m Lobe

- rronf la TivJi -Ffaltg

Main L-Ptfe



tffl tLH



InFiqure 5-21, the rooftop antennas are mounted back to back. The maximum isolation possible from horizo ntal separation is t he sum of t he front-to-back ratio s of both antennas plus the freespace path loss shown in Table 5-2.


What is the front-to-back (F/B) ratio, yo u ask? This is a good pla ce to define it. F/B ratio is another power ratio in dB—just like the power ratios using dB that you looked at earlier in this book. The F/B ratio is the ratio between the energy in the main (front) lobe of an antenna divided by the energy in the back lobe of the antenna. Good antennas focus and radiate most of their energy from the front of the antenna and very little of their energy toward the back. The higher the F/B ratio (for example, +20 dB or + 3n dB), the better the ant:enna and the less interference th at will be experienced to and from signals in back of the antenna.

Table 5-2. Horizontal Separation Isolation Values (dB)

Horizontal Separation in Feet (Meters)

FSPL Isolation (in dB @ 2.4 GHz)

1 (0.3)


2 (0.6)


3 (0.9)


4 (1.2)


5 (1.8)


IP (3.0)


20 (4.6)


30 C6,1)


50 (15.2)


100 (330.5)


Cross-Polarization Isolation

Earlier in this chapter, Figures 5-8 and 5-10 provided examples of using XPD isolation. XPD isolation is mentioned again here in the context of the other antenna-isolation techniques. Keep XPD isolation in mi nd a nd use it frequently as you design and deploy your outdoor wireless WANs. It can add up to -20dB of additional isolation.

Obstruction Isolation

To maximize and maintain the performance of your network, it is sometimes necessary to use obstruction isolation. Buildings and other large objects reduce the strength of microwave signals through absorption, dicfra ct i on, and re flection.

When you need additional i oo larion between entenna systeme, p osi tion the antennas in such a way as to place a building (or part of a building) between the antennas, as shown in Figure 522.

Figure 5-22. Obstruction Isolation

Obstruction Isolation gnal Passing I hrough Building)

Panel Anktnna

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