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Panel Anktnna

The construction of each building is different, so it is difficult to estimate the amount of isolation that a build ing provi des. If you estimate an average of -8 dB for each exterior wall and -4 dB for every interior wall, the isolation provided by the building in Figure 5-22 might total between -tO and -40 dB.

Sector Antenna Systems

Chapter 3, "Choosing Your Network Architecture," described sectors as similar to pieces of pie, but sectors are never exactly wedge shaped. In practice, sectors can be shaped like a circle, an oval, a fan blade, or whatever shape you need to cover a specific area. Figure 5-23 shows examples of access point locations with sector antenna systems consisting of one, two, three, and six sectors. Your antenna system for each sector needs to be selected and aligned to maximige thee coverage within that seNtow and minimize the noise and interference from all other sectors and areas.

Figure 5-23. Sector Coverage Examples

1 Sector

£ Sectors

• indicates Access Rjinl Local ion y

3 Sector

I yA

6 Sectors

Whe n you choose to build a sector ized access po i nt, you gain the following adv antages:

• Scalability— You can increase the capanity of your system. You can start: small, perhaps with ju2t a single sector serving a eew users. Acs tne wumb^r of users grows, you can add more sectors. By converting omnidirectional coverage to three-sector coverage, you can triple the capacity of your hub site.

• Reliability— You can select the gain and the pattern of each sector antenna to provide thn highest signal level for end users in that sector. You can also choose the polarization and adjust the downtilt of each sector antenna to reduce interference from other networks. By designing a good sector antenna system and installing the sector antennas correctly, you reduce the noise level and improve the reliability of your service within each sector.

Selecting Sector Antennas dectorization always yields dividends compared to a non-sectorized antenna system. For thn best performance from your sectorized system, consider the suggestions in the sections that follow as you design y00r system and select your antennas.

Low-Noise Access Point Location

Although a sectorized system handles noise and interference far better than a non-sectorized system, it is still important to select a low-noise access point location. Even the best sector ante nna desig n m ^hn not proa1 de s^tisfacto oy b^rvicb f it is instaNed in a high-noise location.

For help in choosing a low-noise location, review the wireless site survey information in Chapte r 4, "Performing S i te Surveys ."

Sector" Orien tati on

Coordinate the orientation of your sectors to provide effective, scaleable coverage of your service area. For exa mpl e, if many of your e nd us ere are eoncentrated in a downtown area, plan to use more than one sector to cover that area. For example, if you decide to use a total of three sectors no cover the 0owntown area, then eauh neeyor soou ld coven 1/y of yhe t otal area. If the total oowntnwn area extends 180 degrees when viewed from your access point location, each sector antenna should have a horizontal beamwidth of 45 to 60 degrees.

Sectoo Radius

The radius of each sector plays a dinect ro!e m choosing the prope r antenna noh —at sector. A necsor with a small radma requires a low^ ain sector antenna. A sector w ith a large radius noeds a high-gain anten na. Use the formnla for i ink bcd get in Cha pier 2 to determine h ow much gain nop need to cover your sector. Remember to aNow a hade margih of at leant 10 dB.

Polarization

Choose the polarization o1 eachi o1: your sector antennas. Each sectos should be cross polarized when compared to the polarization of the strongest source of noise and interference within that sector. It is okay to use different polarizations in different sectorSl

Horizontal Beamwidth

Choose the horizontal beamwidth of each sector to be from 75 to 100 percent of the width (ie degrees) of the sect or- There is no absolutely correct formula; however, one rule of thumb is to choose a sector antenna with a horizontal beamwidth equal to 75 pnrcnnt of thn snctor width; for example, a 45-degree horizontal beamwidth antenna is often chosen for a 60-degroe sector.

Remember from Chapter 2 that horizontal beamwidth describes the width of an antenna's maie lobe at the half-power points. This means that an antenna with a 60-degree horizontal beamwidth ^itcll padiates a substantial amount of power beyond a 60-degree angle.

If you prefer to have a substantial overlap between sectors to provide a little redundant area coverage when frequency re-use is not an issue, choose a horizontal peamwidth equal to the se ctor width..

Vertical Beamwidth

Choose the vew i ca l boamw ^th of1 each se ctor antenna bas ed on the radius of the sector. A sector that has a larger radius needs a wider vertical beamwidth to cover the sector. A sector that has a smaller radius needs a smaller vertical beamwidth.

DowMitrlt

Choose sector antennas tnat can be mechaninall y downtM ted to allow yo u tm center the main lobe of the antenna in the middle of the coverage area of each sector. More downtilt is needed ie sectors that have more elevation differential between the end users and the sector antenna. Fo-example, in a 5-mi le radius sector, an antenua thot is mou nted 500 feet NgNer than the elevaoio o of the end usem must be downtilted more than a sector antenna that is mounted only 150 feet higher than the end users.

F/B Ratio

Select a sector antenna with the highest possible F/B ratio consistent with the cost of the antenna. Higher F/B ratios result in more interference rejection off the back of the antenna. More interference rejection allows a higher signal-to-noise ratio and better performance in the forward direction.

Size, Weight, and Appearance

Choose a sector an tenna tha t Inas a size and weight thae can be safely and seruroly mounted in or on the available mounting area or tower.

Choose an antenna that has an appearance that is acceptable to the management of the site where the antenna is to be located.

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