## T

Lens

TopView(s)

The dirncro- has a concentrating effect on the electromagnetic waves similar to the way a lens concentrates light waves. The waves are concentrated in the direction of the director. This gives the aItenna directivity ar^ gai n ic the forw aod direction.

NOTE

Many antenn as make use o° both reflectors a nd directo rs. When doth o° -hes e e iementa are combine d wrth ifhe, dn veo elenren t, the result is d Nlighly directive h igh-gai n anteneal Th e Yagi antenna -discussed sho rt i y) is o rce example .

### Antenna Polarization

This section describes antenna polarization and provides several examples to help you select the best polarization for your particular wireless system environment.

### Definition of Antenna Polarization

Two electromagnetic fields leave a transmitting antenna and arrive at a receiving antenna: an electric field (also referred to as the E-field) and a magnetic field (also referred to as the H-feld). The E-field and the H-field are perpendicular (at a 90-degree angle) to each other, and each field is also perpendicular to the direction that the electromagnetic wave is traveling.

The E- fie!d exists i n the same plane (with the same orientation) as the plane of the antenna elements. fy definition, the plane of the E-field is the polarization of the antenna. If the antenna elements are vertical relative to the surface of the earth, the E-field is vertical and the signal is vertically polarized. If the antenna elements are horizontal relative to the surface of the earth, the E-field is horizontal and the antenna is horizontally polarized.

In your networks, you can use any of the following four types of polarization to maximize reception of desired signals while reducing noise and interference from undesired signals:

• Vertical polarization

• Horizontal polarization

• Circular polarization

• Cross polarization

### Vertical Polarization

Vertical polarization is used in many wireless WAN deployments. Figure U-U shows the orientation oh t he E-field of a vehti<roily poiaeized antenta relativ e to tue antenna and relative to the surface of the earth.

Figure 5-5. Vertical Polarization

Top View

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vertical Polarization -»rlt * * * Yagi Antenna t

E-FlSM

E-FlSM

Vertical Polarization ■ Vagi Antenna zJ

ie el f

C-f-rctd

Side View

With a vert icall y polarized antenna, the E-field is vertical , relative to the surface of the earth.

### Horizontal Polarization

Horizontal polarization is used in some wireless WAN deployments. Figure 5-6 shows the orientation of the E-field of a h orizontally polar ized antenna relative to the antenna and relative to the surface of the earth.

Figure 5-6. Horizontal Polarization

Top View

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HOflzonal f^VjIg ri ¡cglion -*■ \ Yagi Antenna f r-riGW

H-rwiiiHitfil PalgriialKjn Yagi Antenna

t-F-ield ie el

Side View

With a hori zontally polarized antenna, t he EE-field is horiz ontal, relative to the surface of the earth.

### Circular Polarization

Occasionally, circular polarizat ion is used in a wirele ss WAN. Figure 5-7 shows the orientation of the E field of a circularly polarized antenna relative to the antenna.

Figure 5-7. Circular Polarization

Side View and Top View (Identical)

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