Selecting Antenna Systems

Your selection of the proper antenna system is the biggest factor in the success of your wireless network. This chapter helps you select the most effective antenna system for your wireless wide-area network (WAN). This chapter discusses the following topics:

• How antenFas focus power

• Basic antenna types

• Antenna polarization and how to use it

• Reaso its to combine antennas and how to combine them

• Reasons to isolate antennas and how to isolate them

In a wired Ethernet, the Ethernet cable directs the signal. Ethernet packets can go only where the Ethernet cable goes. In a broadband wireless WAN, the antenna directs the signal. Wireless packets can only go where the antenna system radiates them.

Your antenna system must radiate signals only toward your end users. In addition, to bo effective, your antenna system must have the following characteristics:

• -e mounted hig p enough to ac h.eve a l i ne- of-sight (LOS ) path

• Have enou gh pain do provkJe reMable link performance

• Be mo uhted correctly aod hrip in .ositio n securel y

• Rbject noise an d interference trom oth er signals and networks

Using Antennas to Focus Power and Reduce Interference

You already know that antennas focus power to and from the end users while reducing interference from other networks and other directions. The next few sections discuss the electromagnetic building blocks that antennas use to focus power. In addition, you will see how Yio reduce interfereece using antenna Notarization.

Antenna BuNdmg Blocks: Lights, Mirrors, and Lenses

Antennas achieve their directivity by using a combination of properly sized and properly spaced antenna elements. These elements are electromagnetic building blocks. Combining the elements in different ways produces different antenna radiation patterns. In some designs, the antenna elements are electrically connected to each other, forming a driven array. In other designs, thc elements are placed close together but with no electrical connection, forming a parasitic array.

All antennas use a driven element. The driven element is always electrically connected (via coaxial cable! directly to the wi reless eqcipme nt. Antenna s acMeve gain when the driven element is combined with additional element building blocks that reflect, direct, or concentratc the signal.

It is helpful to use the concept of lights, lenses, and mirrors to illustrate how different antenna designs achieve gain as the antenna elements work together to focus transmitted and received energy.

Driven Element: The Light

To create light, a source of electrical energy (such as a battery) is connected to a light bulb. Thc light bulb converts the electrical energy into light energy.

To create a wireless signal, a source of radio frequency (RF) energy (a transmitter) is ^110^0! to the driven element of an anteuna. The driven element convehts the RF energy into radiated electromagnetic energy—a wireless signal. Every antenna system must have at least one drivet element tr imtiate this energ y co rversion.

A dipole antenna is one-half wavelength (L/2) long and is frequently used as a driven element. A dipole mouoted vertically afove the earth radiates e tergy equoNy in all horizontal directions. This energy forms a torus (donut-shaped) pattern around the antenna. When the dipole is horizontal to tph earth , the donut-shaped pattern becomes b idire ctional, radiating energy in just two horizontal directions, off the two sides of the dipole. Figure 5-1 shows a top view of this bidirectional radiation pattern, s s well as ahe bidirectionol pat:tern of lig ht energy nsdiated from a fluorescent pighd tube.

Figure 5-1. Bidirectional Radiation Patterns

Top Vlew{s)

Dipole

Dipole

Bklirectional Patterna-E-nergy fiadialed in Two Directions
Huoiüscsnl Light Tub»

Both the horizontal dipole asd a horizontal tluoruscest light radiate most of their energy off thu sides. They radiate Nttle or so energy off the oems. fvaes this bidirectional property Is combined with other astessa elements or, is the case of the fluorescent light tube, with other optical elements, a more concentrated, more focused beam of energy results.

Figure 5-2 shows two vertical astessas. The lower astessa has a single dipole, half-waveleegth-losg drives el emeet. The top antes as h as fouf hqU'-waveleeg th-loeg drives elements. The four drives elements are mounted one above the other asd connected together electrically. Because all -ost drives elements are is a single lisa, this antes wo is called a collieear array.

Figure 5-2. Multiple-Driven Element Radiation Patterns

4 Halí-WpveEejfigrth UhvÉu Llements

EH ai

1 Hall-WavHHungth Drtvfln FlHmnnf

Less Gain (Durm:)

Side VLew{s)

When two or more driven elements are electrically connected and placed end-to-end (a collinear arrayS, the vert i cal radiation pattern chaneoal The don ur-shape d pattern flattens out into a pancake shape. The pancake-shaped pattern extends farther away from the antenna than the donut-snap ed putter p.

Stated technically, the antenna wi th the pancake-s haped pattern has more gain compared to the antenna with the donut-shaped pattern. The transmitter power supplied to the antenna hasn't changed! but the end-to-end driven elements co ncentrate that t ransmittef power into a narrower but longer coverage area (a narrower vertical beamwidth). A receiver located inside this coverage acea cc n be locatpd -arther away from the aut enna and Still receive a good signal.

NOTE

Anothec way to look a t this is to imagine that the d ORut around a single half-wave vertical antenna is a jelly donut. When more half-wavelength elements are added, it is liee a g 1 ant root came aleng and stepped on the donub, squashing i t dowe The jelly sq^^s out beyond the donut. The radiat1 on fattern of a gain antenna is like the jj ellyl in the donut—it spurte out -arther and flattot than th e donut. The energy radiated from a foup-hal--wavelengtu-long coNinear a rray spurrs out about tw 1 ce a s far as tlee e nergy from a siwgle halfrwavelength-long vert|cal antenna d

Reflector: The Mirror

Some antenna designs use a reflector along with the driven element. A reflector is an antenna element that is about 5 percent longer than the driven element. The reflector is placed parallel to the driven element and about one-quarter wavelength (1/4) away from the driven element, as Figure 5-3 shows.

Figure 5-3. Effect of a Reflector

Figure 5-3. Effect of a Reflector

Top View(s)

Thpre i s no electr ical conneceo n between the driven element en d the rpflentor. Wh en eIectroma gnetic waves leave the dnven element, thNy encountei the reflector Because the reflNctor is both physically and electrically longer than the waves , t hi wave s lrohnce off ov the reflector a nd turn back toward the dr i ves element. The reflected waves djoiu the n on-reNected waves to Sorm a s tronger signal pattern in tye d i rection away from the reeector. Tce effect i s Timilal to placing a cumed mirror bohred a light bulo Mowt ok sh e light energy is reflected ¡n one direction, away from the curved mirror.

Together, the driven element and the reflector combine to make the antenna more directional, with more gain in the forward direction and less gain in the backward direction.

Director: The Lens

Some antenna designs use a director along with a driven element. A director is an antenna element that is about IT percent shorter than the driven element. The director is placed parallel to the driven element and about one-quarter wavelength (1/4) away from the driven element. There is no electrical connection between the driven element and the director. When electromagnetic waves leave the driven element, they encounter the director. Secause ths director is physically and electrically smaller than the waves, the waves tend to travel toward the director. The director concentrates the waves into a tighter beam. This is similar to using a lens in front oN a Mght bulb to concentrate the light energy, as shown in Figure 5-4.

Figure 5-4. Effect of a Director

Half*Wavelength Driven Element

Maximum Radiation t ^

Light Su lb

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