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Sloppy Physics:

The Yagi-UdaAntenna

Date: 4/30/08

Authors: Alan Taylor & Ben Harris

Advanced Lab I Talk PHYS 3200

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Outline

Our Wireless World What is an antenna?

How does it work? Oscillating Dipole Radiation Fields

Basic dipole omni-directional antennas Uses Limitations

Multiple dipoles The Yagi-Uda Antenna

How a simple Yagi works Parasitic dipoles

HistoryApplicationsAdvantages

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Our Wireless World

The world is becomingincreasingly wireless, ten years

ago, who had heard of a

wireless network? Now,

corporate and college

campuses, coffee shops,

bookstores, homes, etc. feel

incomplete without a wireless

internet signal.

People without cell phonestoday are viewed as archaic.

What enables our wirelessworld?

ANTENNAS!

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The Dipole Antenna

Same idea as the single vibrating electron, but now many movingtogether

More oscillating electrons means a stronger radiation fieldThe frequency of the oscillation of the electrons is the frequency of the

waves emitted by the antenna

As these electrons move together they alternately build up charge on eitherend of the antenna. A oscillating dipole!

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An Omni-directional Dipole Antenna

Omni-directional is a bit misleading2-D doughnut radiation pattern

These are your common TV/radioantennas

They are also used in cell phones,computers, and GPS

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Limitations of the Omni-directional Antenna

What if we have a limited amount of power with which to driveour antenna, but our target is out of range?

What if we dont want everyone within radius to be able to pickup our signal?

Then, we want to be able to focus our radiation field directionally.How can this be achieved?Multiple dipoles

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Multiple Dipoles

What difference do multiple dipoles make?Now picture two dipole antennas.

They are driven at the same frequency, but the spacing between them and how far out ofphase they are will create interference patterns.

The interference pattern created is analogous to two slit interference of a beam of visiblelight.

For our purposes this is bi-directional

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Multiple Dipoles, Continued

How does the change in dipole separation affect the interferencepattern/gain?

Gain is defined as ratio of the signal output of a system to the signal inputof the same system(Wikipedia)

If we look at two driven dipoles separated by a distance of/4 and /4 out ofphase we see an interesting result

Two Driven Dipole Antennas, Quarter Wavelength Separation,Quarter Wavelength Out of Phase

-1.5

-1

-0.5

0

0.5

1

1.5

1 viewed from 1

2 viewed from 2

1 viewed from 2

2 viewed from 1

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The Yagi-Uda Antenna

Simple YagiTwo dipole antennas butonly one is drivenThe other is a parasitic element

Current is induced in the parasitic element by the radiation field of the driven element Spacing is important ~ /2 This creates a somewhat different interference pattern, much less of our radiation field goes

in the negative direction

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The Yagi-Uda Antenna, Continued

More generally, and more commonly, the Yagi will have severalelementsDriven elementDirector elements

Sometimes upwards of fifteen of theseReflector element

The reflector causes destructive interference in the negative direction Acts as a sort of mirror, creating image elements behind, effectively increasing the number

of directors

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History

Invented by two Japanese Profs. in 1926Hidetsugu Yagi, Shintaro Uda

British implemented technology in WWII radarsJapanese were unaware of this technology until late in the war

Initially designed for wireless power transmission

CommunicationProvides high-speed internet access to rural communitiesConnects business networks between sitesAllows the receiving of VHF and UHF TV bands in low signal zones

TrackingYagi arrays allow enhanced radar imagesExtends the range of RFID

Current Applications

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Practical Implementation (Bens Project)

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Sources

WikipediaAntenna Theory and Design, Robert Stratman ElliottPhysics for Scientists and Engineers, Randall D. Knighthttp://web.mit.eduhttp://www.mwcircle.orghttp://www.arrl.org/tis/info/whyantradiates.htmlhttp://pan.fotovista.com/dev/8/0/04020008/g_04020008.jpg