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Fundamentals of Antennas

Prof. Ely Levine

levineel@zahav.net.il

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Chapter 4

Antenna Arrays and Apertures

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Antenna Arrays

• Simple antennas can be combined to

achieve desired directional effects

• Individual antennas are called elements and

the combination is an array

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Types of Arrays

• Broadside: maximum radiation at right angles to main axis of antenna

• End-fire: maximum radiation along the main axis of antenna

• Phased: all elements connected to source

• Parasitic: some elements not connected to source

– They re-radiate power from other elements

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Array configurations

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Yagi-Uda Array

• Often called Yagi array

• Parasitic, end-fire, unidirectional

• One driven element: dipole or folded dipole

• One reflector behind driven element and

slightly longer

• One or more directors in front of driveh

element and slightly shorter

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Yagi-Uda Array

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Yagi for 14, 21, 28 MHz Amateur Bands

Yagi-Uda Array

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Stacked Yagis

• Stacking in-phase Yagis with half-

wavelength vertical spacing

• Reduces radiation above and below horizon

• Increases gain in plane of the antenna

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Stacked Yagis

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Log-Periodic Dipole Array

• Multiple driven elements (dipoles) of

varying lengths

• Phased array

• Unidirectional end-fire

• Noted for wide bandwidth

• Often used for TV antennas

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Dual Polarized Log Periodic

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Monopole Array

• Vertical monopoles can be combined to

achieve a variety of horizontal patterns

• Patterns can be changed by adjusting

amplitude and phase of signal applied to

each element

• Not necessary to move elements

– Useful for AM broadcasting

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Collinear Array

• All elements along same axis

• Used to provide an omnidirectional

horizontal pattern from a vertical antenna

• Concentrates radiation in horizontal plane

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Broadside Array

• Directional Array

• Uses Dipoles or microstrip or slots

fed in phase and separated by 0.5-0.8

wavelength

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Microstrip Array LP

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Microstrip Array CP

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Microstrip

Array

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Slotted Waveguide Array

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Reflectors

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Plane Reflector

• Mount antenna 1/4 wavelength from flat

metallic surface

• Reflected wave and direct wave are in phase

along normal to survace

• Increases radiation in that direction

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Plane Reflector

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Corner Reflector

• More focused radiation than plane reflector

• Often used with UHF TV antennas

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UHF-TV Antenna: Yagi with Corner Reflector

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Parabolic Reflectors

• All radiation emitted at focus emerges in a

beam parallel to the axis

• Gives a narrow beam

• Suitable mainly at microwave frequencies

because it must be large compared with the

wavelength

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Parabolic Reflector Beamwidth

• Beamwidth at half-power points

• θθθθ = 70 λλλλ / D

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Parabolic Reflector Gain

• As a power ratio (not dB)

• With respect to isotropic

• G = ππππ² D²/ λλλλ²

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Horns

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Reference Books

Balanis C.A Antenna Theory and Design,

Wiley Inter-science, 2005

Kraus J.D. and Marhefka R.J. Antennas for All Applications,

McGraw Hill, 2002

Elliot R.S. Antenna Theory and Design,

Wiley and Sons, 2003

Volakis J.L. (Ed) Antenna Handbook.

McGraw Hill, 2007