ECE 5318/6352 Antenna Engineering - University of Houstoncourses.egr.uh.edu/ECE/ECE5318/ANTENG...

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Chapter 15Chapter 15ReflectorReflectorAntennasAntennas

ECE 5318/6352ECE 5318/6352Antenna EngineeringAntenna Engineering

Dr. Stuart LongDr. Stuart Long

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GEOMETRICAL CONFIGURATIONSGEOMETRICAL CONFIGURATIONS

large flat sheetlarge flat sheet

corner reflectorcorner reflector

small flat sheetsmall flat sheet

parabolic reflectorparabolic reflector

(wide freq. range) (narrow freq. range)

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FLAT REFLECTORFLAT REFLECTOR

image drivenelement

d

Image Theory AnalysisImage Theory Analysis

Source polarization and spacing used to control radiating properties

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FLAT REFLECTORFLAT REFLECTOR

Gain relative to a Gain relative to a λλ//22 dipole in free spacedipole in free space

losses decrease gain at small spacing

larger spacing – less gain– more bandwidth

.5λ λ

d/λ

G

imperfect conductors1

2

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CORNER REFLECTORCORNER REFLECTOR

Electrostatic ImagingElectrostatic Imaging

nfor 180

=α ,

,

3#1 4# 2 3

imagesin phasein phase but negative

φ1

2

3

4

⎟⎠⎞

⎜⎝⎛−⎟

⎠⎞

⎜⎝⎛≈ φ

λπφ

λπ

φ sin2coscos2cos ddE

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CORNER REFLECTORCORNER REFLECTOR(CONT)(CONT)

GainGain

region w/ gain < 0 dB

⇒ main lobe in another direction

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if spacing gets too large, multilobe patterns in real plane

d = 0.5λ

d = 1.5λ

d = 1.0λ

G = 3.1

G = 3.8

G = 4.4

MultilobesMultilobes

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MultilobesMultilobes appear whenappear when

1.2λ2.5λ30°

0.85λ1.2λ45°

0.65λ0.95λ60°

0.5λ0.7λ90°

0.2λ0.3λ180°

1st. max. at d=(multilobe)α

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Design and GeometryDesign and Geometry

NOTEpoint A is 1.4 S from corner

the only waves reflectedfrom an infinite casebut not from the finite caseare those radiated intosector η

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absence of reflectorbeyond point B does not have a large effect-slightly larger beamwidth-null not exactly at 45°(at a somewhat larger φ)

L ≈ 2 d is good practical size


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NOTE

for smaller α gain goes up;but need to use larger dand point A is now at1.73 d from corner

⇒ larger reflector size neededonly small increase in gain


LA

B

1.73dη

φα=60°

d

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Practical DesignPractical Design

L= 2d d = 0.35λ⇒ aperture of 1.0λ

2d

0.7

λ

0.35λ1.0 λ

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Practical DesignPractical Design

L= 2d d = 0.5λ⇒ aperture of 1.4λ

2d

1.0

λ

0.5λ1.4 λ

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WireWire--grid arrangementgrid arrangement

to reduce wind resistanceand weight a grid of wires can be used for the reflector

g

h2d

2d

g < 0.1λh > 0.6λ

(to prevent “spill-over”)

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PARABOLIC REFLECTORPARABOLIC REFLECTOR

given a point sourcewant to produce a planewave front over an aperture

want path lengths from source to reflectorto aperture plane to be equal

aperture

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eqn. of a parabola with focus at F

)cos1(2

θ+=⇒

LR

)cos1(2want θ+= RL

pathFOF

pathFPB

Design and Geometry Design and Geometry

P

R

B’

B

A’

A

S Q

R cosθ

θL O F

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⇒ also means that distancefrom point P on the parabola to the focus at F is equal to the ┴ distanceto a fixed line directrix

P

R

B’

B

A’

A

S Q

R cosθ

θL O F

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- thus all waves from an isotropicsource at the focus, reflectedfrom a parabola arrive at lineAA’ with equal phase

- wave AA’ appears to have come from a plane wave at thedirectrix Q

QSPQQSPFPSPFQPQSPS

PQPF

=−+=+−=

=

P

R

B’

B

A’

A

S Q

R cosθ

θL O F

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PARABOLOIDPARABOLOID(PARABOLA OF REVOLUTION)(PARABOLA OF REVOLUTION)

a portion A of the sourceradiation is interceptedby the paraboloid and reflectedas a plane wave of circularcross-section


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PARABOLOIDPARABOLOID(CONT)(CONT)

wavesreflected of region central reinforces source feed

from radiation direct so

odd with choose nnλ L4

=

feed source


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feed source

LD

LD

LD

∠

⇒

for small, outer edges of

paraboloid get smaller illumination

for more uniform, make illumination smaller by increasing and keeping

the same

large


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feed source

largevery is

telescopes optical for

DL

F D L


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feed source

Polarization depends on primary feedtypically – horn – linearly polarized

presence of primary source in path of reflected waves-cause mismatch-obstruction (blockage)

Can displace feed


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feed source

front-fed Cassegrain-fed

Feeds Feeds

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CORNER REFLECTORCORNER REFLECTORVS.VS.

PARABOLAPARABOLAfeed

source

NOTE

wave from corner reflectortravels shorter distance by (OO’)

patternssamefor

patternsdifferentfor

⇒=⇒=

⇒=⇒=

12'35.

5.0'2

λλ

λλ

OOAF

OOAF

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CORNER CORNER VS. VS. PARABOLAPARABOLA(CONT)(CONT) feed

source

if AF is small, the exact shape of the reflector is unimportant

practical advantage of corner is itssimplicity and ease of construction

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YAGIYAGI--UDA ARRAYSUDA ARRAYS

llrld

sr

sd

directors

reflectordriven

element

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YAGY UDA YAGY UDA (CONT)(CONT)

[ ]

[ ]

[ ]dBSLR

dBBtoF

dBG

10

3020

128

=

−=

−=

TypicalForward gain G

Input impedance Z

Front to back ratio F-B

Sidelobe ratio SLR

Characteristics

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YAGY UDAYAGY UDA(CONT)(CONT)

impedance and gain affect and

gain not but affects

and affect does

much gain affect tdoesn'

dd

r

r

ls

Zl

F-BZs

s

Design

λ25.0=rs

λ45.04.0 →=dl

λ55.05.0 →=rl

)(2

lessslightlyl λ=

λ4.03.0 →=ds

ECE 5318/6352 Antenna Engineering - University of Houstoncourses.egr.uh.edu/ECE/ECE5318/ANTENG...

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