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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

Lecture 36

Reflector Antennas

In this lecture you will learn:

• Reflector antennas

• Dish Antennas

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A Short Dipole Antenna

x

y

φ

( ) ( )θφθ 2sin23, =G

( ) ( )θφθ 2sin, =p

Pattern:

Gain:

( )φπθ ,2=p

In the θ = π/2 plane the radiation goes everywhere – not a very efficient antenna

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A Plane Reflector with a Short Dipole Feed - I

x

y

The basic idea behind reflector antennas is that metal surfaces can be used to direct radiation from antennas in desired directions (or vice versa in reception)

Question: How does one get all the radiation from a Hertzian dipole to go in the +x direction?

Answer: Use a reflector

x

y

a

a

image dipole

φ

φ

( ) ( ) ( )[ ]⎩⎨⎧

≤≤≤≤−=

232for022forcossinsin4,

22

πφππφπφθφθ kaF

Array Factor:

{ ,.......5,3,12

== nnka π

For maximum gain in the θ = π/2, φ = 0 direction,choose:

feed

{ ,.......5,3,14

== nna λ

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

x

y

a

image dipole

φ

( ) ( ) ( )22 ,sin41, φθθφθ Fp =

Pattern:

{ ,.......5,3,12

== nnka π( )φπθ ,2=p for

( ) ( ) ( )22 ,sin23, φθθφθ FG =

Gain:

A plain reflector has increased the maximum gain by a factor of 4

A Plane Reflector with a Short Dipole Feed - II

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A short dipole with a plane reflectord

A Plane Reflector with a Short Dipole Feed

4λ

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A Corner Reflector with a Short Dipole Feed - I

x

φ

a

y

xa

y

Array Factor:

( ) ( ) ( )[ ] ( ) ( )[ ]22 sinsincos2cossincos2, φθφθφθ kakaF −=

44 πφπ ≤≤−For and 0 otherwise

{ ,.......5,3,1== nnka π

For maximum gain in the θ = π/2, φ = 0 direction, choose:

image dipole

image dipole

image dipole

feed

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

xa

y

image dipole

image dipole

image dipole

( ) ( ) ( )22 ,sin161, φθθφθ Fp =

Pattern:

( ) ( ) ( )22 ,sin23, φθθφθ FG =

Gain:

A corner reflector has increased the maximum gain by a factor of 16

Corner reflectorPlane reflector

{ ,.......5,3,12

== nnka π

( )φπθ ,2=p

{ ,.......5,3,1== nnka π

44 πφπ ≤≤−For and 0 otherwise

44 πφπ ≤≤−For and 0 otherwise

A Corner Reflector with a Short Dipole Feed - II

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A Corner Reflector with a Short Dipole Feed

A short dipole with a corner reflector

d

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

How to Improve the Reflector Shape?

x

φ

a

y

Corner reflector

x

a

y

All waves emerging from the source and ending at the dashed line must have traveled equal distance to be in phase at the dashed line

The shape of the reflector must then be a parabola given by the equation:

fyx4

2= fa = = focal distance

Improvement

x

a

y

Radiation is collimated by the parabolic reflector

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A Parabolic Reflector with a Short Dipole Feed

d

A short dipole with a parabolic reflector

Problem:

Radiation is collimated in only one plane by the reflector

Solution:

Use a parabolic dish ……..

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A Parabolic Dish Reflector with a Short Dipole Feed

Parabolic dish reflector

Feed antenna (short dipole)

Nice design but the feed is problematic ……..

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

x

a

y

How to Improve the Feed for a Parabolic Reflector?

Problem:

• All waves from the feed that reflect off from the parabolic reflector are in-phase at the aperture (which is marked by the dashed black line)

• But all waves that reach the aperture directly might not be in-phase and in-line with the ones that reflect off from the reflector

Solution:

The feed must not send out waves directly – all waves must reflect off the reflector

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

A Parabolic Dish Reflector with a Short Dipole Feed

Parabolic Dish Reflector

Feed antenna (short dipole with a plane reflector)

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

Commercial Dish Antennas for Satellite/TV

A DIRECT-TV dish antenna

Feed

Feed (off-axis)

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University

The NASA Deep Space Network (DSN) is an international network of antennas that provide the communication links between Earth and space. DSN consists of three deep-space communications facilities placed approximately 120 degrees apart around the world:

2001 Mars Odyssey

High gain dish antenna

High Gain Dish Antennas for Deep Space

ECE 303 – Fall 2007 – Farhan Rana – Cornell University

Maximum Gain of a Dish Reflector

Parabolic Dish Reflector

Feed

z

y

D

Assumption: The feed antenna “illuminates” the dish uniformly

Under this assumption the radiation crossing the y = 0 plane (at the face of the dish) can be thought of as coming out of an aperture (or hole) of diameter D

The radiation pattern of the dish antenna in the far-field can then be determined by diffraction theory

The maximum Gain G(θ,φ) will occur in the direction along the y-axis, and from diffraction theory it will be:

( )

( )

radradDD

AA

GG

ηλ

πηπλπ

πφπθλπφθ

λπ

πφπθφθ

2

22

2

2

2max2

max

44

2,

24,4

2,

2,

=⎟⎟⎠

⎞⎜⎜⎝

⎛=

⎟⎠⎞

⎜⎝⎛ ====

⎟⎠⎞

⎜⎝⎛ ===

To get larger gain, increase the ratio D / λ

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ECE 303 – Fall 2007 – Farhan Rana – Cornell University