ANTENNA ANTENNA ARRAYARRAYSS

A A Short Short RRevieweview

Array Factor (1)Array Factor (1)

)ˆ.exp(),(1

rrjkaf mo

n

mm

),( fHH ref

),( fEE ref

Uniform, Linear ArrayUniform, Linear Array Equally spaced elements along the z-axisEqually spaced elements along the z-axis

n

m

kdmjen

f1

)cos)(1(1),(

Different radiation patterns can be obtained by changing d and

Phased Array AntennasPhased Array Antennas Each antenna element can be controlled Each antenna element can be controlled

individually by phase or time delayindividually by phase or time delay.. By changing the feeding it is possible to By changing the feeding it is possible to

construct a directive beam that can be construct a directive beam that can be repositioned electronicallrepositioned electronically.y.

AAmplitude control mplitude control can becan be used for pattern used for pattern shapingshaping

The beam can be pointed to new direction, The beam can be pointed to new direction, narrowed or widened in microseconds.narrowed or widened in microseconds.

An array that has aAn array that has a main main peak at peak at a certain a certain angleangle can also have other peak values can also have other peak values depending on the spacing between depending on the spacing between the the antenna elements.antenna elements.

Grating LobesAF for uniform excitation:

))(exp()( oom uudjmkaf

oou sin sinuAF will have a maximum when exponent is a multiple of 2

pd

o

2)sin(sin2

grating lobes will occur at:d

pop

sinsin

to avoid grating lobes:

oo

d

sin1

1

8 element array with /d=1

and for uo=0.5 (scan angle of 30o)

uo=0 (broadside) uo=0.5 (scan angle of 30 degrees)

030030030

Mutual CouplingMutual Coupling

element pattern of the antenna changes element pattern of the antenna changes from its free space (isolated) value when from its free space (isolated) value when it is inserted into an arrayit is inserted into an array

this coupling effect will be different for this coupling effect will be different for each element of the array. each element of the array.

iit may be necessary to use the concept t may be necessary to use the concept of “of “active element patternactive element pattern””

Element pattern of a dipole located as a center element of a 7X9 array

Analysis Including Mutual CouplingAnalysis Including Mutual Coupling

In a strong mutual couping environmentIn a strong mutual couping environment

array pattern = element pattern X array array pattern = element pattern X array factor factor

does not workdoes not work ! Solving ! Solving the the problemproblem using using numerical methods is not practical.numerical methods is not practical.

Therefore other Therefore other effective methods are effective methods are needed to needed to account foraccount for mutual coupling mutual coupling effects.effects.

Mutual Coupling (Mutual Coupling (cont.cont.))

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iitot EE

1

Finite Array Approach:Finite Array Approach:

Used for small and medium arraUsed for small and medium arrays.ys. AActive element patternctive element pattern is calculated separately for is calculated separately for

each element in the arrayeach element in the array.. these patterns are added up to obtain thethese patterns are added up to obtain theoveralloverall array array

pattern.pattern.

may imply simultaneous solution of thousands of equations

Mutual Coupling (Mutual Coupling (cont.cont.))

Infinite array assumption:Infinite array assumption: For large arraysFor large arrays,, the central elements that are the central elements that are

far away from edges are affected lessfar away from edges are affected less

infinite arrayinfinite array concept concept can can then then be be usedused

It is assumed that for all elements the It is assumed that for all elements the currents are currents are similarsimilar except except for for some complex some complex constants. constants.

When thisWhen this approach is used approach is used,, it is it is sufficientsufficient to to analyze only one element completely analyze only one element completely

Array BlindnessArray Blindness

• Direct consequence of mutual coupling

• Can result in complete cancellation of the radiated beam at some scan angle

• Occurs when most of the central elements of the array have reflection coefficients close to unity