ANTENNA ARRAYS

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ANTENNA ARRAYS
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Array Factor (1)
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Phased Array Antennas

• Each antenna element can be controlled
  individually by phase or time delay.
• By changing the feeding it is possible to
  construct a directive beam that can be
  repositioned electronically.
• Amplitude control can be used for pattern shaping
• The beam can be pointed to new direction,
  narrowed or widened in microseconds.
• An array that has a main peak at a certain angle
  can also have other peak values depending on the
  spacing between the antenna elements.

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Grating Lobes
AF for uniform excitation
AF will have a maximum when exponent is a
multiple of 2?
grating lobes will occur at
to avoid grating lobes
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8 element array with ?/d1 and for uo0.5
(scan angle of 30o)
uo0.5 (scan angle of 30 degrees)
uo0 (broadside)
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Mutual Coupling

• element pattern of the antenna changes from its
  free space (isolated) value when it is inserted
  into an array
• this coupling effect will be different for each
  element of the array.
• it may be necessary to use the concept of active
  element pattern

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Element pattern of a dipole located as a center
element of a 7X9 array
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Analysis Including Mutual Coupling

• In a strong mutual couping environment
• array pattern element pattern X array factor
• does not work ! Solving the problem using
  numerical methods is not practical.
• Therefore other effective methods are needed to
  account for mutual coupling effects.

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Mutual Coupling (cont.)

• Finite Array Approach
• Used for small and medium arrays.
• Active element pattern is calculated separately
  for each element in the array.
• these patterns are added up to obtain theoverall
  array pattern.

may imply simultaneous solution of thousands of
equations
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Mutual Coupling (cont.)

• Infinite array assumption
• For large arrays, the central elements that are
  far away from edges are affected less
• infinite array concept can then be used
• It is assumed that for all elements the currents
  are similar except for some complex constants.
• When this approach is used, it is sufficient to
  analyze only one element completely

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Mutual Coupling (cont.)

• For medium size arrays, the exact AEP methods
  are difficult to use and average AEP method
  yields in errors in calculating the array pattern
  
• For these arrays the combination of the two
  methods are used to obtain more accurate results
  for the array pattern

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Array 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

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Array Performance

• Array Lattice
• Array Bandwidth
• Differences Between Single Element and Array
  Performances
• Amplitude Tapering For Sidelobe Level Control
• Wide-Angle Impedance Matching (WAIM)

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Array PerformanceArray Lattice

• The position of the array elements describes the
  array lattice and there are basically three types
  for planar arrays

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Array PerformanceArray Bandwidth

• The bandwidth of the array depends on the
  radiators, phase shifters, feeding networks etc.
• Phase shifters and feeding networks possess
  error transfer functions which grows with
  increasing bandwidth.
• The error analysis of the effect on the pattern
  will determines the bandwidth.

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Array PerformanceSingle Element and Array
Performance

• Due to the mutual coupling effects in the array
  environment the single element performance and
  the array performance of most antennas are
  different

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Array PerformanceAmplitude Tapering for Sidelobe
Level Control

• The amplitude tapering in the excitation of the
  array elements determines the array sidelobe
  level, array gain and the beamwidth.
• Stronger tapering results in reduced sidelobe at
  the expense of increased beamwidth and reduced
  gain.
• - Powers of cosine
• - Taylor distributions
• - Modified Sin ?u/?u taper of Taylor
  distributions
• - Dolph-Chebyshev distributions

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Array PerformanceModified Sin?u/?u taper of
Taylor Distributions
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Array PerformanceDolph-Chebyshev Distributions

• Is the optimum distribution in the sense of
  narrowest beam for a given SLL
• Sidelobes do not decay in amplitude.
• The power of percentage in the main beam
  varies with the number of elements in the array
  for a given SL

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Example of illumination coefficients and array
pattern for a 20 dB taper applied to a 16 element
array
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Array PerformanceWide-Angle Impedance Matching
WAIM

• Scan impedance is the impedance of an element as
  a function of scan angle with all elements
  excited with proper amplitude and phase.
• For wide scan angles another mismatch due to the
  scan angle occurs.
• WAIM techniques are used to overcome this
  problem
• - Transmission line region techniques
• - Free space WAIM techniques

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Array PerformanceWide-Angle Impedance Matching
WAIM

• Transmission Line Techniques
• Passsive circuits to control higher order
  modes in the aperture
• - separate interconnections between the
  elements
• - active tuning circuits
• Free Space Techniques
• - Reduced element spacing
• - Dielectric slabs or dielectric sheets