Final Microwave
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Transcript of Final Microwave
Antenna Arrays
Submitted To
Sumita Shekhawat
Submitted By
Utkarsh Chinmay Paliwal
2011BTechECE024
Dinesh Kumar Goyal
2011BTechECE009
Contents
1.Antenna Arrays
2.Design Principles of Arrays
3.Types of Arrays
4. Isotropic Points
5. Radiation Pattern & Array Factor
6. Array pattern
7. Application of Arrays
Antenna Arrays
• An antenna array (often called a 'phased array') is a set of
2 or more antennas. The signals from the antennas are
combined or processed in order to achieve improved
performance over that of a single antenna.
• An antenna array is a cluster of antennas arranged in a
specific physical configuration (line, grid).
• Each individual antenna is called an element of the array.
• We initially assume that all array elements (individual
antennas) are identical.
• The excitation (both amplitude and phase) applied to each
individual element may differ.
• The far field radiation from the array in a linear medium
can be computed by the superposition of the EM fields
generated by the array elements.
Design Principles of Arrays
Array Design Variables:
1. General array shape (linear, circular, planar)
2. Element spacing.
3. Element excitation amplitude.
4. Element excitation phase.
5. Patterns of array elements.
Types of Arrays
• Linear array - antenna elements arranged along a straight line.
• Circular array - antenna elements arranged around a circular ring.
• Planar array - antenna elements arranged over some planar surface
(example - rectangular array).
• Conformal array - antenna elements arranged to conform two some
non-planar surface (such as an aircraft skin).
• 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
Types of Array
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 driven element and slightly shorter
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 ends
Types of Array 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
Collinear Array
• All elements along same axis
• Used to provide an omnidirectional horizontal pattern from a
vertical antenna
• Concentrates radiation in horizontal plane
Broadside Array
• Bidirectional Array
• Uses Dipoles fed in phase and separated by 1/2 wavelength
End-Fire Array
• Similar to broadside array except dipoles are fed 180 degrees out of phase
• Radiation max. off the
Isotropic Points
The individual elements are characterized by
their element patterns F1(,).
At an arbitrary point P, taking into account the
phase difference due to physical separation
and difference in excitation, the total far zone
electric field is:
2 2
1 2( ) ( ) ( )j jE r E r e E r e
Field due to antenna 1 Field due to antenna 2
Here: coskd
The phase center is assumed at the array center. Since the elements are identical
2 2
1 1( ) 2 ( ) 2 ( )cos2 2
j je eE r E r E r
Relocating the phase center point only changes the phase of the result but not its
amplitude.
Radiation Pattern & Array Factor
• The radiation pattern can be written as a product of the radiation pattern of an
individual element and the radiation pattern of the array (array pattern):
1( , ) ( , ) ( , )aF F F
• The array factor is:
cos( , ) cos
2a
kdF
Here is the phase difference between two antennas. We notice that the array
factor depends on the array geometry and amplitude and phase of the excitation
of individual antennas.
Case 1: 2 isotropic point sources of same amplitude and phase
Case 1: 2 isotropic point sources of same amplitude and phase
Phase Difference =
Path Difference =
Case 1: 2 isotropic point sources of same amplitude and phase
The total field strength at a large distance r in the direction is :
Therefore:
Case 2: 2 isotropic point sources of same amplitude and opposite phase
Case 2: 2 isotropic point sources of same amplitude and opposite phase
Array pattern
Applications of Arrays
• To cancel co channel interferences : an array works on the
premise that the desired signal and unwanted co channel
interferences arrive from different directions.
• The beam pattern of the array is adjusted to suit the requirements
by combining signals from different antennas with appropriate
weighting.
• An array of antennas mounted on vehicles, ships, aircraft, satellites, and base stations is expected to play an important role
in fulfilling the increased demand of channel requirement for
these services
Application of Arrays
Use of an antenna array is to enhance the efficiency of
mobile communications systems. It presents an overview of
mobile communications as well as details of how an array may be used in various mobile communications systems,
including land-mobile, indoor-radio, and satellite-based
systems.
In a mobile communications system, highlights
improvements that are possible by using multiple antennas
compared to a single antenna in a system, and provides
details on the feasibility of antenna arrays for mobile
communications applications.
References
Wikipedia
CiteSeerX
E-Learning VTU
Antenna Theory