array and phased array antennna

INTRODUCTION
• For some applications single element antennas are unable to
meet the gain or radiation pattern requirements.
• So, the concept of an antenna array was first introduced in
military applications in the 1940‟s .
• This development was significant in wireless communications
as it improved the reception and transmission patterns of
antennas used in these systems.
• The array also enabled the antenna system to be electronically
steered – to receive or transmit information primarily from a
particular direction without mechanically moving the structure.

ANTENNA ARRAY
• An antenna array is a set of N spatially separated antennas.
• Arrays in practice can have as few as N=2 elements, which is common for
the receiving arrays on cell phone towers.
• In general, array performance improves with added elements; therefore
arrays in practice usually have more elements.
• The signals from the individual antennas are combined or processed in
order to achieve improved performance over that of a single antenna.

Advantages of using antenna arrays
• They can provide a high gain (array gain) by using simple
antenna elements.
• They can provide the capability of a steerable beam, “steer”
the array so that it is most sensitive in a particular direction
• Provide diversity reception
• Cancel out interference from a particular set of directions
• Determine the direction of arrival of the incoming signals

Azimuthal patterns of two element isotropic array with different distance and
phase difference

d=0.25*lambda & pd=0






d=lambda & pd=0

d=lambda & pd=90

d=lambda & pd=90

Influence of the number of elements and element spacing on the
array factor
• The array directivity increases with the number of elements
• The element spacing also has a large influence on the array factor as well. A
larger element spacing results in a higher directivity. However, the element
spacing is generally kept smaller than lambda/2 to avoid the occurrence of
grating lobes.

Directivity of a 5 elements array with 0.2 (red), 0.3 (green) and 0.5
(blue) element spacing

Directivity of a 5 elements
array with 0.5 (red), 0.75
(green) and 1 (blue) element
spacing

Directivity of a 5 elements
array with 1 (green), 1.5 (red)
and 2 (blue) element spacing

Phased array antenna
 Phased array is a directive antenna made with individual radiating
sources (several units to thousands of elements).
 Radiating elements might be: dipoles, open-ended waveguides, microstrip
antennas, helices etc.
 The shape and direction of pattern is determined by relative phases
amplitudes applied to each radiating element.
 A phased array antenna offers the possibility to steer the beam by means of
electronic control (a dedicated computer is required).
 By altering the relative phase of the signal fed to each element the
direction of the beam can be moved because the direction of constructive
interference will move.



Because Phased array radars require no physical movement, the beam can
scan at thousands of degrees per second, fast enough to irradiate and track
many individual targets, and still run a wide-ranging search periodically.
 By simply turning some of the antennas on or off, the beam can be spread
for searching and narrowed for tracking, or even split into two or more
virtual radars.

Possible arrangements for phased array
 Linear Arrays

• A Linear array consists of elements arranged in a straight line in one
dimension. These antenna consists of lines whose elements are fed about a
common phase shifter. A number of vertically about each other mounted
linear arrays form a flat antenna.
• Advantage: simple arrangement
• Disadvantage: Ray deflection only in a single plane possible

 PLANAR ARRAY
• A Planar array is a two-dimensional configuration of elements arranged to lie in a plane.
The planar array may be thought of as a linear array of linear arrays.
• These antenna arrays completely consist of singles radiating elements and each of it
gets an own phase shifter. The elements are ordered in a matrix array. The planar
arrangement of all elements forms the complete phased-array antenna.
• Advantages: Beam steering in two planes is possible
• Disadvantage: complicated arrangement and more electronically controlled phase shifter
needed

 Conformal arrays

• In a conformal array, they are mounted on a curved surface, and the phase
shifters also compensate for the different phase shifts caused by the varying
path lengths of the radio waves due to the location of the individual antennas
on the curved surface.
• Because the individual antenna elements must be small, conformal arrays are
typically limited to high frequencies in the UHF or microwave range, where
the wavelength of the waves is small enough that small antennas can be used.

Passive Electronically scanned array(PESA)
• It is simply phased array radar. It takes a signal from a single source, split it
up into hundreds of paths, some of them selectively delayed, and sends to
individual antennas.
•

The resulting broadcasts overlapped in space, and the interference patterns
between the individual signals are controlled in order to reinforce the signal in
certain directions, and mute it down in all others.

• The delays can be easily controlled electronically, allowing the beam to be
steered very quickly without having to move the antenna. It can scan a
volume of space much more quickly than a traditional mechanical system.

Active Electronically scanned array(AESA)
• It also known as active phased array radar is a type of phased
array radar whose transmitter and receiver functions are composed of
numerous small solid-state transmit/receive modules (TRMs).
•

AESAs aim their "beam" by broadcasting radio energy that interferes
constructively at certain angles in front of the antenna. They improve on
the older passive electronically scanned radars by spreading their
broadcasts out across a band of frequencies, which makes it very difficult
to detect over background noise.

• In Terms of Advantages the AESA system is typically better than PESA
because of following factors :
• because the Antenna Elements contain its own Transmitter and Receiver
modules and located Right behind the Radiator, it would have lower or even
No accidental loss , unlike her PESA sister which still require a connection to
be made between its RF source to Radiating elements , incidental loss may
occur.
• High fault tolerance, failure of some modules will not hamper the operation
of Radar (typical AESA Radar may contain 100++ Elements) however the
entire device would fail if the failure occurs in 10% of total module counts.
• No Single Point Failure , since basically each modules in active Version
contains its own transmitter and Receiver modules, the passive device however
since it still use single Transmitter device like Traveling Wave tube , failure of
the transmitter may hamper the RADAR operations

APPLICATIONS
• Phased array antennas are electrically steerable, which means the physical
antenna can be stationary. This concept can eliminate all the headaches of a
gimbal in a radar system. It can keep an antenna locked onto a satellite, when
the antenna is mounted on a moving platform.
• It is what allows a satellite to steer its beam around your continent without
having to deal with the "slight problem" associated with trying to point things in
space where every movement would require an equal and opposite mass to
move in order to keep the satellite stabilized.
• A phased array receiver can be mounted on the top of a commercial airplane's so
that all of the happy passengers can receive satellite television!
• It is also used for weather forecast and tracking missiles and aircrafts; such as
search radar and tracking radar.

REFERENCES

 Antenna theory , Constantine A. Balanis, Wiley, 1997
 Phased array antenna handbook, Robert J. Mailloux, Artech House, 1994
 Hansen R. C., „Phased Array Antennas‟, Wiley
 http://www.radartutorial.eu/06.antennas/an16.en.html

array and phased array antennna

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