PHASED ARRAY ANTENNA

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

• The most basic property of an array is that the relative displacements of the antenna elements with respect to each other introduce relative phase shifts in the radiation vectors, which can then add constructively in some directions or destructively in others.

• This is a direct consequence of the translational phase-shift property of Fourier transforms: a translation in space or time becomes a phase shift in the Fourier domain.

• The current density of the translated antenna will be Jd(r) = J(r-d).

• By definition, the radiation vector is the three-dimensional Fourier transform of the current density, as in Eq.1. Thus, the radiation vector of the translated current will be

• Figure below shows on the left an antenna translated by the vector d, and on the right, several antennas translated to different locations and fed with different relative amplitudes.

• The current density of the nth antenna will be Jn(r) = an J(r – dn) and the corresponding radiation vector:

• The total current density of the array will be

And total radiation vector

• The factor F(k) due to a single antenna element at origin is common to all terms. Thus, we obtain the array pattern multiplication property:

• Where A(k) is array factor: • Which incorporates all the translational phase shifts and relative weighting

coefficients of the array elements.

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

d=0.25*lambda & pd=0 d=0.25*lambda & pd=90 d=0.25*lambda & pd=180

d=0.50*lambda & pd=0 d=0.50*lambda & pd=90 d=0.50*lambda & pd=180

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

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