Antenna array

Concepts of Antenna Array and its applications

1. Concepts of Antenna Array and its applications Dr.A.Amsaveni Professor Department of ECE Kumaraguru College of Technology Coimbatore, Tamilnadu Email id: [email protected]

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3. Historical Perspective: Jill Tarter (1944) is an American astronomer who uses arrays of radio antennas to search for extraterrestrial life. She is the former director of the Center for SETI Research and was the inspiration for the main character in the movie Contact .

4. Presentation Outline Antenna Array Need for Antenna Array Types of Antenna Array Antenna Array parameters Pattern multiplication Applications of Antenna Array Smart Antennas

5. Antenna An antenna is a passive structure that serves as a transducer between transmission line and air in order to receive and transmit electromagnetic waves. An Antenna is an electrical device which converts electric energy into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. An antenna is a device used for sending or receiving Electromagnetic waves.

6. Antenna Array An antenna array is group of antennas connected and arranged in a regular structure to form a single antenna that is able to produce radiation patterns not produced by individual antennas. Antenna arrays are groups of isotropic radiators which radiate or receive electromagnetic energy. They provide a solution to the problems caused by single antennas. Antenna array composed of several similar radiating elements (e.g., dipoles or horns). Element spacing and the relative amplitudes and phases of the element excitation determine the array’s radiative properties.

7. Generally any combination of elements can form an array. However equal elements of regular geometry are usually used. Antenna array is used to modify the radiation pattern. Generally it is used to increase the directivity of the antenna. Usually, arrays have many applications. But, these are most often used to improve the gain and shape the radiation pattern of an antenna system effectively

8. Examples of Antenna array

9. Owens Valley Radio Observatory

10. The New Mexico Very Large Array 27 antennas along 3 railroad tracks provide baselines up to 35 km. Radio images are formed by correlating the signals collected by each antenna.

11. Need for Antenna Array Single element Provides Wide beam and low directivity To have very high gain and long distance antenna, Increase the electrical size of the antenna .But Increasing size -> difficult to build and expensive Useful especially when the element gain is low. Advantages of Antenna Array over single element Higher directivity Narrower beam Lower side lobes Electronic steerable beam The total field is determined by the vector addition of the fields radiated by the individual elements

12. In general antenna array can be used to : increase the overall gain provide diversity reception cancel out interference from a particular set of directions "steer" the array so that it is most sensitive in a particular direction determine the direction of arrival of the incoming signals maximize the Signal to Interference Plus Noise Ratio (SINR)

13. Controlling Parameters that can be used to shape the overall pattern of the antenna Geometrical configuration (linear, circular, etc.) Relative spacing between elements Excitation amplitude of individual elements Excitation phase of individual elements Relative pattern of individual elements Frequency of operation( wavelength ), f = c/ λ

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15. TYPES Of ANTENNA ARRAY 1.Linear Antenna Array Broad side array. End fire array. Parasitic array. Driven array. 2.Circular Antenna Array 3.Planar Array 4.Phased Array

16. Linear Array An antenna array is said to be linear, if the individual antennas of the array are equally spaced along a straight line. Uniform Linear Array A uniform linear array is one in which the elements are fed with a current of equal magnitude with uniform phase shift along the line.

17. Broadside array The broadside array is one in which a number of identical parallel antennas are set up along a line drawn perpendicular to their respective axes. In the broadside array, the individual antennas are equally spaced along a line and each element is fed with current of equal magnitude, all in the same phase. By doing so, this arrangement radiates in broadside directions ( i.e.,perpendicular to the line of array axis) where there are maximum radiation and relatively a little radiation in other directions. Radiation pattern of broadside array is bidirectional (radiates equally well in either direction of maximum radiation).

18. The bidirectional pattern of broadside array can be converted into unidirectional by installing an identical array behind this array at a distance  /4 and exciting it by a current leading in phase by 90° or  /2 radians.

19. End-fire array The end fire array is nothing but broadside array except that individual elements are fed in, out of phase (usually 180º). End fire array is one in which a number of identical elements are fed with currents of equal magnitude but their phases varies progressively along the line in such a way that it provides unidirectional pattern. End fire array may be defined as the arrangement in which the principal direction of radiation coincides with the direction of array. The radiation pattern of end fire array is unidirectional The end fire array may be bidirectional. Example, two elements array, fed with equal current, 180º out of phase.

20. Parasitic Array Antenna ( yagi-uda antenna) used for controlling radar and various other narrow beams such as the microwave communication system . Driven Array Antenna (Log –periodic antenna) Radiating elements are connected to an energy source and have less loss when compared to parasitic arrays, but still maintain the narrow beam characteristics, are used as search radar antennas where narrow beams are less critical when compared to low losses.

21. Circular Array Antenna elements arranged around a circular ring. Unlike linear arrays, circular arrays can provide a 2D angular scan, both horizontal φ and vertical θ scans. Unlike 2D planar arrays, circular arrays are basically 1D linear arrays but in a circular form. Unlike linear arrays, a circular array can scan horizontally for 360° with no distortions near the endfire directions. Applications are radio direction finding, air and space navigation, underground propagation, radar, sonar. Circular Microstrip Array

22. Binomial Array To reduce the sidelobe level, John Stone proposed that sources have amplitudes proportional to the coefficients of a binomial series of the form, where n is the number of sources. Thus for arrays of 2 to 6 sources, the relative amplitudes are given by,

23. These coefficients for any number of radiating sources can also be obtained from Pascal’s triangle. In Pascal’s triangle, each internal integer (leaving the side integer) is the sum of the above adjacent integers. However , the elimination of secondary lobes takes place at the cost of directivity .

24. But the half power beam width (HPBW) of binomial array is more than that of uniform array for the same length of array. Example: for n = 5, spaced  /2 apart, HPBW of binomial array is 31° with no side lobes, But HPBW of uniform array is 23°. Thus, in uniform array secondary lobes appear but principle lobe is sharp and narrow where as in Stone’s binomial array, width of beam widens but without secondary lobes. Disadvantages 1. HPBW increases and hence the directivity decreases. 2. For design of a large array, larger amplitude ratio of sources is required.

25. Array Antenna Characteristics Gain An antenna array can achieve higher gain (directivity), that is a narrower beam of radio waves, than could be achieved by a single element. In a broadside array, the array gain is almost exactly proportional to the length of the array. Radiation pattern The radiation pattern of such an antenna consists of a strong beam in one direction, the main lobe, plus a series of weaker beams at different angles called sidelobes , usually representing residual radiation in unwanted directions. Directivity Directivity is also defined for an antenna receiving electromagnetic waves its directivity when receiving is equal to its directivity of transmitting. The directivity of an actual antenna can vary from 1.76 dBi for a short dipole, to as much as 50 dBi for a large

26. Beam-width The beam width is the product of the individual antenna pattern and the "array factor" shape. Bandwidth It is is affected by many factors, Including impedances with frequency, change of array spacing in wavelengths that may allow grating lobes, change in element beam width, and so on… When an array is scanned with fixed units of phase shift, provided by phasers , there is also a band width limitation as the position of the main beam will change with frequency.

27. Directivity In an antenna array, the directivity of the entire array is the multiplicative sum of the individual antenna's directivity function with a mathematical expression known as the array factor which typically depends on the location, the excitation and the phase of each antenna element. The overall directivity function is given by, Darray ( β,Φ)= AF * D( β,Φ) Where D(β,Φ) is the directivity of a single element. This single element term is sometimes referred to as the element pattern.

28. Phased Arrays Array of N antennas in a linear or two-dimensional configuration + beam-forming & control device The amplitude and phase excitation of each individual antenna controlled electronically (“software-defined ”) Phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas. In phased array, the power from the transmitter is fed to the antennas through devices called phase shifter. High gain low sidelobes Multi-target handling multifunction by radiating several beam More reliable

29. Consist of multiple (usually identical) antennas (elements) ‘collaborating’ to synthesize radiation characteristics not available with a single antenna. They are able to match the radiation pattern to the desired coverage area to change the radiation pattern electronically (electronic scanning) through the control of the phase and the amplitude of the signal fed to each element to adapt to changing signal conditions to increase transmission capacity by better use of the radio resources and reducing interference Complex & costly Intensive research related to military, space, etc. activities

30. Phased array antennas are an antenna that has multiple radiating elements each connecting to a phase shifter. The phase shifting allows the radiation pattern to be “steered” towards a certain direction. The lobe is increased upward as a result of the phase shift demonstrating by changing the phase one can steer the beam in desired location without actually moving the element. A  phased array antenna  is an array antenna whose single radiators can be fed with different phase shifts. As a result, the common antenna pattern can be steered electronically. The electronic steering is much more flexible and requires less maintenance than the mechanical steering of the antenna. Enabled - electronically steered – to receive or transmit info from a particular direction without mechanically moving the structure. Appln : Used in Radars as it requires antennas with steering capabilities.

31. Advantages and Disadvantages of Phased Arrays Advantages: high antenna gain with large side-lobe attenuation very fast change of beam direction (in range of microseconds) high beam agility arbitrary space scanning multi-function operation by simultaneous generation of multiple beams failure of some components does not result in a complete system failure. Disadvantages: limited scanning range (up to max. 120° in azimuth and elevation) Deformation of the antenna pattern during beam steering low frequency agility very complex structure (computer, phase shifter, data bus to each radiator) high cost

32. Possible Arrangements of Phased Array Linear Array These phased array antennas consist of lines, which are commonly controlled by a single phase shifter. (Only one phase shifter is needed per group of radiators in this line.) A number of linear arrays arranged vertically on top of each other form a flat antenna. Advantage :  simple arrangement; Disadvantage :  beam steering only in a single plane; Planar Array These phased array antennas consist completely of single elements with a phase shifter per element. The elements are arranged like a matrix, the flat arrangement of all elements forms the entire antenna. Advantage:  beam deflection possible in two planes Disadvantage :  a large number of phase shifters

33. Frequency Scanning Array The frequency scanning array is a special case of the phased array antenna, in which the beam steering is controlled by the transmitter's frequency without use of any phase shifter. The beam steering is a simple function of the frequency. This type of phased array antenna was often used in older radar sets.

34. Types of phased array Passive phased array(PESA ) Active electronically scanned array(AESA ) Hybrid beam forming phased array Digital beam forming(DBF) array Disadvantages Complex structure High cost Low frequency bandwidth 44

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36. Planar Array An antenna in which all of the elements, both active and parasitic, are in one plane. A  planar array  provides a large aperture and may be used for directional beam control by varying the relative phase of each element. Antenna elements arranged over some planar surface (example - rectangular array). Planar arrays are more versatile; They provide more symmetrical patterns with lower side lobes, much higher directivity (narrow main beam). They can be used to scan the main beam toward any point in space. much higher directivity (narrow main beam). Applications – tracking radars, remote sensing, communications, etc.

37. Smart antenna systems

38. Why Smart antennas? Increases the number of subscribers(Higher system capacity) Higher data transmission rate Better spectral efficiency Extended range Processing of multipath propagating signal TYPES: Switched beam- a finite number of fixed, predefined patterns or combining sectors Adaptive array - an infinite number of patterns (scenario-based ) that are adjusting in real time. 48

39. Smart antenna is not smart, it is the digital signal processing doing with antenna which makes the system smart. It is the antenna phased array with DSP processor High gain for desired direction Interference Rejection Increases the system capacity 49

40. Adaptive (“Intelligent”)Antennas Array of N antennas in a linear, circular, or planar configuration Used for selection signals from desired sources and suppress incident signals from undesired sources. The antenna pattern track the sources. It is then adjusted to null out the interferers and to maximize the signal to interference ratio (SIR). Able to receive and combine constructively multipath signals.

41. Adaptive antenna array A set of 48 2GHz antennas An adaptive antenna is an antenna of which the antenna radiation (or reception) pattern can be adjusted dynamically. An adaptive antenna consists of a number of antennas(called as elements). Adaptive antennas use signal processing algorithms to combine the signals from the different elements in such a way that the desired signal is received (or sent in a particular direction) or that interfering signals or multipath signals are optimally suppressed.

42. Switched beam array Switched beam  systems have several available fixed  beam  patterns . A decision is made as to which  beam  to access, at any given point in time, based upon the requirements of the system . Overall goal of switched beam system to increases gain according to the location of the user  

43. ANTENNA ARRAY APLICATIONS An array widely used as a base-station antenna for mobile communication. Yagi- Uda array is used to TV and radio application Log periodic antenna is used for TV with wider bandwidth Broadcasting Beam forming Naval usage optical phased arrays Radio-frequency identification (RFID) Weather research usage Missiles guidance Fire radar aircraft…etc.. Are the areas in which array antennas are used

44. Application of Antenna Arrays to Mobile Communications Array processing is expected to play an important role in fulfilling the increased demands of various mobile communications services Array processing involves manipulation of signals induced on various antenna elements. Its capabilities of steering nulls to reduce co-channel interferences and pointing independent beams toward various mobiles It has the ability to provide estimates of directions of radiating sources, make it attractive to a mobile communications system designer

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