Antenna Array

Chapter 3

1 Array Antenna Introduction

• Usually the radiation patterns of single-element antennas are relatively wide. i.e., they have relatively low directivity (gain). • In long distance communications, antennas with high directivity are often required. Such antennas are possible to construct by enlarging the dimensions of the radiating aperture (maximum size much larger than λ ). • This approach however may lead to the appearance of multiple side lobes. Besides, the antenna is usually large and difficult to fabricate.

2 Array Antenna Introduction cont.

• Another way to increase the electrical size of an antenna is to construct it as an assembly of radiating elements in a proper electrical and geometrical configuration – as known as antenna array. • Usually, the array elements are identical. • This is not necessary but it is practical and simpler for design and fabrication. • The individual elements may be of any type (wire dipoles, loops, apertures, etc.)

3 Array Antenna Five basic methods

• To control the overall antenna pattern: a) the geometrical configuration of the overall array (linear, circular, spherical, rectangular, etc.) b) the relative placement of the elements. c) the excitation amplitude of the individual elements. d) the excitation phase of each element. e) the individual pattern of each element.

4 Array Antenna Radiation Pattern

• Radiation pattern of array antenna is called an Array Factor (AF). • Array factor can be expressed using this formula:  sin(N ) 2 Where : AF = N = Total Element  N sin( ) 2 k = 2π/ λ is the polar angle is the difference of phase between any two successive elements forming the array.

5 Array Antenna Array Antenna

• At the end of the chapter, you should able: – What is the array antenna application – What is array factor and how to determine it – What is the radiation pattern for array antenna look like

6 Array Antenna Radiation pattern for vertical plane

(a) Single halfwave dipole (a) Single halfwave dipole

(b) two-elemen array (b) two-elemen array

7 Array Antenna Why An Array of Antenna?

• Single element is relatively wide and low gain (directivity) • To have very high gain and long distance antenna, this can be accomplished by increasing the electrical size of the antenna • Enlarge dimensions of the antenna without increasing the size of individual elements is to form an assembly of radiating elements in an electrical and geometrical configuration • This new antenna, formed by multielements, is referred to as an array • Most cases, the elements of an array are identical • The total field is determined by the vector addition of the fields radiated by the individual elements

8 Array Antenna Antenna Array Application

• An array is widely used as a base-station antenna for mobile communication • Each four-element array is used to cover an angular sector of 120o

9 Array Antenna Antenna Array Application

• Yagi-Uda array is used to TV and Amerteur radio application • Log periodic antenna is used for TV with wider bandwidth

Yagi Uda Log periodic

10 Array Antenna Examples of antenna arrays

• Four-element microstrip antenna array (phased array).

11 Array Antenna

• Cell-tower Antenna Array. These Antenna Arrays are typically used in groups of 3 (2 receive antennas and 1 transmit antenna)

12 Array Antenna Array Theory

• Configuration of individual radiating elements that are arranged in space and can be used to produce a directional radiation pattern • Allows shaping of radiation pattern – Narrow beam – Low sidelobe – Higher gain & directivity • Arrays usually employ identical antenna elements

13 Array Antenna Advantages of using antenna arrays

Antenna arrays are becoming increasingly important in wireless communications. 1. They can provide the capability of a steerable beam (radiation direction change) as in smart antennas. 2. They can provide a high gain (array gain) by using simple antenna elements. 3. They provide a diversity gain in multipath signal reception. 4. They enable array signal processing.

14 Array Antenna Far-Field Expression of An Antenna Array

15 Array Antenna Uniform Liner Arrays (ULAs)

16 Array Antenna Two element array

• Let us assume, that two infinitesimal horizontal dipole antennas positioned along the z-axis as depict in figure below. • The total field of the array is determined by the vector addition of the fields radiated by the individual elements. • The electric field pattern in the y-z plane for one element is given by:

17 Array Antenna

18 Array Antenna The far-field approximation

• The far field approximation of this two element array can be illustrated as in figure below:

19 Array Antenna Derivation Array Factor (1)

20 Array Antenna Derivation Array Factor (2)

21 Array Antenna Pattern Multiplication (1)

22 Array Antenna Pattern Multiplication (2)

• The concept of pattern multiplication valid for arrays with any number of identical elements. • So each array has its own array factor (AF). • The total pattern, therefore, can be controlled via the single- element pattern or via the AF of an array can be obtained by replacing the actual elements with isotropic sources. • The AF, in general, depends on: – Number of elements. – Relative excitation (magnitudes and phases). – Spacing between the elements.

23 Array Antenna N-element Linear Array with Uniform Amplitude and Spacing

24 Array Antenna

25 Array Antenna

26 Array Antenna

As we aim at obtaining the normalized AF, we will neglecting the phase factor, which gives

27 Array Antenna Typical Radiation Pattern

28 Array Antenna Direction of Maximum Radiation

29 Array Antenna

• An array is said to be End-fire array if the main beam is along the axis of the array. • An array is said to be Broadside array if the main beam is perpendicular to the axis of the array. • There are two end-fire directions for an array but the broadside is a plane perpendicular to the array axis (see Fig below)

30 Array Antenna Directions of Nulls

31 Array Antenna Terminology Antenna – structure or device used to collect or radiate electromagnetic waves Array – assembly of antenna elements with dimensions, spacing, and illumination sequence such that the fields of the individual elements combine to produce a maximum intensity in a particular direction and minimum intensities in other directions Beamwidth – the angle between the half-power (3-dB) points of the main lobe, when referenced to the peak effective radiated power of the main lobe Directivity – the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions Effective area – the functional equivalent area from which an antenna directed toward the source of the received signal gathers or absorbs the energy of an incident electromagnetic wave Efficiency – ratio of the total radiated power to the total input power Far field – region where wavefront is considered planar Gain – ratio of the power at the input of a loss-free isotropic antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance Isotropic – radiates equally in all directions Main lobe – the lobe containing the maximum power Null – a zone in which the effective radiated power is at a minimum relative to the maximum effective radiation power of the main lobe Radiation pattern – variation of the field intensity of an antenna as an angular function with respect to the axis Radiation resistance – resistance that, if inserted in place of the antenna, would consume that same amount of power that is radiated by the antenna Side lobe – a lobe in any direction other than the main lobe 32 Array Antenna Tutorial for Array Antenna (1)

A three elements array of isotropic sources has the phase and the magnitude relationship as shown in figure below. The λ spacing between the elements is d = . 2 (i) Find the array factor z (ii) Find the nulls #2 -1 d -j #1 y

d #3 +1

33 Array Antenna Tutorial for Array Antenna (2) four isotropic sources with spacing d between them are placed along the z-axis as shown in figure below. Assuming that the amplitudes of elements #1 and #2 are +1 and the amplitudes of elements #3 and #4 are -1, find the, z (i) The array factor. #2 -1 (ii) The nulls when d= λ/2 d d/2 #1 -j -j d/2 y #3 d #4 +1