1. Chapter 4 Antenna Arrays

2. Introduction
Antenna arrays refer to groups of similar antennas, each with its own amplitude and phase relation, that are arranged in certain configurations to produce certain desired radiation characteristics.
Generally, an array consists of similar radiating elements.

4. Printed Circuit Dipole Array

5. Two-dimensional Arrays

6. Two-Dimensional Array of Dipoles

7. In the far-field region of any radiating element, the phasor electric-field intensity (R, , ) may be expressed as

9. The far-zone field due to element i at range Ri from Q
where Ai = ai eji is a complex feeding coefficient

11. Array-antenna power density is given by

12. The array factor represents the far-field radiation intensity of the N elements and it is denoted by

13. The power density of the antenna array is then written as
This equation is called the pattern multiplication principle

14. The array factor is governed by two input functions:
the array amplitude distribution given by the ai’s and
the array phase distribution given by the i’s.
The amplitude distribution serves to control the shape of the array radiation pattern, while the phase distribution can be used to steer its direction.

15. Example Array of Two Vertical Dipoles
An AM radio station uses two vertically oriented half-wave dipoles separated by a distance of /2, as shown in Fig. 9-28(a). The vector from the location of the first dipole to the location of the second dipole points toward the east. The two dipoles are fed with equal-amplitude excitations, and the dipole farther east is excited with a phase shift of -/2 relative to the other one. Find and plot the antenna pattern of the antenna array in the horizontal plane.

18. Example Pattern Synthesis
In the previous example, we were given the array parameters a0, a1, 0, , and d, and we were then asked to determine the pattern of the two element dipole array. We will now consider the reverse process; we will be given specifications on the desired pattern, and we are then asked to specify the array parameters to meet those specifications.

19. Given two vertical dipoles, as depicted in Fig
Given two vertical dipoles, as depicted in Fig. 9-28(b), specify the array parameters such that the array exhibits maximum radiation toward the east and no radiation toward the north or south.

22. Electronic scanning of Arrays
The normalized array factor is given by
where g= d + k d cosq

23. Uniform phase distribution
The normalized array factor is given by
where g= k d cosq

24. The normalized radiation intensity can be expressed as
where Fe() is the normalized radiation intensity of one element

25. Example
Discuss the radiation pattern of a linear array of two isotropic sources spaced a distance d apart and have a phase difference d for the following cases
d=0o
i) d= 0.2 l ii) d= 0.5 l iii) d= 1.5l
d= l/3
i) d= 0o ii) d = 60o iii) d =120o

26. d=0.2l

27. d=0.5l

28. d=1.5l

29. d=0o

30. d=60o

31. d=120o

33. Broadside arrays are arrays that generate their maximum radiation perpendicular to the array axis. (In this case q=90o).
End-Fire arrays are arrays that directs its main lobes along the array axis (In this case q=0o)

34. Example: Multiple-Beam Array
Obtain an expression of the array factor of a two-element array with equal excitation and a separation d = 7/2, and then plot the array pattern.