Chapter 4 Antenna Arrays

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.

Printed Circuit Dipole Array

Two-dimensional Arrays

Two-Dimensional Array of Dipoles

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

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

Array-antenna power density is given by

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

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

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.

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.

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.

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.

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

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

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

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

d=0.2l

d=0.5l

d=1.5l

d=0o

d=60o

d=120o

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)

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.