Antenna Array

NUS/ECE

EE4101

Antenna Arrays
1 Introduction Antenna arrays are becoming increasingly important in wireless communications. Advantages of using antenna arrays: 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.
Hon Tat Hui

1

Antenna Arrays

NUS/ECE

EE4101

An important characteristic of an array is the change of its radiation pattern in response to different excitations of its antenna elements. Unlike a single antenna whose radiation pattern is fixed, an antenna array’s radiation pattern, called the array pattern, can be changed upon exciting its elements with different currents (both current magnitudes and current phases). This gives us a freedom to choose (or design) a certain desired array pattern from an array, without changing its physical dimensions. Furthermore, by manipulating the received signals from the individual antenna elements in different ways, we can achieve many signal processing functions such as spatial filtering, interference suppression, gain enhancement, target tracking, etc.
Hon Tat Hui

2

Antenna Arrays

NUS/ECE

EE4101

2 Two Element Arrays z Far field observation point

θ
Dipole 2

r1

I 2 = Ie jβ d θ
I1 = I

r

r1 = r − d cos θ , 0 ≤ θ ≤ π

Dipole 1

x

Two Hertzian dipoles of length dℓ separated by a distance d and excited by currents with an equal amplitude I but a phase difference β [0 ~ 2π).
Hon Tat Hui

3

Antenna Arrays

NUS/ECE

EE4101

E1 = far-zone electric field produced by antenna 1 = E2 = far-zone electric field produced by antenna 2 =

ˆ aθ E1 ˆ aθ E2

η kI1d ⎛ e− jkr ⎞ ⎛ π ⎞ η kd ⎛ e − jkr ⎞ E1 = j ⎜ r ⎟ sin ⎜ θ + 2 ⎟ = j 4π ⎜ r ⎟ cos θ I1 4π ⎝ ⎠ ⎠ ⎝ ⎝ ⎠ η kI 2 d ⎛ e − jkr ⎞ ⎛ π ⎞ η kd ⎛ e− jkr ⎞ E2 = j ⎜ r ⎟ sin ⎜ θ