ICT083 Antenna Design

Linear Array Antennas, Basic Level

I. Theory

1. Introduction

1) Why use array antenns?

- Narrow beam or high gain antenna

- Electronic beam scanning: Phased array antenna

- Smart antenna: Adptive digital beamforming

Figure: Antenna arrays and their directivity patterns [O'Donnell]

2) Linear array:

- Place many antenna elements on a straight line (= array axis).

- Narrow beam in the plane of the array axis.

- Wide beam in the orthogonal plane

3) Applications of linear array antennas

- Mobile communication base station antennas

- Marine navigational radar antennas

- Broadcast transmitting antennas

- Radio astronomy antennas

4) Constituents of an antenna array

- Array elements: Radiators, radiating elements

- Feed network: Series, parallel, series/parallel, space-fed

- Array beam control unit: Passive, active

2. Theory of Antenna Arrays

2.1 Two-element array

Figure: Two-element array of an ideal dipole

An ideal dipole with z-directed current at origin:

Two-element array of an ideal dipole:

(element factor)

(array factor)

(pattern factor) : principle of pattern multiplicaton

EF(element factor): Radiation pattern of a single element of the array

AF(array factor): Radiation pattern due to the element arrangement

PF(pattern factor): Radiation pattern of an array antenna

Figure: Principle of pattern multiplication [O'Donnell]

Simple examples of array factor:

- Element: Isotropic radiator

Figure: Simple examples of array factor

2.2 Linear Arrays

2.1 Array factor of a linear array

Figure: Uniformly spaced linear array [Stutzman]

: element position

: element current

A_n : current magnitude

nα : current phase

Beam scanning:

: phase difference between elements

(Important) The main lobe is tilted in the direction of decreasing phase.

: Scan to the left

: Scan to the right

The case is symmetric to the case .

Broadside, scanned and endfire array:

- Broadside array:

- Endfire array: or

- Scanned array:

Figure: Broadside, endfire, and scanned arrays [Balanis]

Figure: Broadside, scanned, and endfire arrays (N = 20, d = λ/4) [O'Donnell]

2.2 Graphical Methods of Array Pattern Plotting

Uniformly excited linear arrays:

: normalized array factor

Properties of :

- Maximum at ψ = 2nπ (n = 0, ±1, ±2, ...)

- Period of ψ : 2π

- Symmetric with respect to ψ = π

- Distance between nulls:

: major lobes

: minor lobes

Figure: Array factor of uniformly excited linear arrays with N = 3 and 5 [Stutzman]

Graphical Methods for Plotting the Array Factor:

- First plot the array factor

- Next plot a circle for the polar radiation pattern diagram with radius and center at .

Figure: Graphical method for plotting a polar pattern of the array factor [Stutzman]

Uniform two-element array:

Figure: Pattern plot for uniform two-element array. (b) d = 0.5λ, α = 0, (c) d = 0.5λ, α = π, (d) d = 0.25λ, α = –π/2

Four-element scanned array:

Figure: Examplex of array factor plotting (N = 4, α = π/2, d = λ/2) [Stutzman]

2.3 Grating Lobes

- Grating lobe: More than one major lobe

AF is maximum when

: Main lobe

: Grating lobes

No grating lobe condition:

: Visible region

Figure: Grating lobes in 10-element broadside (top) and enfire (bottom) arrays with element spacing of λ/4, λ/2, and λ [O'Donnell]

II. Exersice Problems

Element: Half-wave dipole

Element spacing: d

Dipole current: In y direction

Array current distribution: Magnitude = uniform with phasing to be designed for beam scanning

Array axis: In z direction

Design the following array plot the element factor, the array factor, and the pattern factor in the E- and H-plane.

No part of the grating lobe appears in the visible region.

1. N = 2,

2. N = 5,

3. N = 6,

III. Examples of Linear Array Antennas

Figure: Linear array antennas in mobile communicatios base stations [ESTEL, Wirelesse Adviser.com, PPG Cuming Microwave]

Figure: Linear array of circularly polarized antennas for terrestrial TV broadcast transmission at 50-200MHz (Channels 2 to 6) [Alan Dick]

Figure: Linear array of patches. 16.25GHz 18 elements, 0.48λ spacing, 35° scan