(PDF) Antennas: Radiation mechanism

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 1

July, 2019

Teaching

INP of Toulouse, ENSEEIHT

EEEA Department

Research

LAPLACE, UMR 5213

Research group in Electromagnetism

Hamza KAOUACH

Assistant Professor

hamza.kaouach@laplace.univ-tlse.fr

Antennas: Radiation mechanism

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 2

Contents of lecture

1. Introduction

2. Types of antennas

3. Radiation mechanism

4. Current distribution on a thin wire antenna

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Introduction

An antenna is defined by Webster’s Dictionary as “a usually metallic

device (as a rod or wire) for radiating or receiving radio waves.” The IEEE

Standard Definitions of Terms for Antennas defines the antenna or aerial as

“a means for radiating or receiving radio waves.”

In other words the antenna is the transitional structure between free-

space and a guiding device, as shown in Figure 1.1.

The guiding device or transmission line may take the form of a coaxial

line or a hollow pipe (waveguide), and it is used to transport

electromagnetic energy from the transmitting source to the antenna, or from

the antenna to the receiver.

In the former case, we have a transmitting antenna and in the latter a

receiving antenna.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 4

Introduction

A transmission-line Thevenin equivalent of the antenna system of Figure 1.1 in the

transmitting mode is shown in Figure 1.2 where the source is represented by an ideal

generator, the transmission line is represented by a line with characteristic impedance Zc,

and the antenna is represented by a load ZA[ZA= (RL+ Rr) + jXA] connected to the

transmission line.

A transmission-line Thévenin equivalent

of the antenna system

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Introduction

The load resistance RLis used to represent the conduction and dielectric

losses associated with the antenna structure, while Rrreferred to as the

radiation resistance, is used to represent radiation by the antenna.

The reactance XAis used to represent the imaginary part of the

impedance associated with radiation by the antenna.

Under ideal conditions, energy generated by the source should be totally

transferred to the radiation resistance Rr, which is used to represent radiation by the

antenna.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 6

Introduction

For wireless communication systems, the antenna is one of the most

critical components. A good design of the antenna can relax system

requirements and improve overall system performance.

A typical example is TV for which the overall broadcast reception can be

improved by utilizing a high-performance antenna.

The antenna serves to a communication system the same purpose that

eyes and eyeglasses serve to a human.

The field of antennas is vigorous and dynamic, and over the last 60 years

antenna technology has been an indispensable partner of the communications

revolution.

Many major advances that occurred during this period are in common use today;

however, many more issues and challenges are facing us today, especially since the

demands for system performances are even greater.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 7

Types of Antennas

Wire Antennas

Slot Antennas

Microstrip Antennas

Horn Antennas

Reflector Antennas

Dielectric Resonator Antennas

Leaky Wave Antennas

In this section, we will introduce and discuss some forms of the

various antenna types.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 8

Types of Antennas

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Types of Antennas

Today this is called a

MIMO array

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Types of Antennas

1. Wire Antennas

Wire antennas are familiar to the layman because they are seen virtually

everywhere - on automobiles, buildings, ships, aircraft, spacecraft, and so on.

There are various shapes of wire antennas such as a straight wire (dipole),

loop, and helix which are shown in Figure 1.3. Loop antennas need not only

be circular. They may take the form of a rectangle, square, ellipse, or any

other configuration. The circular loop is the most common because of its

simplicity in construction.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 11

Types of Antennas

2. Aperture Antennas

Aperture antennas may be more familiar to the layman today than in the

past because of the increasing demand for more sophisticated forms of

antennas and the utilization of higher frequencies. Some forms of aperture

antennas are shown in Figure 1.4.

Antennas of this type are very useful for aircraft and spacecraft

applications, because they can be very conveniently flush-mounted on the skin

of the aircraft or spacecraft.

In addition, they can be covered with a dielectric material to protect them

from hazardous conditions of the environment. Waveguide apertures and

horns will be discussed in more detail in this course.

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Types of Antennas

3. Microstrip Antennas

Microstrip antennas became very popular in the 1970s primarily for

spaceborne applications. Today they are used for government and

commercial applications. These antennas consist of a metallic patch on a

grounded substrate. The metallic patch can take many different

configurations, as shown in Figure below.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 13

Types of Antennas

3. Microstrip Antennas (cont.)

The microstrip antennas are:

low profile,

comformable to planar and nonplanar surfaces,

simple and inexpensive to fabricate using modern printed-circuit

technology,

mechanically robust when mounted on rigid surfaces,

compatible with MMIC designs,

and very versatile in terms of resonant frequency, polarization,

pattern, and impedance.

These antennas can be mounted on the surface of high-performance

aircraft, spacecraft, satellites, missiles, cars, and even handheld mobile

telephones.

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Types of Antennas

3. Microstrip Antennas (cont.)

The rectangular and circular patches, shown in Figure 1.5, are the most

popular because of ease of analysis and fabrication, and their attractive

radiation characteristics, especially low cross-polarization radiation.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 15

Types of Antennas

4. Array Antennas

Many applications require radiation characteristics that may not be

achievable by a single element. It may, however, be possible that an

aggregate of radiating elements in an electrical and geometrical arrangement

(an array) will result in the desired radiation characteristics.

The arrangement of the array may be such that the radiation from the

elements adds up to give a radiation maximum in a particular direction or

directions, minimum in others, or otherwise as desired.

Typical examples of arrays are shown in Figure 1.6.

Usually the term array is reserved for an arrangement in which the

individual radiators are separate as shown in Figures 1.6(a–c).

However the same term is also used to describe an assembly of

radiators mounted on a continuous structure, shown in Figure 1.6(d).

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 16

Types pf Antennas

4. Array Antennas (cont.)

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Types of Antennas

5. Reflector Antennas

The success in the exploration of outer space has resulted in the

advancement of antenna theory. Because of the need to communicate over

great distances, sophisticated forms of antennas had to be used in order to

transmit and receive signals that had to travel millions of miles.

A very common antenna form for such an application is a parabolic

reflector shown in Figures 1.7(a) and (b).

Antennas of this type have been built with diameters as large as 305 m.

Such large dimensions are needed to achieve the high gain required to

transmit or receive signals after millions of miles of travel.

Another form of a reflector, although not as common as the parabolic, is

the corner reflector, shown in Figure 1.7(c).

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 18

Types of Antennas

5. Reflector Antennas (cont.)

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Types of Antennas

6. Lens Antennas

Lenses are primarily used to collimate incident divergent energy to

prevent it from spreading in undesired directions. By properly shaping the

geometrical configuration and choosing the appropriate material of the

lenses, they can transform various forms of divergent energy into plane

waves. They can be used in most of the same applications as are the

parabolic reflectors, especially at higher frequencies. Their dimensions and

weight become exceedingly large at lower frequencies. Lens antennas are

classified according to the material from which they are constructed, or

according to their geometrical shape. Some forms are shown in Figure 1.8.

In summary, an ideal antenna is one that will radiate all the power

delivered to it from the transmitter in a desired direction or directions. In

practice, however, such ideal performances cannot be achieved but may be

closely approached. Various types of antennas are available and each type

can take different forms in order to achieve the desired radiation

characteristics for the particular application.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 20

Types of Antennas

6. Lens Antennas (cont.)

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 21

Radiation Mechanism

First Question ?

One of the first questions that may be asked concerning antennas would

be “how is radiation accomplished?”

In other words,

How are the electromagnetic fields generated by the source,

contained and guided within the transmission line and antenna, and

finally “detached” from the antenna to form a free-space wave?

The best explanation may be given by an illustration.

However, let us first examine some basic sources of radiation.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 22

Radiation Mechanism

1. Single Wire

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 23

Radiation Mechanism

1. Single Wire (cont.)

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Radiation Mechanism

1. Single Wire (cont.)

Equation (1-3) is the basic relation between current and charge, and

it also serves as the fundamental relation of electromagnetic radiation.

It simply states that to create radiation, there must be a time-varying

current or an acceleration (or deceleration) of charge.

We usually refer to currents in time-harmonic applications while charge

is most often mentioned in transients.

To create charge acceleration (or deceleration) the wire must be curved,

bent, discontinuous, or terminated.

Periodic charge acceleration (or deceleration) or time-varying current is

also created when charge is oscillating in a time-harmonic motion, as shown

in Figure 1.17 for a λ/2 dipole.

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Radiation Mechanism

1. Single Wire (cont.)

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Radiation Mechanism

1. Single Wire (cont.)

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Radiation Mechanism

1. Single Wire (cont.)

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Radiation Mechanism

1. Single Wire (cont.)

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Radiation Mechanism

2. Two Wire

Let us consider a voltage source connected to a two-conductor T-line

which is connected to an antenna. This is shown in Figure 1.11(a).

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Radiation Mechanism

2. Two Wire (cont.)

The creation of time-varying electric and magnetic fields between the

conductors forms electromagnetic waves which travel along the transmission

line, as shown in Figure 1.11(a).

The electromagnetic waves enter the antenna and have associated with

them electric charges and corresponding currents. If we remove part of the

antenna structure, as shown in Figure 1.11(b), free-space waves can be

formed by “connecting” the open ends of the electric lines (shown dashed).

The free-space waves are

also periodic but a constant

phase point P0moves outwardly

with the speed of light and travels

a distance of λ/2 (to P1) in the

time of one-half of a period.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 31

Radiation Mechanism

2. Two Wire (cont.)

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Radiation Mechanism

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Radiation Mechanism

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Radiation Mechanism

3. Two Wire

Now let us attempt to explain the mechanism by which the electric lines

of force are detached from the antenna to form the free-space waves.

This will again be illustrated by an example of a small dipole antenna

where the time of travel is negligible.

This is only necessary to give a better physical interpretation of the

detachment of the lines of force.

Although a somewhat simplified mechanism, it does allow one to

visualize the creation of the free-space waves.

Figure 1.14(a) displays the lines of force created between the arms of a

small center-fed dipole in the first quarter of the period during which time the

charge has reached its maximum value (assuming a sinusoidal time variation)

and the lines have traveled outwardly a radial distance λ/4.

Antennas and Propagation Course Chapter 02 Dr. Hamza KAOUACH 35

Radiation Mechanism

3. Two Wire (cont.) For this example, let us assume that the number of lines

formed are three.

During the next quarter of the period, the original three lines

travel an additional λ/4 (a total of λ/2 from the initial point) and

the charge density on the conductors begins to diminish.

This can be thought of as being accomplished by introducing

opposite charges which at the end of the first half of the period

have neutralized the charges on the conductors.

The lines of force created by the opposite charges are three

and travel a distance λ/4 during the second quarter of the first

half, and they are shown dashed in Figure 1.14(b).

The end result is that there are three lines of force pointed upward

in the first λ/4 distance and the same number of lines directed

downward in the second λ/4. Since there is no net charge on the

antenna, then the lines of force must have been forced to detach

themselves from the conductors and to unite together to form closed

loops.

This is shown in Figure 1.14(c).

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