Antennas Lecture 1

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November 14 Chapter 1: Introduction to Antennas 1

Antennas and Wave

propagation

Lecture 1


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Chapter 1: Introduction to Antennas 2November 14

Antennas definition

An antenna is defined by Webster's Dictionary as

A useful 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 antenna is atransitionalstructure between

free-space and a guiding device


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Antenna

Form system point of view, an antenna is a transducer

that changes energy from one form to another

As a receiver it changes the energy from

electromagnetic to electric or magnetic energy

As a transmitter it changes the energy from electric or

magnetic to electromagnetic energy


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EM energy

From electromagnetic theory:

The electromagnetic energy consists of two packets of

energy: the magnetic and electric (one does not exist with outthe other)

Half of the energy is in the electric-field and half of the energyis in the magnetic-field. One gives rise to other.


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Transmission medium The wave guiding device being an interface between

source and the antenna is called the transmissionmedium and it can appear in the form of

A Coaxial cableOR a Waveguide(hollow pipe)

For transmitting antenna the transmission mediumtransports energy from transmitter to the antenna.

While for a Receiving antenna the transmission mediumtransport the energy from receiver to the source.


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Radiation resistance From circuit point of view, the antennas appear to the

transmission line as a resistance, Rrad, called Radiationresistance.

Radiation resistance is used to represent the radiation bythe antenna.

The radiation resistance is caused by the power radiatedfrom the antenna Prad.

Prad= I2Rrad

OR Rrad = PradI2


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Radiation resistance

In effect, the radiation resistance represents the power

lost by radiation from the antenna (similar to heat lost)

The greater the radiation resistance, the more the

energy is radiated.

Radiation resistance is not related to any resistance inantenna itself, but a Virtual resistance(does not exist

physically) that represents the radiation by the antenna.


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Transmission line Thevenin Equivalent

The antenna in a transmittingmode can be expressed

as a Thevenin equivalent circuit.

Where

Vg= voltage source generator (transmitter)

Zg= impedance of generator (transmitter)

Rrad= radiation resistance (related to the radiation power

as Prad= IA2Rrad)

RL= Load resistance (represent the conduction and dielectric losses)

jXA= antenna reactance

Antenna impedance: ZA= (Rrad+ RL) + jXA


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Optimization of the antenna system

The antenna is said to be optimized when the energygenerated by the transmitter is totally transferred to the

antenna.

In ideal case, the energy generated should be totally

transferred to the Rrad. However, in practical system the due to lossy nature of

transmission lines and antennas losses occur, such as;

Conduction loss, dielectric loss and losses due to

reflections (mismatch) at the interface between thetransmission line and the antenna.

Hence, energy generated is not totally transferred.


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Impedance Matching

Practically, it can be said that the system is optimized(Maximum power is delivered to the antenna) under

impedance (conjugate) matching.

Conjugate matching condition:

RL+ Rr= Rc and XA= -Xc


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Standing waves

The EM waves (incident waves) while passing throughthe transmission line are reflected back due to mismatch.

Consequently, the reflected waves create constructive

and destructive interference patterns referred to asstanding wave, inside a transmission line.

This standing wave represent pockets of energy

concentrations and storage.

The losses due to the transmission line, antenna, andstanding waves are undesirable.


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Reduction of losses How can we reduce the losses?

1. Losses of Lines.

2. Loss in Antenna.

3. Standing waves.

By utilizing low-loss lines.

By the reduction of Loss resistance represented by RL

Through matching the impedance of antenna to thecharacteristic impedance of the line. (Smith Chart)


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Radiation Mechanism (single wire)

Conducting wires are material

whose prominent characteristic

is the motion of electric

charges and the creation ofcurrent flow.

Let us assume that an electric

volume charge density,

represented by qv (columbs/m3),

is distributed uniformly in a

circular wire of cross section

area A and volume V.


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Radiation through single wire

The total chargeQwithin volumeVis moving inz

direction with a uniform velocity ofvz(meters/sec).

It can be shown that the current densityJZ(amperes/m2)

over the cross section of the wire is given by

Jz= qvvz

If the wire is made of an ideal electric conductor, the

current densityJs(amperes/m) over the surface of the

wire and it is given by

Js= qsvz

whereqs(coulombs/m2) is the surface charge density.


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If the wire is very thin (ideally zero radius), then the

current in the wire can be represented by

Iz= ql vzwhereql(coulombs/m) is the charge per unit length

If the current is time varying, then the derivative of the

currentIzcan be written asdIz = ql d(vz) = ql az

dt dt

wheredvz/dt = az(meter/sec2) is the acceleration. If the wire

is of lengthl, thenl dIz = l ql d(vz) = l ql az

dt dt


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l dIz = l ql d(vz) = l ql az

dt dt

This is the basic relation between current and charge,and it also serves as the fundamental relation of

electromagnetic radiation.

According to this equation:To create radiation, there must be a time varying current

OR an acceleration (or deceleration) of charge.

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 time

harmonic motion.


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Conditions for radiation

If a charge is not moving, current is not created and

there is no radiation.

If the charge is moving with a uniform velocity:1. There is no radiation if the wire is straight, and infinite is extent.

2. There is radiation if the wire is bent, curved, discontinuous or

truncated.

If charge is oscillating in a time-motion, it radiates even

if the wire is straight.


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Wire configurations for radiation


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Example:

By energizing the source, charges are accelerated in the

source-end of the wire.

At the other end of the wire deceleration of chargesoccur due to reflection.

Due to accelerated and decelerated charges radiated

fields are produced at each end and along the remaining

part of the wire Shorter or more compact duration pulses produces stronger

radiation with a broad frequency spectrum.

while continuous time-harmonic oscillating charge produces,

ideally, radiation of single frequency.

Pulse source Load


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Radiation mechanism (two-wires)

The guided wave traveling along a transmission linewhich opens out tends to be radiated and tends to

launch a free space wave as the separation approachesthe order of wavelength or more

The guided wave is planer while free-space wave isspherically expanding.


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a. Antennas and electric field lines

b. Antennas and free space waves


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Dipole (t=T/4)

The figure displays the lines

of force created between the

arms of a small center-fed

dipole.

These lines are created in

first quarter of the period,

and the lines have traveledoutwardly a radial distance

l/4.


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Types of Antennas According to our desired transmission and reception

through antennas make them to vary in shape and size.

Some time, while transmitting through antenna directivityof energy in certain direction is desired e.g. radio link.

Some time, while receiving energy through antennasuppression at certain angle is required in order to avoidinterference.

This must then take various forms to meet the particularneed at hand, and it may be a piece of conducting wire,an aperture, a patch, an assembly of elements (array),reflector, a lens and so forth.

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Antennas Lecture 1