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Antenna Fundamentals Basics antenna theory and concepts

M. Haridim Brno University of Technology, Brno

February 2017

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Topics

• What is antenna

• Antenna types

• Antenna parameters: radiation pattern, directivity, efficiency, gain, radiation resistance, bandwidth, beamwidth

• Antenna array

• Dipole

• Monopole

• Loop antennas

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What is antenna?

A passive device acting as transition between some

form of transmission line and the space.

Signals from an RF transmitter are delivered via a

transmission line to the antenna, which launches

them as electromagnetic waves in the space, or

vice versa.

The shape and size of the current carrying structure

determine how much energy is radiated, direction of

propagation and how well the radiation is captured.

Reciprocity property: The same antenna can be

used in both transmitting and receiving modes.

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Antenna types Antennas can be of either wire or aperture types.

Wire type:

dipole, monopole, loop, helix, Yagi-Uda.

Aperture type:

horn, reflector.

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Common types of antennas:

Fundamentals of Electromagnetics With Engineering Applications by Stuart M. Wentworth

Copyright © 2005 by John Wiley & Sons. All rights reserved.

Figure 8-1 (p. 389)Common single-element antennas.

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Radiation intensity

I( , ) [W/str] is the average power radiated per unit solid

angle in direction (, ).

Integration of I over the whole space (4 Str) gives the total

radiated power.

Power density [w/m2]

is the time-average of the Poynting vector

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Reactive Near- Field Region (‘Rayleigh’ Zone)

Radiating Near- Field Region (Fresnel’ Zone)

Radiating Far- Field Region (‘Fraunhofer’ Zone)

infinity

֎

֎

֎

Near/far field regions

The boundary of near/far field zones is commonly

accepted as

22Dr

D is the largest dimension on the antenna element, and

assumed D>λ.

Near/far field regions

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1 2 2

1 1H ja a

r r

r 1 2 1 22 3 2

1 1E  

rb jb b  jb

r r r

1 2 32 3

1 1E jc c jc

r r r

To feature the spatial dependence of the field intensity, we

substitute all antenna and media parameters with constants

I II III

F I E L D Z O N E S

+

–

Iant Hrad

Erad

Near-Field Reactive

Near-Field

Radiating

Far-Field

radiating

Radiating Antenna

Wire Antenna

Near/Far Field Spatial Distribution

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The “fictional” HARD field

limits between the near and

far field zones

Near/Far Field of Aperture Antennas

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( a )

Radiation pattern

2D vs. 3D E-plane (E and boresight direction) and H-plane

Main , side and back lobes

Isotropic Radiation Pattern, constant in both azimuth

and elevation planes.

Omni Directional Radiation Pattern.

Directional Radiation Pattern- contains one main beam

(lobe) in both azimuth and elevation planes.

Radiation patters shown in polar plots

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Directivity D(, )

An antenna’s directivity is determined by the

directionality of its radiation pattern.

It is defined as the ratio between its radiation

intensity and that of an isotropic antenna, radiating

the same total power.

Proportional to the radiation intensity.

Max. directivity Dmax is the Max. value of D(, ).

Not related to matching level

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Efficiency

The average power delivered to the antenna’s

input is

Pinc = Prad + Ploss

Antenna’s efficiency is defined as

rad rad

inc rad loss

P P

P P P

Loss mechanisms: dielectric losses, conduction

losses, and reflection (mismatch) losses

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Gain G(, )

Antenna’s directivity has no information about the antenna’s

efficiency. The gain of an antenna is a function of both

directivity and efficiency.

The max gain occurs when the directivity is max.

It’s expressed in dBi,

with respect to an isotropic antenna,

or in dBd.

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Antenna's Input Impedance and Radiation Resistance

a a aR jX a rad lossR R R

The real part of the antenna impedance consists of

radiation resistance Rrad and a dissipative resistance

Rloss (ohmic and dielectric losses).

Rrad at represents the useful part of the input power.

In order to avoid reflection of the incident power, the

antenna input impedance must be equal to the line's

characteristic impedance, for the whole operation

bandwidth.

The mismatch is measured by the Voltage Standing

Wave Ratio- VSWR.

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Example

A /2 dipole with a total input impedance of 75 +j40

and loss resistance 2 is fed by a power amplifier

whose output voltage and impedance are 5cos0t and

50+j30, respectively.

Find the power delivered to antenna, the radiated

power and the amount of power dissipated by the

antenna (ohmic losses).

Find the power delivered to antenna, the radiated

power and the amount of power dissipated by the

antenna (ohmic losses).

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Solution First, we find the current (phasor) in the equivalent circuit

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50 30 75 40

35 29 2 mA

s ant loss

o

VIZ Z R j j

I .

The power delivered from the power amplifier is

1 15 0 035 29 2 76 7

2 2in

*sp Re{ v I } Re{ . . } . mW

The radiated power 2 21 1

0 035 73 44 72 2

rad radp I R ( . ) . mW

And the dissipated power

2 211 0 035 2 1 2

2 2diss lossP I R . . mW

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The Effective Area of a Receiving Antenna

The antenna's gain applies to both transmitting and

receiving modes.

However, it is more convenient to characterize a

receiving antenna by its effective area that is given by

the ratio between the power flux incident on the

antenna and the power delivered to a matched load.

It can be shown that the following relationship between

antenna's gain and effective area prevails:

2

4 eAG

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Beam-width

The directivity of an antenna increases as its

beamwidth is made smaller, as the energy

radiated is concentrated into a smaller solid angle

The are some zeros and nulls in

radiation pattern, indicating no

radiations.

A directional antenna directs radiation

in one or more directions.

The width of this beam is defined as

the angle between its half-power

points.

A half-wave dipole has beam-widths

of about 78º and 360º in horizontal

and vertical directions.

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Polarization

The polarization of EM waves is determined by the direction of

the electric field, and that of an antenna is defined as the

polarization of its far field.. A dipole antenna, for example,

produces a linearly polarized wave aligned with its axis.

It picks-up maximum power energy in co-pol (co-polarization)

operation, i.e. when its axis is aligned to the polarization of the

impinging waves.

In the cross-pol operation, on the other hand, it receives no

radiation, as its axis is orthogonal to the polarization of the

incident waves.

The cross-pol level (in dB) of an antenna denotes the ratio

between the maximum received power in the co-pol and

cross-pol operations.

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Antenna arrays

Antenna arrays are formed by an assembly of

identical (in most cases) radiating elements,

arranged in either a one-dimensional (1D) or two-

dimensional (2D) structure.

The design of a single antenna, such as a resonance

wire antenna, allows for limited control of its

properties, especially its gain and radiation pattern.

Greater design flexibility as well as improved

performance can be achieved by arranging multiple

spatially separated antennas that are separately

driven, at the cost of increased size and complexity.

Antenna arrays are used for increasing the antenna

gain, combatting interference from certain directions,

steering the boresight direction, and direction finding.

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Dipole Dipole is the simplest antenna consisting of two straight

pieces of wire or metal rods, fed at the center. The length of

the dipole arms determines its characteristics, e.g.

impedance, operating frequency, etc.

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The dipole antenna radiates energy in all directions

perpendicular to its axis, and no radiation along its axis.

Thus, dipole has some directionality.

In the case of a Hertzian dipole it is given by

21 5D( , ) . sin

So the maximum directivity is

1 5maxD .

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For a half-wave dipole the directivity is slightly

higher, that is Dmax =1.64,

its HPBW is 78 degree

and its radiation resistance is 73.13 .

2280

radR

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Elementary dipole

An elementary dipole or Hertzian dipole, is a wire

segment whose length L is very small compared to

wavelength

Assuming a vertical Hertzian dipole placed at the origin

and excited by a sinusoidal current 0I I sin t

00

4

j rI LeE j sin

r

0r rE H H

0

E

H

Where is the intrinsic impedance of the medium.

In free space 0 376 7.

.

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2 2 220 0

20 5

4av

L IP . sin

( r )

2 2 220 0

232

L II( , ) sin

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If the wire diameter d << λ, the wave pattern along the dipole

arms is sinusoidal with a null at the end.

For center-fed dipoles:

L << L = /2

Current distribution

No phase reversal No phase reversal

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/2 < L <

< L < 3/2

Phase reversal No phase reversal

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Monopole

Monopole are unbalanced antenna, and

hence they can be fed with an unbalanced

feed lines such as coaxial cables.

Like dipoles, monopoles are omnidirectional,

but in contrast to dipoles that are balanced

antenna, monopoles are,

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Whip

Whip antenna is the most common example of a

monopole radio antenna, almost always vertically

mounted onto a base vehicle, resulting in vertical

polarization.

Whip antennas are easy to install and operate, but

suffer low efficiency when they lack a stable ground.

The length of the whip determines its operating

wavelength.

It is possible to shorten the whip by inserting a coil

anywhere along the antenna.

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Loop is a very versatile

antenna.

It has different forms: circle,

rectangle, square, triangle,

ellipse, etc.

A small loops is equivalent to

an infinitesimal

dipole, whose axis is

perpendicular to the loop’s

plane.

Loop is used as field probes at

both low and high frequencies.

Loop antenna

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Electrically small loop antennas are radiation resistances lower

than their loss resistances, hence they have poor performance.

The radiation pattern of electrically small loops is similar to that

of an infinitesimal dipole.

It has a null perpendicular to the loop’s plane and its maximum

is along the plane of the loop.

As the loops perimeter increases and approaches , the

maximum of the pattern shifts from the loop’s plane to the axis

of the loop which is perpendicular to its plane.

Loop antenna