mobile antennas v1

8/12/2019 Mobile Antennas v1

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Basic Antenna Theory and

Concepts

1


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Introduction

Antenna or aerial is a transducer

antennas convert electromagnetic waves into electrical

currents and vice versa

An antenna is an electrical conductor or system of

conductors

Transmission - radiates electromagnetic energy into

space Reception - collects electromagnetic energy from space

Antenna application


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Introduction

Two important points:

Antennas are passive devices

Antennas are reciprocal devices


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Antenna Definition

An antenna is a circuit element thatprovides a transition from a guided

wave on a transmission line to a freespace wave and it provides for thecollection of electromagnetic energy.


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Antenna Definition-contd

In transmit systems the RF signal isgenerated, amplified, modulated and

applied to the antenna In receive systems the antenna collects

electromagnetic waves that arecutting through the antenna andinduce alternating currents that areused by the receiver


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Reciprocity

An antenna ability to transfer energy form

the atmosphere to its receiver with the same

efficiency with which it transfers energyfrom the transmitter into the atmosphere

Antenna characteristics are essentially the

same regardless of whether an antenna issending or receiving electromagnetic energy


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Simple Antennas

Isotropic antenna (idealized)

Radiates power equally in all directions

it has zero size

efficient

used as reference antenna i.e. gain and directionality of

other antennas are compared with isotropic antenna

isotropic antenna cannot be built and tested but itscharacteristics are simple and easy to derive


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Simple Antennas..

Dipole antennas

has two parts: two wires pointed in opposite directions

arranged either horizontally or vertically, with one end

of each wire connected to the device and the other endhanging free in space.

Half-wave dipole antenna (or Hertz antenna)

Quarter-wave vertical antenna (or Marconi antenna)


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Length of half wave dipole antenna

Typically the length of a half-wave dipole is95% of one-half the wavelength measured infree space

Assumption: antenna conductor diameter ismuch less than the length of the antenna.

The free space wavelength is given by


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Length of half wave dipole antenna


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Problem 1

Calculate the length of a half-wavedipole for an operating frequency of200MHz


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Dipole Antenna

Hertz antennas are not found at frequenciesbelow 2MHz because of the physical sizeneeded of the antenna to represent a half-

wave Vertical Antennas or marconi are used for

frequencies under 2 MHz. It uses aconducting path to ground that acts as

wavelength portion the antenna above theground. The above ground structurerepresents a /4 wavelength


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Antenna Characteristics

Radiation Pattern: The radiation pattern or antennapattern describes the relative strength of the radiated field

in various directions from the antenna, at a constant

distance. The radiation pattern is a reception pattern as well, since it

also describes the receiving properties of the antenna.

The radiation pattern is three-dimensional, but usually the

measured radiation patterns are a two-dimensional slice of

the three-dimensional pattern, in the horizontal or vertical

planes.


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Radiation Pattern... Horizontal Plane Vertical Plane

Angle of Elevation: Vertical

angle measured upward fromthe ground

Representation of radiationpattern

The three dimensionalrepresentation

Two dimensional horizontal andvertical plane representation


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3dimensional representation


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2-dimensional pattern


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2-dimensional pattern


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Far-field and Near-field

Near-field region: the observer close to theantenna

Far-field region: an observer must be farenough away from the antenna that any localcapacitive or inductive coupling is negligible

Radiation patterns are valid only in the far-

field region;


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Comparison of different antennas

Reference antennas : isotropic or half-wave dipoleantennas are used

The gain of the antenna may be expresses indecibels with respect to a half wavelength dipole or

isotropic antenna dBdgain of an antenna expressed in decibels with

respect to a half wave dipole antenna dBi gain of an antenna expressed in decibels with

respect to a half wave isotropic antenna


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Comparison of different antennas

Since the gain of a dipole is 2.14 dBi, the gainof any antenna in dBd is 2.14 dB less than thegain of the same antenna expressed in dBi.

That is,


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Problem 2

Two antennas have gains of 5.3 dBi and4.5 dBd, respectively. Which hasgreater gain?


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Antenna Gain

Antenna gain

Power output, in a particular

direction, compared to that

produced in any direction by a

perfect omnidirectional

antenna (isotropic antenna)

Antenna gain is themeasure in dB how much

more power an antenna willradiate in a certain directionwith respect to that whichwould be radiated by a

reference antenna


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Antenna Gain

Effective area

Related to physical size and shape of antenna

Relationship between antenna gain and effective area

G = antenna gain

Ae= effective area f = carrier frequency

c = speed of light ( 3 108m/s)

= carrier wavelength

2

2

2

44

cAfAG ee


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Directivity

Sometimes the term directivity is used.

This is not quite the same as gain. Directivity is thegain calculated assuming a lossless antenna. Real

antennas have losses, and gain is simply thedirectivity multiplied by the efficiency of theantenna, that is:

G =D

where

D = directivity, as a ratio (not in dB)

G = gain, as a ratio (not in dB)

= efficiency


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Directivity

When an antenna is used for transmitting, the totalpower emitted by the antenna is less than thatdelivered to it by the feedline.

the efficiency as = Pr / Pt

where = antenna efficiencyPr = radiated powerPt = power supplied to the antenna

The figure of 2.14 dBi we have been using for thegain of a lossless dipole is also the directivity for anydipole.

To find the gain of a real (lossy) dipole, it isnecessary first to convert the decibel directivity to apower ratio, then to multiply by the efficiency.


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Problem 3

A dipole antenna has an efficiency of85%. Calculate its gain in decibels.


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Solution


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Beamwidth

Beamwidthis the angular separation ofthe half-power points of the radiatedpattern

half-power pointsare also the points atwhich the power density is 3 dB less than it isat its maximum point.

Half-wave dipole has a beamwidth of about78 in one plane and 360 in the other.

Many antennas are much more directionalthan this, with a narrow beam in both planes.


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beamwidth

antenna

A

Power 3dB down

from maximum

point A

Max power

2 dipole

Beamwidth of Directional Antenn

Radiated energy is

focused in a specific

direction

R di ti tt f


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Radiation pattern ofunidirectional antenna


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Major and Minor lobes

Polar Plot


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Major and Minor lobes

The main beam(or main lobeor major lobe) is the regionaround the direction of maximum radiation (usually theregion that is within 3 dB of the peak of the main beam).

The sidelobesare smaller beams that are away from the

main beam. These sidelobes are usually radiation inundesired directions which can never be completelyeliminated.

The sidelobe level (or sidelobe ratio) is an importantparameter used to characterize radiation patterns. It is themaximum value of the sidelobes away from the main beamand is expressed in Decibels.

One sidelobe is called backlobe. This is the portion ofradiation pattern that is directed opposing the main beam

direction.


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Home work

Polar plot

Rectangular plot

what is sidelobe ratio?

what is front to back ratio?

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