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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