introduction to antenna fundamentals
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Set III. Dr. Zuhair M. Hejaz Antennas and Wave Propagation1
Introduction to Antenna Fundamentals
A Review
How an Antenna Radiates
In order to know how an antenna radiates, let us first
consider how radiation occurs. A conducting wire
radiates mainly because of time-varying current or an
acceleration (or deceleration) of charges. If there is no
motion of charges in a wire, no radiation takes place,
since no flow of current occurs. Radiation will not occur
even if charges are moving with uniform velocity along a
straight wire. However, charges moving with uniform
velocity along a curved or bent wire will produce
radiation. If the charge is oscillating with time, then
radiation occurs even along a straight wire.
If a source is connected to a transmission line with a
sinusoidal voltage applied across the transmission line,
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an electric field is created which is sinusoidal in nature
and results in the creation of electric lines of force. The
free electrons on the line are forcibly displaced by the
electric lines of force and the movement of these charges
causes the flow of current which in turn leads to the
creation of a magnetic field. In turn it creates electric
field and so on as long as the source alternating
voltage is sustained.
The wave transition into free space is show below.
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Due to the time varying electric and magnetic fields,
electromagnetic waves are created and travel along the
line until they approach open space, free space waves are
formed by connecting the open ends of the electric lines
in closed loops.
Consider a two-wire transmission line (balanced). the
direction of current flow is always opposite so the fields
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strengthen one other between the conductorsbut cancel
each other out away from the conductors. (See below).
If a parallel wire transmission line is left open, then
electric and magnetic fields do escape from the end of
the wire and radiate into space. This is rather inefficient
because most of the incident energy is reflected and a
great deal of cancellation of the electric and magnetic
fields still occurs.
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However, if the last quarter-wavelength (/4) of each
conductor is turned at a right angle then:
The electric fields spread out from the conductor.
The magnetic fields reinforce one another.
This results in efficient radiation.
Such arrangement is called a /2Dipole.
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The intensity/strength of the radiated power Prad(related
to the cross product EH) as a function of direction is
called radiation pattern. For the above dipole, it looks
like a doughnut. See below.
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Spherical Coordinate System
Azimuth and Elevation: These are angles used to
describe a specific position in an antenna's radiation
pattern. Azimuth is a horizontal angle ranging from 0 to
360 degrees or . The elevation angle is a vertical
angle, ranging from 0 degrees (horizon) to 90 degrees.
The radius vector of the distance r from the antenna to
an infinitesimal surface area Ais denoted as
r .
Or it can be presented for a point in space as:
d
d
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Theser , and are commonly termed (spherical polar
co-ordinates) which allows us to specify any radiation
direction uniquely (individually).
,,r are unit vectors
of the coordinates.
The zaxis is taken to be the vertical direction and the xy
plane is horizontal. The
denotes the elevation angle
and denotes the azimuth angle. The xz plane is the
elevation plane ( = 0 ) or the E-plane which is the
plane containing the electric field vector and the
direction of maximum radiation. The xy plane is the
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Set III. Dr. Zuhair M. Hejaz Antennas and Wave Propagation10
azimuth plane ( = /2 ) or the H-plane which is the
plane containing the magnetic field vector and the
direction of maximum radiation.
Near and Far Field Regions
The field patterns, associated with an antenna, change
with distance and are associated with two types of
energy: - radiating energy and reactive energy. Hence,
the space surrounding an antenna can be divided into
three regions as shown below.
Field regions around an antenna
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1.
Reactive Near-Field RegionR1: In this region, the
reactive field dominates. The reactive energy
oscillates towards and away from the antenna, thus
appearing as reactance. In this region, energy is only
stored and no energy is dissipated. The outermost
boundary for this region is at a distance
Where R1is the distance from the antenna surface,
D is the largest dimension of the antenna and is
the wavelength.
2.
Radiating Near-Field Region (also called Fresnel
region): This is the region which lies between the
reactive near-field region and the far field region.
Reactive fields are smaller in this field as compared
to the reactive near-field region and the radiation
fields dominate. In this region, the angular field
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distribution is a function of the distance from the
antenna.The outermost boundary for this region is
where R2is the distance from the antenna surface.
3.
Far-field region (also called Fraunhofer region):
The region beyond (R3> R2)
3 > 22/
is the far field region. In this region, the reactive
fields are absent and only the radiation fieldsexist.
The angular f ield distr ibution is not dependent on
the distance from the antenna in
this region and the
power density varies as the inverse square of the
radial distance 21r
.
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Plane and Spherical Waves
Plane waves: The front is a plane (see plot below)
Spherical waves: The front is spherical.
However, at a large distance in the far field region
from the source, the phase front becomes locally a
plane (seen by a receiver)as shown in the plots below).
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Radian and Steradian (Definition)
In a plane angle, the measure is radian (rad) and is
defined from the arc length l and radiusr
of the circle as: rl/
So if rl , 1 rad
l
r
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In a solid angle the measure is steradian (sr) and is
defined for a sphere from the area of the arc Aand2
r of
the radius as shown below:
2/ rA .
If2
rA , then srsteradian1
Infinitesimal Area is equivalent to an area of a square2
r
The area of a sphere of radius ris given by
so in a closed sphere there are 2/ rA sr
steradians (sr)
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For a sphere of radius r, an infinitesimal surface area dA
is given as:
and hence the element of solid angle of a sphere is
given from2
/ rA by:
This formula is of prime importance for calculation of
beam solid angle, Radiated power, directivityand gain
of an antenna which will be discussed later.
Other Classification of Antennas by Size
Let l be the antenna dimension:
1.
Electrically small, l
Primarily used at low frequencies where the
wavelength is long as in telephony, broadcasting,
marine and navigation.
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2.
Resonant antennas, 2/l
Most efficient; examples are slots, dipoles, patches and
have a wide range of communication applications in the
HF band, beamed arrays of dipoles.
3.
Electrically large, l
Can be composed of many individual resonant antennas;
good for radar applications (high gain, narrow beam, low
side-lobes).
Other Terms
Reciprocity
Reciprocityis an antenna ability to transfer energy from
free space to its receiver with the same efficiency with
which it transfers energy from the transmitter into free
space. Or in short: Antennas exhibit the same radiation
pattern for transmission and reception (see Fig. below).
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Polarization
The polarizationof an antenna refers to the direction of
the electric field it produces with reference to the Earths
surface. It can be linear (horizontal/vertical), circular, or
elliptical (right hand polarization or left hand
polarization). See figs below.
Polarization is important because the receiving antenna
should have the same polarization as the transmitting
antenna to maximize received power.
Example of a half-dipole antenna- verticallypolarized.
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Example of a circularly polarized radiation is
produced by helical antennas
In general, the common types of polarizations can be
summarized as shown below.