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INTRODUCTIONAntenna or Aerial :
- is a transducer that transmits or receives
electromagnetic waves
- converts the voltage and current into the
electromagnetic radiation and vice-versa
- is a transitional structure between free-space and a
(Transmission Line) guiding structure
- The American Heritage Dictionary:A metallic
apparatus for sending and receiving electromagnetic
waves.
- Websters Dictionary:A usually metallic device (as a
rod or wire) for radiating or receiving radio waves
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INTRODUCTION
What is an Antenna?
An antenna is a device for radiating and receiving radiowaves. The antenna is the transitional structure betweenfree-space and a guiding device.
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INTRODUCTION
A transmission-line Thevenin equivalent of the antennasystem
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INTRODUCTION
A transmission-line Thevenin equivalent of the antennasystem
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.
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. 1. Low loss line
2.reduce RL
ZL=ZA
INTRODUCTION
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TYPES OF ANTENNAS
Wire antennas
Aperture antennas
Microstrip antennas
Array antennas
Reflector antennas Lens antennas
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Wire antennas
seen virtually everywhere- on automobile,
building, ships, aircraft, and so on. Shapes of wire antennas:
straight wire (dipole), loop (circular), and helix,
Loop antenna may take the shape of a rectangle ,
ellipse or any other shape configuration
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Aperture-antenna
Note: The aperture concept is applicable also to wired antennas.
For instance, the max effective aperture of linear/2
wavelength dipole antenna is 2/8
EM wave
Power
absorbed: P [watt]Effective
aperture: A[m2]
Aperture antennas derivedfrom waveguide technology
(circular, rectangular)
Can transfer high power
(magnetrons, klystrons)
Utilization of higherfrequencies
Applications: aircraft, and spacecraft
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Microstrip antenna
consist of metallic patch on a grounded substrate
Examples: rectangular and circular shapeApplications: aircraft, spacecraft, satellite, missiles,
cars etc
RECTANGLE
CIRCLE
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Patch Antennas Radiation is from two slots on left and right edges of patch where slot is
region between patch and ground plane
Length d = /r1/2 Thickness typically 0.01 The big advantage is conformal, i.e. flat, shape and low weight
Disadvantages: Low gain, Narrow bandwidth (overcome by fancy shapes and
other heroic efforts), Becomes hard to feed when complex, e.g. for wide
band operation
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Patch Antenna Pattern
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Array antennasa collection of simple antennas
- gives desire d radiation characteristics
- The arrangement of the array may be such that the radiation from the
elements adds up to give a radiation maximum in a particular
direction or directions, minimum in others, or otherwise as desired
- Examples: yagi-uda array, aperture array, microstrip patch array,
slotted waveguide array
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Array of patch Antennas
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Reflector antennas
- used in order to transmit and receive signals that had to travel
millions of miles
- A very common reflector antenna parabolic reflector
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Parabolic Reflectors A parabolic reflector operates
much the same way a reflectingtelescope does
Reflections of rays from the feedpoint all contribute in phase to aplane wave leaving the antenna
along the antenna bore sight(axis)
Typically used at UHF andhigher frequencies
L t
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Lens antennas lenses are primarily used to collimate incident divergent energy to
prevent it from spreading in undesired directions
Transform various forms of divergent energy into plane waves
Used in most of applications as are the parabolic reflectors,
especially at higher frequencies. Their dimensions and weight
bec0me exceedingly large at lower frequencies.
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18
Lens antennas
Lenses play a similar role to that of reflectors in reflector antennas:
they collimate divergent energyOften preferred to reflectors at frequencies > 100 GHz.
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PHYSICAL CONCEPT OF RADIATION
or RADIATION MECHANISMHow is Radiation Accomplished?
Principle of radiationWhen electric charges undergo acceleration or deceleration,
electromagnetic radiation will be produced. Hence it is the
motion of charges (i.e., currents) that is the source of radiation
basic equation of radiation
IL = Qv
Where
Itime varying current
Qcharge
Llength of current element
vtime change of velocity
Radiation Mechanism in a) Single wire, b) Two wire and c) Dipole
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Radiation Mechanism
Single wire:
Conducting wires are characterized by the motion ofelectric charges and the
creation of current flow.electric volume charge density, qv (coulombs/m3),
is distributed uniformly in a
circular wire of cross-sectional area A
and
volume V
qv - volume charge density
A- cross-sectional area
V-Volume
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Radiation Mechanism Single wire:
Instead of examining all three current densities, we will primarily
concentrate on the very thin wire.
The conclusions apply to all three. If the current is time varying.
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Radiation Mechanism
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Radiation Mechanism
Thin wire
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Radiation Mechanism Single wire:
CURVED
BENT DISCONTINUOUS
TERMINATED
TRUNCATED
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Radiation MechanismTwo-Wires:
Applying a voltage across the two-conductor transmission line
creates an electric field between the conductors. The movement of the charges creates a current that in turn
creates a magnetic field intensity.
The creation of time-varying electric and magnetic fields between
the conductors forms electromagnetic waves which travel along
the transmission line.
Voltage creates electric field
Current that produced produces Magnetic field
Together that forms Electro Magnetic Waves that travels along the
transmission line
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Radiation MechanismTwo-Wires:
The electromagnetic waves enter the antenna and have
associated with them electric charges and correspondingcurrents.
If we remove part of the antenna structure, free-space waves can
be formed by connecting the open ends of the electric lines
If we remove the antenna then in free space it will be like this bcoz they
are actually electromagnetic waves
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Radiation MechanismTwo-Wires:
If the initial electric disturbance by the source is of a short duration,
the created electromagnetic waves travel inside the transmission
line, then into the antenna, and finally are radiated as free-space
waves, even if the electric source has ceased to exist.
They radiate in free space atleast once evn if the elect field get ceased
If the electric disturbance is of a continuous nature, electromagneticwaves exist continuously and follow in their travel behind the others.
pattern will follow one after another
However, when the waves are radiated, they form closed loops and
there are no charges to sustain their existence.
No electrical presence since they forms closed loop
Electric charges are required to excite the fields but are not needed
to sustain them and may exist in their absence.
Electric field is required to excite not to continue or sustain the field
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Radiation MechanismDipole Antenna:
A radio antenna that can be made of a simple wire, with a
centre-fed driven element
Consist of two metal conductors of rod or wire,
oriented parallel and collinear
with each other (in line with each other), Small Space
with a small space between them.
Consider the example of a small dipole antenna where the
time of travel is negligible
Driven Element
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Radiation MechanismFormation and detachment of electric field line for short Dipole Antenna
T=T/4,T/2,T/2+
Di l
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Dipole
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Current Distribution on a thin wire antenna
Let us consider the geometry of a lossless two-wire
transmission line
The movement of the charges creates a traveling wave
current, of magnitude I0 /2, along each of the wires.
When the current arrives at the end of each of the wires,
it undergoes a complete reflection (equal magnitude and
180 phase reversal)
The reflected traveling wave, when combined with the
incident traveling wave, forms in a each wire a pure
standing wave pattern of sinusoidal form.
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current = I/2+I/2
reflected and 180 phase reversal
incidented wave forms standing waves
b h
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Current Distribution on a thin wire antenna
For the two-wire balanced (symmetrical) transmission line, thecurrent in a half-cycle of one wire is of the same magnitude but 180
out-of-phase from that in the corresponding half-cycle of the
otherwire
Current same but 180 out of phase
b h
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Current Distribution on a thin wire antenna If s is also very small, the two fields are canceled
The net result is an almost ideal, non-radiating transmission line.
When the line is flared, because the two wires of the flared section
are not necessarily close to each other, the fields do not cancel each
other
Therefore ideally there is a net radiation by the transmission linesystem
C Di ib i hi i
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Current Distribution on a thin wire antenna When the line is flared into a dipole, if s not much less than , the
phase of the current standing wave pattern in each arm is the same
through out its length. In addition, spatially it is oriented in the same
direction as that of the other arm
Thus the field s radiated by the two arms of the dipole (vertical parts
of a flared transmission line) will primarily reinforce each other
toward most directions of observation
C Di ib i hi i
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Current Distribution on a thin wire antenna The current distributions we have seen represent the maximum
current excitation for anytime. The current varies as a function of time
as well.
T=T/8,T/4,T/2,T
R di ti tt
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Radiation pattern
Mathematical function or Graphical representation of radiation
properties of an antenna as a function of space coordinates.
Radiation properties include power flux density, radiation intensity,field strength, directivity, phase or polarization.
(RIFSD PHASE)
Radiation pattern usually indicate either electric field intensity or
power intensity. Magnetic field intensity has the same radiation
pattern as the electric field intensity
A directional antenna radiates and receives preferentially in some
direction
Depicted as two or three-dimensional spatial distribution ofradiated energy as a function of the observers position along a
path or surface of constant radius
Lobes are classified as: major, minor, side lobes, back lobes
R di ti tt
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Radiation pattern
Coordinate system for antenna analysis
Graphical representation of radiation properties of an antenna
R di ti tt
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Radiation pattern
For an antenna
The Field pattern(in linear scale):
typically represents a plot ofthe magnitude of the electric or
magnetic field as a function of the angular space.
The Power pattern(in linear scale):
typically represents a plot of the square of the magnitude of theelectric or magnetic field as a function of the angular space
The Power pattern(in dB):
represents the magnitude of the electric or magnetic field, in
decibels, as a function of the angular space.
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Radiation pattern
HPBW(FIELD,
POWER,DB )
R di ti tt
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Radiation pattern
various parts of a radiation pattern are referred to as a lobes
Total electric fieldis given as
Three dimensional polar pattern
R di ti tt
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Radiation pattern
various parts of a radiation pattern are referred to as a lobes
Two dimensional polar pattern
Radiation pattern lobes
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Radiation pattern lobes
Various parts of a radiation pattern are referred to as lobes
Major lobe (main lobe):
The radiation lobe containing the direction of maximum
radiation
Major lobe is pointing at =0 direction in figure
In spilt-beam antennas, there may exist more than one
major lobes
Minor Lobe is any lobe except a major lobe
all the lobes exception of the major lobe
Side lobe: a radiation lobe in any direction other than intended lobe
Usually it is adjacent to main lobe
Back lobe:
a radiation lobe whose axis makes an angle of
approximately 1800 with respect to the beam of antenna
usually it refers to a minor lobe that occupies the
hemisphere in a direction opposite to that of major lobe
R di ti P D it
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Radiation Power Density
Poynting Vector or Power density(w)
The instantaneous poynting vector describe the power
associated with electromagnetic wave
Poynting vector defined as
W = E x H
W- instantaneous poynting vector (W/m2)
E- instantaneous electric field intensity (V/m)H-instantaneous magnetic field intensity (A/m)
R di ti P D it
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Radiation Power Density The total power crossing a closed surface can be obtained by
integrating the normal component of poynting vector over the entire
surface.
p= instantaneous total power (W)
n = unit vector normal to the surfaceda = infinitesimal are a of the closed surface (m2 )
we define the complex fields E and H which are related to theirinstantaneous counter parts Eand Hby
R di ti P D it
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Radiation Power Densityidentity
W = E x H =The first term of is not a function of time, and the time variations of the
second are twice the given frequency.
Average Power Density:
The average power density is obtained by integrating the
instantaneous Poynting vector over one period and dividingby the period.
Wav=the real part of represents the average (real) power densitythe imaginary
part Must represent the reactive (stored) power density associated with the
electromagnetic fields
Radiation Po er Densit
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Radiation Power DensityThe average power radiated by an antenna (radiated power)
can be written as
Radian and Steradian
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Radian and Steradian
A radian is defined with the using Figure ( a)
It is the angle subtended by an arc along the
perimeter of the circle with length equal tothe radius.
A steradian may be defined using Figure (b)
Here, one steradian (sr) is subtended by an
area r2 at the surface of a sphere of radius r.
The infinitesimal area dA on the surface of
radius r is defined as
dA =r2 sin d d (m2)
A differential solid angle, d, in sr, is given
by
d = dA/r
2
= sin d d (sr)= sin d d
Unit of plane Angle is a radian
Unit of Solid Angle is a steradian
Isotropic antenna
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Isotropic antenna Isotropic antenna orisotropic radiatoror
isotropic source oromnidirectional radiatororsimple unipole
is a hypothetical (not physically realizable)lossless antenna having equal radiation in all
directions
used as a useful reference antenna todescribe real antennas.
Its radiation pattern is represented by a
sphere of radius (r) whose center coincides
with the location of the isotropic radiator.
All the energy(power) must pass over the
surface area of sphere=4r2
Isotropic antenna
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Isotropic antenna Poynting vector or power density(w) at any point on the sphere
power radiated per unit area in any direction
The magnitude of the poynting vector is equal to the radial componentonly(because p = p=0)
|w|=wr
The total radiated power
PTrad = w.ds
= wr.ds= wrds
= wr4r2
or wr= PTrad /4r2 watt/m2
where
Wr radiated power of average power density
PTrad total power radiated
Isotropic antenna
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Isotropic antenna The total power radiated by it is given by:
The power density is given by:
which is uniformly distributed over the surface of a sphere of radius r.
Isotropic means ALL OVER THE SURFACE THEREFORE SS ds =
4 |=| R^2
0 ---- 2pi and 0 ---- pi
a^r r^2 sinx dx dphi
Isotropic antenna
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Isotropic antenna
g
Directional antenna
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Directional antenna
is an antenna, which radiates (or receives) much more
power in (or from) some directions than in (or from) others
RECEIVES OR RADIATES IN SOME SPECIFIED
DIRECTION ONLY
Note:
Usually, this term is applied to antennas whosedirectivity is much higher than that of a half-wave dipole
Principal Patterns
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For linearly polarized antenna performance is often described
in terms of its principal E-and H-Plane patterns
E- Plane : the plane containing the electric field vector and the
maximum radiationH- Plane : the plane containing the magnetic field vector and the
maximum radiation x-z elevation plane contain principal E- planex-y azimuthal plane contain principal H- plane
Principal Patterns
Principal E and H plane pattern for a pyramidal horn antenna
Field Regions
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The space surrounding an antenna is usually subdivided into three
regions:
Field Regions
DISTIBUTION)
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The space surrounding an antenna is usually subdivided into three
regions:
Reactive near-field region:That portion of the near-field region
immediately surrounding the antenna wherein the reactive field
predominates.
Radiating near-field (Fresnel) region:That region of the field of an
antenna between the reactive near-field region and the far-field region
wherein radiation fields predominate and wherein the angular field
distribution is dependent upon the distance from the antenna
Far-field (Fraunhofer) region:That region of the field of an antenna
where the angular field distribution is essentially independent of the
distance from the antenna.
DISTIBUTION)
Field Regions
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The space surrounding an antenna is usually subdivided into three
regions:
Field Regions
Radiation Intensity
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Radiation intensity in a given direction is defined as the power
radiated from an antenna per unit solid angle.
U = Prad/d
Where
Pradradiated power
=d solid angel
The radiation intensity is a far-field parameter, and it can be obtainedby simply multiplying the radiation density by the square of the
distance.
U = r2WradWrad radiation density (W/m
2)
r distance (m)U - radiation intensity (W/ unit solid angle)
Radiation Intensity
Radiation Intensity
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The radiation intensity is also related to the far-zone electric field of
an antenna, by
The total power is obtained by integrating the radiation intensity,
over the entire solid angle of 4. Thus
Radiation Intensity
Radiation Intensity
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Radiation patterns may be functions of both spherical coordinate
angles and
Let the radiation intensity of an antenna be of the form
The maximum value of radiation intensity
The total radiated power is found using
Radiation Intensity
DirectivityDi ti it (D)
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yDirectivity (D):
the ratio of the radiation intensity in a given direction from the antenna to
the radiation intensity averaged over all directions
the directivity of a nonisotropic source is equal to the ratio of its radiation
intensity in a given direction over that of isotropic source
The average radiation intensity (U0) is equal to the total power radiated by
the antenna divided by 4.
If the direction is not specified, it implies the direction of maximum radiation
intensity (maximum directivity) expressed as
U = radiation intensity (W/unit solid angle)
Umax = maximum radiation intensity (W/unit solid angle)
U0= radiation intensity of isotropic source (W/unit solid angle)
Prad = total radiated power (W)
D = directivity(dimensionless)
D0= maximumdirectivity(dimensionless)
Directivity
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yThe general expression for the directivity and maximum
directivity (D0) using
Directivity
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Directivity
,,
,
n
n avg
P
PD
The directive gain,, of an antenna is the ratio of the
normalized power in a particular direction to the
average normalized power, or
max
max max
,
,,
n
n avg
PD D
P
The directivity, Dmax, is the maximum directive gain,
max
4
p
D
Directive gain
,
,4
n p
n avg
P dP
d
Where the normalized powers average value taken
over the entire spherical solid angle is
max
1,nP Using
Directivity
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Partial Directivity of antenna:
Partial directivity of an antenna for a given polarization in a given direction as
that part of the radiation intensity corresponding to a given polarization
divided by the total radiation intensity averaged over all directions.
With this definition for the partial directivity, then in a given direction
the total directivity is the sum of the partial directivities for any two
orthogonal polarizations
For a spherical coordinate system, the total maximum directivity D0 for theorthogonal and components of an antenna can be written as
While the partial directivities D and D are expressed as
Directivity
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Directional Patterns:
In Figure2.14(a). For a rotationally symmetric pattern, the half-power beam widths in
any two perpendicular planes are the same, as illustrated in Figure2.14(b).With this approximation, maximum directivity can be approximated by
Directivity
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Directional Patterns:
With this approximation, maximum directivity can be approximated by
Antenna Gain
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Antenna Gain The ratio of the intensity, in a given direction, to the radiation
intensity that would be obtained if the power accepted by the
antenna were radiated isotropically
The radiation intensity corresponding to the isotropically
radiated power is equal to the power accepted (input) by the
antenna divided by 4
In equation form this can be expressed as
Antenna Gain
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Antenna Gainthe ratio of the Power gain in a given direction to the power
gain of a reference antenna in its referenced direction
The power input must be the same for both antennas
Mostly Reference antenna is a lossless isotropic source. Thus
When the direction is not stated, the power gain is usually taken in
the direction of maximum radiation.
We can write that the total radiated power related to the total input
power
Antenna Gain
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Antenna Gain
Antenna Gain
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Antenna GainPartial gain of an antenna
For a given polarization in a given direction
Total gain
A t G i
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Antenna Gain
Relationship between antenna gain and effective area
G = antenna gain
Ae= effective area f= carrier frequency
c = speed of light (3x108 m/s)
= carrier wavelength
2
2
2
44
c
AfAG ee
Antenna Efficiency
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Antenna Efficiency
In general, the over all
Efficiency can be written as
Radiation Resistance & Antenna Efficiency
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Radiation Resistance & Antenna Efficiency
Radiation resistance (Rrad) is a fictitious resistance,
such that the average power flow out of the antenna isPav = (1/2) I2 Rrad
Using the equations for our short (Hertzian) dipole we find that
Rrad = 80 2 (l/)2 ohms
Antenna Efficiency
o = Rrad/(Rrad+ Rloss)
where Rloss ohmic losses as heat
Gain = ox Directivity
G =o D
Beamwidth
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Half-power beamwidth (HPBW) (H)
is the angle between two vectors from the patterns origin to the
points of the major lobe where the radiation intensity is half itsmaximum
Often used to describe the antenna resolution properties
Important in radar technology, radioastronomy, etc.
Power pattern of U()=
First-null beamwidth (FNBW) (N)
is the angle between two vectors, originating at the patterns origin
and tangent to the main beam at its base.
Often FNBW 2*HPBW
Beam efficiency
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Beam efficiency
To judge the quality of transmitting and receiving antennas
If 1 is chosen as the angle where the first null or minimum
occurs, then the beam efficiency will indicate the amount of
power in the major lobe compared to the total power.
Bandwidth
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Bandwidth: the range of frequencies within which the performance of
the antenna, with respect to some characteristic, conforms to a
specified standard
For broadband antennas, the bandwidth is usually expressed as the
ratio of the upper-to-lower frequencies of acceptable operation
F.E. 10:1 bandwidth indicates that the upper frequency is 10 times
greater than the lower
For narrowband antennas, the bandwidth is expressed as a
percentage of the frequency difference (upper minus lower) over the
center frequency of the bandwidth
F.E. a 5% bandwidth indicates that the frequency difference of
acceptable operation is 5% of the center frequency of the
bandwidth
gain, side lobe level,
beamwidth,
polarization, and
beam direction
Pattern
bandwidth
inputimpedance andradiationefficiency
Impedancebandwidth
Polarization
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Polarization of an antenna:
the polarization of the wave transmitted (radiated) by the antenna
When the direction is not stated, the polarization is taken to be
the polarization in the direction of maximum gain
Polarization of the radiated energy varies with the direction from
the center of the antenna, so that different parts of the pattern
may have different polarizations
Polarization may be classified as linear, circular, or elliptical
Polarization
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Polarization of a radiated wave
is defined as that property of an electro magnetic wave
describing the time varying direction and relative magnitude ofthe electric-field vector; specifically, the figure traced as a
function of time by the extremity of the vector at a fixed location
in space, and the sense in which it is traced, as observed along
the direction of propagation.
Polarization then is the curve traced by the end point of the
arrow (vector) representing the instantaneous electric field. The
field must be observed along the direction of propagation. A
typical trace as a function of time is shown in Figure
Polarization of EM Waves
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Clockwise rotation of the E vector= right-hand polarization
counterclockwise rotation of the E vector = left-hand
polarization
Polarization of EM Waves
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Polarization of EM Waves
Horizontal polarization
Vertical polarization
Circular polarization
(RCP)
Circular polarization
(LCP)
Polarization of EM Waves
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AR: The ratio of the major axis to the minor axis is referred to as the axial ratio (AR)
,and it is equal to
Polarization Electromagnetic wave
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Polarization Electromagnetic wave
Polarization of EM Waves
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Polarization of EM Waves
The instantaneous field of a plane wave, traveling in the
negative z direction:
Instantaneous components are related to their complexcounterparts by
where Exo and Eyo are, respectively, the maximum magnitudesof the x and y components.
Linear Polarization:
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A time-harmonic wave is linearly polarized at a given point in
space if the electric-field (or magnetic-field) vector at that point
is always oriented along the same straight line at every instantof time.
This is accomplished if the field vector (electric or magnetic)
possesses:
Only one component, or
Two orthogonal linear components that are in time phase or 180
(or multiples of 180) out-of-phase.
the time-phase difference between the two components mustbe
linearly polarized plane waves
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linearly polari ed plane waves
)(tan
)cos(e)cos(e
0
01
2
0
2
0
00
x
y
yx
yyxx
E
E
EEEE
kztEkztEE
Any two orthogonal plane waves
Can be combined into a linearly
Polarized wave.
Conversely, any arbitrary linearlypolarized wave can be resolved
into two independent Orthogonal
plane waves that are in phase.
[2-15]
Circular Polarization:
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C cu a o a at o : A time-harmonic wave is circularly polarized at a given point in
space if the Electric (or magnetic) field vector at that point
traces a circle as a function of time
The necessary and sufficient conditions to accomplish this are ifthe field vector (electric or magnetic) possesses all of thefollowing:
a. The field must have two orthogonal linear components,and
b. The two components must have the same magnitude, and
c. The two components must have a time-phase difference ofodd multiples of 90.
Circular Polarization:
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Circular Polarization:
magnitudes of the two
components are same
the time-phase difference
between components is odd
multiples of /2
Circular Polarized wave:
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polarizedcircularlyleft:-polarized,circularlyright:
2&:onpolarizatiCircular 000
EEE yx[2-17]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Elliptical Polarization:
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A time-harmonic wave is elliptically polarized if the tip
of the field vector (electric or magnetic) traces an
elliptical locus in space.
At various instants of time the field vector changes
continuously with time at such a manner as to
describe an elliptical locus.
It is right-hand (clockwise) elliptically polarized if the
field vector rotates clockwise, and it is left-hand
(counterclockwise) elliptically polarized if the fieldvector of the ellipse rotates counterclockwise
Elliptical Polarization:
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magnitudes of the two components are NOT same the
time-phase difference between components is odd
multiples of /2
Or when the time phase difference between the two
components is not equal to multiples of /2
(irrespective of their magnitudes)
Elliptical Polarization:
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The necessary and sufficient conditions
The field must have two orthogonal linear components, and
The two components can be of the same or different
magnitude
(1) If the two components are not of the same magnitude, thetime-phase difference between the two components must not
be 0 or multiples of 180 (because it will then be linear).
(2)If the two components are of the same magnitude, the time-
phase difference between the two components must not be
odd multiples of 90 (because it will then be circular).
Elliptically Polarized plane waves
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2
0
2
0
00
2
00
2
0
2
0
0
cos2)2tan(
sincos2
)cos(e)cos(e
Eee
yx
yx
y
y
x
x
y
y
x
x
yxx
yyxx
EE
EE
E
E
E
E
E
E
E
E
kztkztE
EE
[2-16]
Polarization
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Typical Applications
Vertical polarization is most commonly used when it is
desired to radiate a radio signal in all directions over ashort to medium range.
Horizontal polarization is used over longer distances to
reduce interference by vertically polarized equipmentradiating other radio noise, which is often predominantly
vertically polarized.
Circular polarization is most often used in satellitecommunications.
Polarization Loss Factor & Efficiency
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Polarization Loss Factor & Efficiency
Usually, polarization of the receiving antenna polarization of
the incoming (incident) wavepolarization mismatch.
The amount of power extracted by the antenna from the
incoming signal will not be maximum because of thepolarization loss
Assuming that the electric field of the incoming wave can be
written as
unit vector ofthe wave
Polarization Loss Factor & Efficiency
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Polarization of the electric field of the receiving
antenna
Polarization loss factor (PLF)
angle between the twounit vectors
Polarization Loss Factor & Efficiency
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Polarization efficiency
= Polarization mismatch = loss factor:
the ratio of the power received by an antenna from a given
plane wave of arbitrary polarization to the power that would be
received by the same antenna from a plane wave of the same
power flux density and direction of propagation, whose state of
polarization has been adjusted for a maximum received power
Polarization Loss Factor & Efficiency (Cont)
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PLF for transmitting and receiving aperture antennas
Polarization Loss Factor & Efficiency
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PLF for transmitting and receiving linearwire antennas
Input Impedance (Transmitting mode)
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Input impedance:
the impedance presented by an antenna at its terminals or
the ratio of the voltage to current at a pair of terminals or the ratio of the appropriate components of the electric to
magnetic fields at a point
We are primarily interested in the input impedance at the input terminalsof the antenna
Input Impedance
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Ratio of the voltage to current at these terminals, with no load
attached, defines the impedance of the antenna as
Input Impedance
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Assume that the antenna is attached to a generator with
internal impedance
We can find the amount of power delivered to Rr for radiation by
the amount of power dissipated in RL as heat by
Input Impedance
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Current developed within the loop is
Input Impedance
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Magnitude of current developed within the loop is
The power delivered to the antenna for radiation is given by
and that dissipated as heat by
Input Impedance The remaining power (Pg) is dissipated as heat on the
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The remaining power (Pg) is dissipated as heat on the
internal resistance Rg and it is given by
The maximum power delivered to the antenna occurs when
we have conjugate matching
For this case
Input Impedance From equations (2 81) (2 83) It is clear that
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From equations (2-81)(2-83), It is clear that
The power supplied by the generator during conjugate
matching is
Antenna in the Receiving Mode
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The incident wave impinges upon the antenna, and it
induces a voltage VT
All the formulation is same as the transmitting mode (just
replace subscript g with T)
Antenna in the Receiving Mode Under conjugate matching
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j g g
Powers delivered to Rr ,RL ,and RT are given, respectively,
by
While the induced (collected or captured) is
Under conjugate matching of the total power collected or captured (Pc) half is
delivered to the load RT and the other half is scattered or reradiated through Rr
and dissipated as heat through RL
Antenna Radiation Efficiency
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Remember that antenna efficiency that takes into account the
reflection, conduction, and dielectric losses
The conduction and dielectric losses of an antenna are very
difficult to compute
Even with measurements, they are difficult to separate and theyare usually lumped together to form the ecdefficiency.
The resistance RL is used to represent the conduction-dielectric
losses
Antenna Radiation Efficiency
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power delivered to the radiation resistance
the power delivered to Rrand RL
Conduction-dielectric efficiency
where
RrRadiation ResistanceRLLoss Resistance
Equivalent areas
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With each antenna, we can associate an umber of equivalent areas.
These are used to describe the power capturing characteristics of the
antenna when a wave impinges on itEffective area (aperture) Ae
Scattering area (aperture) As
loss area (aperture) AL
Capture area (aperture) Ac
Effective area (Aperture) Ae (Power available at input/wave
incidented on the antenna)the ratio of the available power at the terminals of a receiving
antenna to the power flux density of a plane wave incident on the
antenna from that direction, the wave being polarization matched tothe antenna. If the direction is not specified, the direction of
maximum radiation intensity is implied.
Equivalent areas
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Effective areaAe
The effective aperture is the area which when multiplied by
the incident power density gives the power delivered to the
load
Equivalent areas
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Effective areaAe
Under conjugate matching
The maximum effective area Aem
Equivalent areas
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The scattering area As
is defined as the equivalent area when multiplied by the
incident power density is equal to the scattered orreradiated power.
Under conjugate matching
As
Equivalent areas
Th l A
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The loss area AL
is defined as the equivalent area, which when multiplied by
the incident power density leads to the power dissipated asheat through RL
Under conjugate matching
Equivalent areas
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Capture area Ac
is defined as the equivalent area, which when multiplied by
the incident power density leads to the total power
captured, collected, or intercepted by the antenna
Under conjugate matching
In general, the total capture area is equal to the sum of theother three
Capture Area = Effective Area+Scattering Area+Loss Area
Ac = Ae + AS+ AL
Equivalent areas
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Aperture Efficiency: apis defined as the ratio of the maximum effective area Aem
and of the antenna to its physical area Ap
the maximum effective aperture of any antenna is related to itsmaximum directivity D0 by
Antenna Temperature
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The brightness temperature emitted by the different sources is
intercepted by antennas, and it appears at their terminals as an
antenna temperature
The temperature appearing at the terminals of an antenna is that
given by
Antenna Temperature
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Antenna temperature (effective noise temperature of the
antenna radiation resistance Rr; K) TA
Assuming no losses or other contributions between the antenna
and the receiver, the noise powertransferred to the receiver isgiven by
Antenna Temperature
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Antenna Noise Power (Pr)
Antenna TemperatureAntenna temperature at the receiver terminals
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Antenna temperature at the receiver terminals
Antenna TemperatureThe effective antenna Temperature (T ) at the receiver
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The effective antenna Temperature (Ta) at the receiver
terminals is given by
Antenna Temperature The antenna noise power
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The antenna noise power
The system noise powerIf the receiver itself has a certain noise temperature Tr(due to
thermal noise in the Receiver components), the system noise
power at the receiver terminals is given by
Antenna Temperature
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Antenna Temperature
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Arrays
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INTRODUCTION
Enlarging the dimensions of single elements often leads to
more directive characteristics (very high gains) to meet the
demands of long distance communication
An other way to enlarge the dimensions of the antenna,
without necessarily increasing the size of the individualelements, is to form an assembly of radiating elements in
an electrical and geometrical configuration. This new
antenna, formed by multi elements, is referred to as an
array
ArraysINTRODUCTION
Th t t l fi ld f th i d t i d b th t dditi f
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The total field of the array is determined by the vector addition of
the fields radiated by the individual elements
To provide very directive patterns, it is necessary that the fields
from the elements of the array interfere constructively (add) in
the desired directions and interfere destructively (cancel each
other) in the remaining space
There are at least five controls that can be used to shape theoverall pattern of the antenna
1. The geometrical configuration of the overall array (linear,
circular, rectangular, spherical, etc.)
2. The relative displacement between the elements
3. The excitation amplitude of the individual elements
4. The excitation phase of the individual elements
5. The relative pattern of the individual elements
ArraysApplications
A th t i id l d
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An array that is widely used
as a base-station antenna for
mobile communication.
It is a triangular array consisting
of twelve dipoles, with four
dipoles on each side of the
triangle.
Each four element array, on
each side of the triangle, is
basically used to cover anangular sector of 120 forming
what is usually referred to as a
sectoral array.
ArraysTWO-ELEMENT ARRAY:
T i fi it i l h i t l di l iti d l th
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Two infinitesimal horizontal dipoles positioned along the z-
axis, as shown in figure 6.1(a)
Figure 6.1 Geometry of a two element array positioned along
the z-axis.
ArraysTWO-ELEMENT ARRAY:
Th t t l fi ld di t d b th t l t i
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The total field radiated by the two elements, assuming no
coupling between the elements, is equal to the sum of the
two and in the y-z plane it is given by
Phase difference between the elements adjacentelement
k=2/
The magnitude excitation of the radiators is identical (I0)
Phase difference=(2/)x Path difference
ArraysTWO-ELEMENT ARRAY:
Assuming far field observations and
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Assuming far-field observations and
referring to Figure 6.1(b)
Equation 6-1reduces to
ArraysTWO-ELEMENT ARRAY:
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The total field of the array is equal to the field of a singleelement positioned at the origin multiplied by a factor
which is widely referred to as the array factor.
Thus for the two-element array of constant amplitude, theArray Factoris given by
which in normalized form can be written as
ArraysTWO-ELEMENT ARRAY:
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The array factor is a function of the geometry of the arrayand the excitation phase.
By varying the separation d and /or the phase between
the elements, the characteristics of the array factor and ofthe total field of the array can be controlled.
ArraysTWO-ELEMENT ARRAY:
Pattern Multiplication:
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Pattern Multiplication:
The far-zone field of a uniform two element array of
identical elements is equal to the product of the field of asingle element, at a selected reference point (usually the
origin), and the array factor of that array. That is,
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and
Spacing
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Spacing
Let us generalize the method to include N elements. Referring to
the geometry of Figure 6.5(a),
Let us assume that all the elements have identical amplitudes
but each succeeding element has a progressive phase lead
current excitation relative to the preceding one.
An array of identical elements all of identical magnitude
and each with a progressive phase is referred to as a
uniform array
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and
Spacing
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Spacing
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and
Spacing
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Spacing
The Array factor (AF)
By applying pattern multiplication rule on arrays of identicalelement . The array factor is given by
Total phase difference = Phase difference due to path difference + =kdcos+Phase difference=(2/)x Path difference=kdcosPhase difference between the elements adjacent element
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and
Spacing
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Spacing
Multiplying both sides by ej
subtracting Eq 6.6 from Eq 6.8
Which can also be written as
6.6
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and
Spacing
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Spacing
AF
If the reference point is the physical center of the array, the array
factor of (6-10) reduces to
if is small
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and
Spacing
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Spacing
maximum value of array factor is equal to N
Normalized Array Factor
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and Spacing
Nulls of the Array:
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Nulls of the Array:
(AF)n=0| = n
To find the nulls of the array,
Eq (6-10c) or (6-10d) is set equalto zero
= 2n/N(kdcosn+) = 2n/N
cosn= (/2d)(- 2n/N)
For n = N,2N,3N,..., (6-10c) attains its
maximum values because it reduces to a
sin(0)/0 form.
The values of n determine the order of the
nulls (first, second, , etc.)
ArraysN-ELEMENT LINEAR ARRAY: Uniform Amplitude and Spacing
Maximum
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Maximum
The maximum values of normalized array factor occur when
sin(/2) =0| = m
First Maximum or Principal Maximum
The first maximum of the array factor occurs when m = 0
/2 = 0 or = 0
ArraysOn the basis of the main lobe array
Broadside Array
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y
Ordinary End-fire Array
Broadside Array:The maximum radiation of an array directed normal to the axis
of the array (broadside; =900)
The first maximum of the array factor occurs when
= 0 or kdcos+ = 0
kdcos900+ = 0 or = 0
Since it is desired to have the first maximum directed toward =900
Thus to have the maximum of the array factor of a uniform linear array
directed broadside to the axis of the array, it is necessary that all theelements have the same phase excitation (in addition to the same
amplitude excitation). The separation between the elements can be of
any value.
ArraysBroadside Array:
To ensure that there are no principal maxima in other directions,
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p p ,
which are referred to as grating lobes, the separation between
the elements should not be equal to multiples of a wavelength
(dn, n=1,2,3,.) and =0.
If d=n, n=1,2,3,. And =0, then
Thus for a uniform array with = 0 and d = n, in addition to having themaxima of the array factor directed broadside ( = 90 ) to the axis of the
array, there are additional maxima directed along the axis ( = 0 ,180 ) ofthe array (end-fire radiation).
ArraysBroadside Array:
Grating Lobes: multiple maxima, in addition to the main maximum
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g p ,
To avoid any grating lobe, the largest spacing between the elements
Should be less than one wavelength(dmax
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where L is the overall length of the array
ArraysBroadside Array:
Directivity:
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y
where L is the overall length of the array
Arrays
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