Transcript Antenna

Radio & Transmission Systems
Antennas
Antenna
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Antennas
INTRODUCTION
Antennas are constructed with conductors in the
form of wires or rods.
An antenna converts electrical energy into EM
waves for transmission and EM waves into
electrical energy for reception.
Tx. and Rx. antennas behave identically. The same
antenna can be used for transmitting or for
receiving.
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EXCITATION
A Tx. antenna is said to be excited when the
electrical o/p of a transmitter is fed to it.
Antenna excitation generates antenna current and
causes radio waves to be radiated into the
atmosphere.
The radiation will be strongest when the antenna
is resonant to the frequency of the antenna
current.
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Half-wave Antenna
A resonant antenna is approximately  /2 long.
A half-wave antenna is called as a dipole, a doublet
or a Hertz antenna.
It is  /2 long at its operating frequency.
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Example of a dipole antenna
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Standing Waves Pattern of Dipole
Open ends have a current minimum (node) and a
voltage maximum (loop).
The center feed point has maximum current and
minimum voltage.
Antenna standing wave current and voltage
curves are commonly called displacement
curves.
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Standing Waves Pattern of Dipole
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Antenna Radiation
An excited dipole develops an electric field
between its ends because of the high voltage
potentials at these points.
A magnetic field develops around the antenna
because of the current flow in the antenna. Since
the current is maximum at the center, so is the
magnetic field.
The combined electric and magnetic field radiate
into space as electromagnetic wave (EM).
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Field Strength
Field strength is a measure of the intensity of an
electric (in v/m), magnetic ( in gauss) or
electromagnetic field (in watts per square
meter).
It depends on the distance from the antenna and
on the antenna radiated power.
It varies inversely with distance and it is evaluated
by the voltage induced across a wire.
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Radiation Patterns
An antenna does not perform equally well in all
directions.
The directivity of an antenna is expressed by its
Radiation Pattern.
The radiation pattern is a graphical plot of the field
strength radiated by an antenna in different angular
direction at equal distance from the antenna.
The bidirectional radiation pattern of a horizontal
dipole shows that maximum radiation occurs
broadside and minimum radiation occurs off the
ends.
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Radiation Pattern of a horizontal half-wave
dipole
Ground plane plot
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The omnidirectional radiation pattern of a
vertical dipole shows that maximum radiation
occurs in all directions in the horizontal plane
and minimum radiation occurs directly above
or below the antenna.
The polar plot in the horizontal plan is a circle.
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Radiation Patterns for a vertical half-wave dipole
a. Ground plane plot
b. Polar plot
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Beamwidth
Beamwidth is a measure of the directivity of an
antenna.
It is the angle subtended by the points at which
the radiated power has fallen to half of its
maximum value ( 3dB points),
or the field strength has fallen to 1/ 2 (0.707) of
its maximum voltage.
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Gain of Antenna
The gain of an antenna is a measure of its directional
properties and indicates the extent to which radiation
is concentrated in a particular direction.
The increased power being radiated in a particular
direction is obtained at the expense of other directions.
Antenna gain is defined relative to a reference antenna.
The reference antenna is either a half-wave dipole or
an isotopic antenna.
An isotropic antenna is one which radiate equally well in
all directions. (in theory)
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• The gain of a Tx. Antenna is the square of the ratio
of the field strength produced at a point in the
direction of maximum radiation from the antenna
to the field strength produced at the same point by
the reference antenna.
• It may also be expressed as the ratio of the powers
required to be transmitted by the two antennas to
produce the same field strength at a particular
point in the direction of maximum radiation.
• The gain of a half-wave dipole relative to an
isotropic radiator is 1.64 times or 2.15dB.
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Example
• An aerial must be fed with 10kW of power to
produce the same field strength at a given point
as a half-wave dipole fed with 20kW of power.
• Calculate the gain of the antenna
a. relative to a half-wave dipole;
b. relative to an isotropic radiator.
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Solution
Gain of antenna relative to an half-wave dipole
= 10 log10 (20000/10000)
= 3dB
Gain of antenna relative to isotropic radiator
= 3dB + 2.15dB
= 5.15dB
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Effective radiated power
• Effective radiated power, e.r.p., is the power that
an isotropic radiator to produce the same field
strength at a particular point in the direction of
maximum radiation.
e.r.p. = total transmitted power x gain of
antenna
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Example
• An antenna with a gain of 20dB relative to an
isotopic radiator radiates a power of 10W.
Determine the effective power of the antenna.
Solution
• 20dB is a power ratio of 100:1
• e.r.p = 100 x 10 = 1kw
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Radiation resistance
• a mathematical quantity that express the
relationship between the antenna current and
the power radiated in the form of radio waves.
Pr = I2Rr
Pr = power radiated
I = antenna current
Rr = radiation resistance
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Antenna efficiency
• All the antenna losses are represented by a loss
resistance RL.
• The efficiency  of antenna is the ratio of the
power radiated to the power fed to the antenna.
 = I2Rr / (I2RL + I2Rr) = Rr / (RL + Rr) x 100%
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Example
An antenna has a radiation resistance of 0.3 and
a loss resistance of 1.5. If the current fed into the
aerial is 50A, calculate the radiated power, the
power input and the antenna efficiency.
Solution
Power radiated = I2Rr = 502 x 0.3 = 750W
Input power = I2Rr + I2RL = 502x0.3 + 502x1.5
= 4.5KW
Efficiency = 750x100/4500 % = 16.67%
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Impedance
• Antenna impedance = voltage / current at the
input
• Due to energy lost to radiation, input impedance
of /2 dipole is not zero, but is approx. 73.
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Marconi Antenna
Physical size is /4 (quarter-wave)
Erected vertically and connected to ground at
one end
A perfect ground will produce a mirror image of
the quarter-wave
The earth acts as additional /4 of the antenna
Marconi behaves electrically as a half-wave
antenna.
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Direct wave
Reflected
wave
/4
antenna
/4
mirror
antenna
Ground
Marconi antenna
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Parasitic Element
• Parasitic elements are secondary antennas which
are placed in close proximity to the main or driven
antenna.
• They are not directly fed, but have currents
induced in them from the main element (or from
the received wave in the case of a receiving
antenna).
• The secondary antennas are tuned so as to cause
energy reradiation from them, and this changes
the radiation pattern of the main antenna
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Parasitic Reflectors
• The reflector element is placed behind the
driven dipole and is made about 5% longer than
the driven /2 dipole.
• Maximum radiation occurs in the front
direction along array axis when the reflector is
placed about 0.2 behind the driven element.
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Parasitic Reflectors
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Parasitic Directors
• The director element, which is placed in front of
the driven dipole, is made about 5% shorter
than the dipole.
• It is spaced to provide maximum radiation in
the forward direction, and optimum spacing is
again found experimentally to be about 0.1.
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Parasitic Directors
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Yagi Antenna
• Yagi antennas are the most commonly used
parasitic arrays, used primary for TV reception.
• A Yagi antenna consists one driven element, one
reflectors and one or more directors.
• One or more directors are placed in front of the
driven element to direct the wave in a forward
direction.
• A reflector is placed behind the driven element to
reflect the wave back to the driven element.
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Yagi Antenna
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Yagi Antenna
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Folded Dipole
• Input impedance of a /2 dipole is 73 resistive.
• Addition of parasitic elements reduces the input
impedance to much lower value.
• Impedance mismatch with a normal 50  or 75 
coaxial feeder.
• It can be overcome by using a higher input
impedance driven element -folded dipole.
• The input impedance of a folded dipole is is four
times of a simple dipole (292  ).
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Folded Dipole
/2
292
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Log Periodic Antenna
• The log periodic antenna is basically an array of
dipoles, fed with alternating phase, lined up along the
axis of radiation.
• The element lengths and their spacing all conform to
a ratio, given as
t=L(n+1)/Ln=X(n+1)/Xn
• Also, the angle of divergence is given as
a=tan-1(Ln/Xn)
• The open-end length L must be larger than 1/2 if high
efficiency (90%) is to be obtained.
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Log Periodic Antenna
• This antenna has the unique feature that its
impedance is a periodic function of the
logarithm of the frequency-hence its name.
• The antenna characteristics are broadband, and
it has the directional characteristics of a dipole
array.
• This type of antenna is often used for mobilebase-station operations
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Dish Antenna
• A dish antenna is a high-gain antenna used for
reception and transmission of UHF and microwave
signals.
• E.g. microwave relay system, satellite communication
system and TV receiving systems.
• It consists of a driven element and a large spherical or
parabolic reflector. The driven element is placed at the
focal point of the reflector.
• Signals arriving from distance and in parallel
waveforms are reflected off the disc and brought
together at the focal point.
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Dish Antenna
• Energy radiated by the driven element is
reflected by the disc and sent out as parallel
wave.
• D = Diameter of the dish
• Gain = 6(D/ )2
• Beamwidth = 70 /D
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Dish Antenna
Reference: http://www.sat-ant-sys.com/frames.htm
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Longwire Antenna
• The long-wire antenna is a wire with several
wavelengths in length that is suspended at some
height above the earth.
• The wire is driven at one end and has a resistive
termination at the remote end which is matched to the
characteristic impedance of the line at that end.
• When an alternating current wave is transmitted
down this line toward the terminated end, about half
of the energy is radiated into space.
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Longwire Antenna
• Longwire antennas are sometimes used in
MF/HF band transmission and reception.
• Gain is proportional to length of wire.
• Inexpensive and easy to install.
• Disadvantages:
– Inconvenient to change direction.
– Large space required.
• The radiation pattern of a longwire antenna
consists of 2 main lobes.
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Longwire Antenna
Longwire antenna
Ground
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Longwire Antenna
Radiation patterns
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Rhombic Antenna
• A wideband antenna used for h.f. links.
• The rhombic antenna consists of 4 long wires
(4 - 8  in length) connected horizontally
together in a geometric shape of a rhombus.
• All the 4 wires will radiate energy in the
directions indicated by its radiation pattern.
• The transmission line feeds one end and
transmits an unreflected current wave down
each side toward the resistive termination at the
far end.
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Rhombic Antenna
 :Title angle
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Rhombic antenna
Reference: http://users.neca.com/cummings/rhombic.html
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Loop Antenna
• A loop antenna consists of one or more turns wound
in form of a circle, a square or a rectangle.
• Small loops, suitable for receiving signals up to
30MHz, have a circumference of less than 0.1 at the
highest operating frequency.
• The voltage delivered to the receiver depends on the
area of the loop and the number of turns of the wire.
• No radiation is received from the direction normal to
the plane of the loop.
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Loop Antenna
• Because of the excellent directivity property,
loop antennas are often used in direction finding
equipment.
No radiation in
direction of normal
to the plane
Square loop antenna
Radiation pattern