advance radiation systems

1. UNIT-II
APERTURE AND REFLECTOR
ANTENNAS
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2. APERTURE ANTENNA:
•An Antenna with an aperture at the end can be termed as
an Aperture antenna.
• Waveguide is an example of aperture antenna.
• The edge of a transmission line when terminated with an
opening, radiates energy.
• This opening which is an aperture, makes it
an Aperture antenna.
• The main types of aperture antennas are :
 Wave guide antenna
 Horn antenna
 Slot antenna
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3. Field Equivalence Principle: Huygen’s
Principle:
• The field equivalence is a principle by which actual
sources, such as an antenna and transmitter, are
replaced by equivalent sources.
• The fictitious sources are said to be equivalent
within a region because they produce the same
fields within that region.
• Huygens’ principle states that “each point on a
primary wavefront can be considered to be a new
source of a secondary spherical wave and that a
secondary wavefront can be constructed as the
envelopeof these secondary spherical waves .”
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4. Huygen’s Principle:(contd…)
consider an actual radiating source, which
electrically is represented by current
densities 𝐽1and 𝑀1.
• The source radiates fields 𝐸1 and 𝐻1
everywhere.
• However, it is desired to develop a
method that will yield the fields outside
a closed surface
• To accomplish this, a closed surface S is
chosen.
• The volume within S is denoted by 𝑉1
and outside S by 𝑉2
• The primary task will be to replace the
original problem by an equivalent one
which yields the same fields 𝐸1 and
𝐻1 outside S (within 𝑉2).
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5. Huygen’s Principle:(contd…)
• The original sources 𝐽1and 𝑀1 are removed,
and we assume that there exist fields E and H
inside S and fields 𝐸1and 𝐻1 outside of S.
• For these fields to exist within and outside
S,they must satisfy the boundary conditions:
• The current densities 𝐽𝑆 and 𝑀𝑆 are said to be
equivalent only within 𝑉2 because they
produce the original fields (𝐸1 𝐻1 ) only
outside S. Fields E, H, different from the
originals (𝐸1 𝐻1) , result within 𝑉1
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6. UNIFORMLY ILLUMINATED
RECTANGULAR APERTURE:
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7. 𝑇𝐸10 and 𝑇𝐸11 Mode For Illuminated
Rectangular Aperture:
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8. 𝑇𝐸10 ILLUMINATED RECTANGULAR
APERTURE:
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9. DIRECTIVITY OF RECTANGULAR
APERTURE:
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11. CIRCULAR APERTURE UNIFORM
ILLUMINATION:
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12. DESIGN CONSIDERATIONS:
• Aperture antennas can be designed to control their radiation
characteristics
• the level of the minor lobes can be controlled by tapering the
distribution across the aperture
• the smoother the taper from the center of the aperture
toward the edge, the lower the side lobe level and the larger
the half-power beamwidth, and conversely.
• An intermediate taper, such as that of a Tschebyscheff
distribution or any other similar one, will have to be selected
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13. DESIGN CONSIDERATIONS:(Contd..)
• Aperture antennas, both rectangular and circular, can also be
designed for satellite applications where the beamwidth can
be used to determine the “footprint” area of the coverage.
• In Such Designs,
 It is important to relate the beamwidth to the size of the
aperture
 It is also important to maximize the directivity of the
antennas within a desired angular sector defined by the
beam width, especially at the edge of coverage.
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14. HORN ANTENNAS
• A Horn antenna may be considered as a flared out wave guide, by which
the directivity is improved and the diffraction is reduced.
• The flared portion can be square, rectangular, or conical.
• The maximum radiation and response corresponds with the axis of the
horn.
• A horn antenna is used for the transmission and reception
of microwave signals.
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15. TYPES OF HORN ANTENNA:
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16. SECTORAL HORN ANTENNA:
• A pyramidal horn with only one pair
of sides flared and the other pair
parallel.
• It produces a fan-shaped beam, which
is narrow in the plane of the flared
sides, but wide in the plane of the
narrow sides.
• These types are often used as feed
horns for wide search radar antennas.
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17. TYPES OF SECTORAL HORN ANTENNA:
• E-plane horn antenna : This form of antenna is one that is flared in the
direction of the electric or E-field in the waveguide.
• H-plane horn antenna : This form of antenna is one that is flared in the
direction of the electric or H-field in the waveguide.
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18. PYRAMIDAL HORN ANTENNA:
• Has flaring on both sides.
• If flaring is done on both the E & H walls of a rectangular
waveguide, then pyramidal horn antenna is produced.
• This antenna has the shape of a truncated pyramid.
• probably the most popular antenna in the microwave
frequency ranges (from ≈1 GHz up to ≈18 GHz).
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19. PYRAMIDAL HORN :(Contd…)
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20. PYRAMIDAL HORN:(Contd…)
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21. PYRAMIDAL HORN:(Contd…)
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22. PYRAMIDAL HORN (DESIGN
PROCEDURE):
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23. PYRAMIDAL HORN(PROBLEM):
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24. PYRAMIDAL HORN(PROBLEM):(Contd..)
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25. PYRAMIDAL HORN(PROBLEM):(Contd..)
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26. PYRAMIDAL HORN(FINAL DESIGN):
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27. CONICAL HORN ANTENNAS:
• When the walls of a circular wave guide are
flared, it is known as a conical horn.
• This is a logical termination of a circular wave
guide.
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31. CORRUGATED ANTENNAS:
• Efficiencies of the order of 75–80% can be
obtained with improved feed systems utilizing
corrugated horns.
• Corrugations with depth λ/2 acts as a conducting
surface while that with λ/4 depth in horn antenna
present a high impedance.
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32. CORRUGATED HORN:
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33. COMPARISON OF HORN ANTENNA:
SECTORAL HORN PYRAMIDAL HORN CONICAL HORN CORRUGATED
HORN
Flares out in only
one direction
Has flaring on both
sides
The walls of a
circular wave guide
are flared.
Has parallel slots or
grooves along the
inside surface of the
horn, transverse to
the axis.
 Flaring in the
direction of
Electric vector
produces
the sectorial E-
plane horn.
 Flaring in the
direction of
Magnetic vector,
produces
the sectorial H-
plane horn.
If flaring is done on
both the E & H walls
of a rectangular
waveguide,
then pyramidal
horn antenna is
produced
This is a logical
termination of a
circular wave guide.
The corrugated
horn provides a
pattern that is
nearly symmetrical,
with the E and H
plane beam-widths
being nearly the
same.
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34. ADVANTAGES OF HORN ANTENNA:
• They can operate over wide ranges of
frequencies.
• Very wide bandwidth, for example allowing it to
operate from 1GHz to 20GHz, 20:1.
• High Directivity.
• High gain.
• Support for wide applications.
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35. APPLICATIONS OF HORN ANTENNA:
• They are used as feeders (called feed horn) for
larger antenna.
• structures such as parabolic antennas, as
directive antennas for such devices as radar guns,
automatic doors openers, microwave radiometer.
• A common element of phase array.
• Satellite and microwave communications.
• Used in the calibration, other high gain antenna.
• Used for making electromagnetic interference
measurement.
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36. BABINET PRINCIPLE:
• Since the impedance for a half-wavelength dipole is about 73 ohms,
the corresponding slot has an impedance of
• The field at any point behind a plane having a screen, if added to the
field at the same point when the complementary screen is
substituted, is equal to the field at the point when no screen is
present.
• Apply this to antennas:
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37. BABINET PRINCIPLE:(contd…)
• Self-complementary antennas – frequency
independent.
• This type of antenna has a constant
impedance of
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38. PARABOLIC REFLECTOR:
• Parabolic Reflectors are Microwave antennas.
• The frequency range used for the application
of Parabolic reflector antennas is above 1MHz.
• These antennas are widely used for radio and
wireless applications.
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39. PRINCIPLE OF OPERATION:
• parabola - Locus of a point, which moves in such a way that its distance
from the fixed point (called focus) plus its distance from a straight line
(called directrix) is constant.
• The point F is the focus (feed is given) and V is the vertex.
• The line joining F and V is the axis of symmetry.
• PQ are the reflected rays where L represents the line directrix on which
the reflected points lie (to say that they are being collinear).
• The distance between F and L lie constant with respect to the waves being
focussed.
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40. PRINCIPLE OF OPERATION:
• The reflected wave forms a collimated wave front, out
of the parabolic shape.
• The ratio of focal length to aperture size (ie., f/D)
known as “f over D ratio” is an important parameter of
parabolic reflector. Its value varies from 0.25 to 0.50.
• The law of reflection states that the angle of incidence
and the angle of reflection are equal. This law when
used along with a parabola, helps the beam focus.
• The shape of the parabola when used for the purpose
of reflection of waves, exhibits some properties of the
parabola, which are helpful for building an antenna,
using the waves reflected.
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41. PROPERTIES OF PARABOLA:
• All the waves originating from focus, reflects back
to the parabolic axis. Hence, all the waves
reaching the aperture are in phase.
• As the waves are in phase, the beam of radiation
along the parabolic axis will be strong and
concentrated.
• the parabolic reflectors help in producing high
directivity with narrower beam width.
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42. CONSTRUCTION AND WORKING OF
PARABOLIC REFLECTOR:
• The waves come out of the focal point and strikes the
Parabolic reflector.
• This wave now gets reflected as collimated wave front,
to get transmitted.
• The same antenna is used as a receiver.
• When the electromagnetic wave hits the shape of the
parabola, the wave gets reflected onto the feed point.
• The dipole or the horn antenna, which acts as the
receiver antenna at its feed, receives this signal, to
convert it into electric signal and forwards it to the
receiver circuitry.
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43. CONSTRUCTION AND WORKING OF
PARABOLIC REFLECTOR:(contd…)
• The gain of the paraboloid is a function of
aperture ratio (D/λ).
• The Effective Radiated Power (ERP) of an antenna
is the multiplication of the input power fed to the
antenna and its power gain.
• Usually a wave guide horn antenna is used as a
feed radiator for the paraboloid reflector
antenna.
• Another type of feed given to the paraboloid
reflector antenna, is called as Cassegrain feed.
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44. CASSEGRAIN FEED:
• Cassegrain is another type of feed given to the reflector antenna.
• The feed is located at the vertex of the paraboloid, unlike in the parabolic reflector.
• A convex shaped reflector, which acts as a hyperboloid is placed opposite to the
feed of the antenna.
• It is also known as secondary hyperboloid reflector or sub-reflector.
• It is placed such that its one of the foci coincides with the focus of the paraboloid.
• Thus, the wave gets reflected twice.
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45. CASSEGRAIN ANTENNA:
• TRANSMITTING ANTENNA:
 The energy from the feed radiates through a horn antenna onto the
hyperboloid concave reflector.
 which again reflects back on to the parabolic reflector.
 The signal gets reflected into the space from there.
 Hence, wastage of power is controlled and the directivity gets improved.
• RECEIVING ANTENNA:
 The electromagnetic waves strike the reflector, gets reflected on to the
concave hyperboloid .
 and from there, it reaches to the feed.
 A wave guide horn antenna presents there to receive this signal and sends
to the receiver circuitry for amplification.
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46. PARABOLIC REFLECTOR WITH
CASSEGRAIN FEED:
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47. ADVANTAGES OF PARABOLIC
REFLECTOR ANTENNA:
• Reduction of minor lobes
• Wastage of power is reduced
• Equivalent focal length is achieved
• Feed can be placed in any location, according
to our convenience
• Adjustment of beam (narrowing or widening)
is done by adjusting the reflecting surfaces
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48. APPLICATIONS:
• Parabolic reflectors with cassegrain feed is
mainly used in satellite communications.
• Also used in wireless telecommunication
systems.
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