1)Draw and explain about yagi uda antenna and derive input impedance for folded dipole?

A)

• A Yagi-Uda antenna has 3 main elements that combinedly form its structure. These 3 major elements are
driven element which is generally a half-wave folded dipole, a reflector and directors.The structure
contains one driven element and a reflector while directors can be more than one.
• Basically, the arrangement is said to be an array of active and parasitic elements.
• The dipole generally a metallic rod acts as the active element as external feeding is provided to it using
transmission lines.
• While reflector and directors are the parasitic elements of the structure.
• The parasitic elements are also metallic rods placed parallelly in the line of sight orientation with respect
to the driven element.
INPUT IMPEDANCE OF FOLDED DIPOLE:

RADIATION PATTERN

Folded dipole Transmission line mode Antenna mode

2. Draw the structure of Yagi antenna and explain its operation?
A:A Yagi-Uda antenna has 3 main elements that combinedly form its structure. These 3 major elements are driven
element which is generally a half-wave folded dipole, a reflector and directors.The structure contains one driven
element and a reflector while directors can be more than one.
The figure below represents the structure of the Yagi-Uda antenna:

• Basically, the arrangement is said to be an array of active and parasitic elements.

• The dipole generally a metallic rod acts as the active element as external feeding is provided to it using
transmission lines.
• While reflector and directors are the parasitic elements of the structure.
• The parasitic elements are also metallic rods placed parallelly in the line of sight orientation with respect
to the driven element.
• It is noteworthy here that no external excitation is provided to the parasitic elements.
• However, when the dipole is excited using a transmission line then the current that flows through the
driven element induces voltages in the parasitic elements.
• All these elements are mounted on a centre rod, that acts as horizontal support.
• The reflector is present at one of the ends of the metallic rod and has length around, 5% greater than the
length of the driven element.
• While the directors are almost 5% shorter than the driven element (i.e., λ/2 at the resonant frequency)
and are placed at the other side of the dipole as these are used to provide maximum directivity to the
antenna.

So, for 3 element aerial, the lengths of the elements can be considered as:

475
Length of driven element = feet
fMHz
500
Length of reflector = 𝑓𝑒𝑒𝑡
fMHz
455
Length of director = feet
fMHz

Working of Yagi-Uda Antenna:

• We know that external excitation is directly provided to the active element of the arrangement i.e., the
dipole.
 • The flow of current through the active element induces a voltage in the parasitic elements that cause
current to flow through it.
• The element having a length greater than λ/2 i.e., the reflector, shows inductive characteristic, therefore,
the current in the reflector lags the induced voltage.
• Whereas, the one shorter than the half-wave dipole i.e., the director is capacitive. So, the current flowing
through it leads the voltage.
• As we know that director is placed in front of the driven elements, so, these directors add the field of the
driven element in the direction away from it.
• When multiple directors are placed in the arrangement then each director will provide excitation to the

next one.
• Also, the reflector in the opposite direction as that of the director when accurately placed adds the field in
the direction towards the driven element.
• This is done in order to reduce the losses due to the back radiated wave as much as possible.
• In order to get the additional gain, multiple directors can be used in the direction of the beam.

The spacing between the elements to form a Yagi -Uda structure is as follows:
 • Basically, the induced voltage and the current flowing due to the induced voltage in the element varies
with the spacing between the active and parasitic elements along with the reactance associated with the
elements.
• It is to be noted here that with the increase in distance between driven element and director, there will be
more need for capacitive reactance in order to provide accurate phasing to the current in the director.
Thus, the length of the director is kept small to get the capacity reactance.

So, we can compile all the above-discussed factors as:

Initially, excitation to the driven element is provided using feed lines. This causes the emission of radiation from
reflector towards the director. Moreover, a portion of the emitted radiation excites the parasitic elements, that
further re-radiate the radiations.The length of the elements and spacing is of great importance here because
radiated energy from each element gets summed in the front direction and so cancels the back radiated wave.

Radiation Pattern of Yagi-Uda Antenna:

Here the major lobe represents the forward radiated wave while the major lobe represents the back radiated
wave.

3. Find the length of reflector, director and driven element operating at

145MHz.
A. Given that frequency(f) = 145MHz.
Reflector length (feet) = 500/f (MHz)
= 500/145
= 3.44 feet
Driven element length (feet) = 475/f (MHz)
= 475/145
= 3.27 feet
Director element length (feet) = 455/f (MHz)
= 455/145
 = 3.13 feet
Therefore, the length of the reflector is 3.44 feet , length of director is 3.27 feet and the length of
the driven element is 3.13 feet
4.Design a six element Yagi array operating at 500MHz and explain?
A. Given Operating frequency (f) = 500MHz
We know that 𝑐 = 𝑓𝜆
Therefore 𝜆 = 𝑐/𝑓
3 ∗ 108 300
𝜆= 6
=( ) = 0.6𝑚
500 ∗ 10 500
The 6 elements in the Yagi-Uda antenna are :1 Reflector
1 Driven Element
4 Directors
The design parameters of 6 element Yagi-Uda antenna is given by:
➢ The Length of the driven element 𝐿𝑎 = 0.46𝜆 = 0.46 ∗ 0.6 = 0.276𝑚
➢ The Length of the reflector 𝐿𝑟 = 0.475𝜆 = 0.475 ∗ 0.6 = 0.285𝑚
➢ The Length of the director 𝐿𝑑1 = 0.44𝜆 = 0.44 ∗ 0.6 = 0.264𝑚
𝐿𝑑2 = 0.44𝜆 = 0.44 ∗ 0.6 = 0.264𝑚
𝐿𝑑3 = 0.43𝜆 = 0.43 ∗ 0.6 = 0.258𝑚
𝐿𝑑4 = 0.40𝜆 = 0.40 ∗ 0.6 = 0.240𝑚
➢ Spacing between reflector and driven element

𝑆𝑙 = 0.25𝜆 = 0.25 ∗ 0.6 = 0.15𝑚

➢ Spacing between director and driven element

𝑆𝑑 = 0.31𝜆 = 0.31 ∗ 0.6 = 0.186𝑚

➢ The diameter of the element is 𝑑 = 0.01𝜆 = 0.01 ∗ 0.6 = 0.006𝑚
➢ The Length of Yagi-Uda array is 𝐿 = 1.5𝜆 = 1.5 ∗ 0.6 = 0.9𝑚

0.186m

0.276m

0.285m 0.264m 0.240m

0.264m

0.258m

0.15m
5)Explain about the importance of folded dipole and derive it’s input impedance?
Ans:

Folded Dipole:

It is an antenna consists of two or more parallelly and closely spaced dipoles connected
together at their edges ends which forms a folded dipole.

In the folded dipole, one dipole must be center fed (connecting signal source) as shown in
figure.

Element ‘1’ is directly connected to source voltage V and the other element ‘2’ is inductively
coupled at their ends.

The radiation pattern at folded dipole is same as half wave dipole and it is like figure of 8.

The folded dipole is mainly used in YAGI UDA antenna as a main element (source element).

The input independence of folded dipole is much higher than half wave dipole.

For ‘n’ element folded dipole, its input independence is given as

Z1n = n2Rr

Where,
Rr -> the radiation resistance of half wave dipole

Rr = 73Ω

Where, ‘n’ is the number of elements in the folded dipole.

Typically, the width d is far less than the length L

Because the folded dipole forms the loop, one might expect input impedance to
depend on the input impedance of a short circuited transmission line of length L.
However a folded dipole antenna can act as two parallel short circuited transmission
lines of length L/2 seperated at the midpoint . It turns the output of the input
impedance of the folded dipole antenna will be function of a transmission line of
length L/2

Also, Folded dipole id “folded” back on itself, the currents can reinforce each other
instead of cancelling each other out, so the input impedance will also depend on the
impedance of a dipole antenna of length L

Let Zd is the impedance of the dipole antenna of length L and Z t is the input impedance of
the transmission line off length L/2,

ZA is the input impedance of the folded dipole given by:

The folded dipole is resonant and radiates well at odd integer multiples of half wavelengths
(0.5λ,1.5λ…..),when antenna is fed I n the center. The input impedance of the folded dipole
antenna is higher than for a regular dipole antenna.
Derivation for input impedance of two element folded dipole :

From the above diagram, it is a two element folded dipole

v
=Z11 I1 + Z12 I2
2
v
= Z21 I1 + Z22 I2
2

If ‘t’ is very small,

I1 = I2 = I
If S is small,

Z11=Z12

Z22=Z21

v
= [Z11 + Z12] I
2
v
= [Z21+ Z22] I
2

v
= 2 IZ11
2

𝑣 = 4 IZ11

V/I = 4*Z11

For a dipole ,The Radiation resistance is 73Ω (Z11=73Ω)

Hence, 𝑍𝑖𝑛 = 4 ∗ 73Ω=292Ω

Derivation of 3 element folded dipole:

From the diagram,

v
=Z11 I1 + Z12 I2+ Z13 I3
3
v
= Z21 I1 + Z22 I2+ Z23 I3
3
v
= Z31 I1 + Z32 I2+ Z33 I3
3

If ‘t’ is very small,

I1= I2=I3=I
If S is small,

Z11=Z12 = Z13

Z21=Z22= Z23

Z31=Z32 = Z33
v
= [Z11 + Z12+Z13] I
3
v
= [Z21+ Z22+Z23] I
3

v
= [Z31+ Z32+Z33] I
3
 v
= 3 IZ11
3

𝑣 = 9 IZ11

V/I = 9*Z11

For an 3 element folded dipole, Zin=9*73Ω=657Ω

Similarly,

For an 4 element folded dipole,Zin=16*73Ω=1168Ω

For an “n” element dipole , Zin= n2Rr

Where Rr = Radiation resistance of half wave dipole.

FORMULAE:
• If d1 is not equal to d2 and s is small, Zin = Z11[1+r2/r1]2 where r1=radius of 1st dipole, r2 = radius of 2nd dipole.
• If s is very small and d1 is not equal to d2, Zin = Z11[1+log(s/r1)/log (s/r2)]2
6. With a neat sketch explain the principle of parabolic reflector?
ANS. The parabolic antenna often referred to as dish antenna, was invented by German physicist Heinrich Herz
during his discovery of radio waves. It uses a parabolic reflector, a curved surface with the cross-sectional shape
of parabola, to direct the radio waves.

• A type of reflector which has a reflecting surface having the shape of a paraboloid that is used to
collect and re-radiated the electromagnetic energy is known as Parabolic Reflector. It basically
transforms the plane wave from the axis towards the focus into the spherical wave. While the spherical
wave from a point source present at focus is transformed into the plane wave and is parallelly
propagated along the axis.
• The operating principle of a parabolic antenna is that a point source of radio waves at the focal
point in front of a paraboloidal reflector of conductive material will be reflected into a collimated plane
 wave beam along the axis of the reflector. Conversely, an incoming wave parallel to the axis will be
focused to a point at the focal point.
• The radiating element used at the focus is generally dipole or horn antenna, which are used to
illuminate the reflecting surface. The waves emitted from the source, incident on the surface of the
reflector and are further reflected back as a plane wave of circular cross-section. The waves from the
feed incidents at different points on the parabolic surface however, all the waves after reflection are
collimated and the plane waves travel in the direction parallel to the axis. Thus, all the collimated
waves from the reflecting surface have equal path length i.e., from the above figure -
• 𝐹𝐸 + 𝐸𝐺 = 𝐹𝐷 + 𝐷𝐻 = 𝐹𝐶 + 𝐶𝐼 = 𝐹𝐵 + 𝐵𝐽 = 𝐹𝐴 + 𝐴𝐾.
• When the incoming waves hit the surface of the parabolic reflector, they are reflected in a way that
the angle of incidence is equal to the angle of reflection. The parabolic shape of the reflector ensures
that all the reflected waves converge at the focal point. At the focal point all the reflected waves come
together and become highly concentrated.

7) Explain about different Parabolic reflector antennas?

Parabolic reflector antenna:
The standard definition of a parabola is - 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 crucial function of the parabolic reflector is to change the spherical wave into a plane wave.So, at the focus
when a feed antenna is placed which is nothing but an isotropic source then the waves are emitted from the source.

The radiating element used at the focus is generally dipole or horn antenna, which are used to illuminate the
reflecting surface.Thus, the waves emitted from the source, incident on the surface of the reflector and are further
reflected back as a plane wave of circular cross-section.
As we can see clearly in the above figure that the waves from the feed incidents at different points on the parabolic
surface. However, all the waves after reflection are collimated and the plane waves travel in the direction parallel
to the axis.

It is to be noted here that, if there is any deviation of the surface of the reflector from an actual paraboloid then it
must not be more than some fraction of wavelength.
Thus, all the collimated waves from the reflecting surface have equal path length i.e., twice the focal length with
a similar phase.

This will lead to provide very high radiation in the direction of the parabolic axis. In this way, the spherical wave
from the feed is converted into a plane wave.

Types of parabolic reflector antennas:

• Focal feed - often also known as axial or front feed parabolic reflector
• Cassegrain feed parabolic reflector
• Gregorian feed parabolic reflector
• Off Axis or offset feed parabolic reflector
1)Focal feed - often also known as axial or front feed parabolic reflector:
The parabolic reflector or dish antenna consists of a radiating element which may be a simple dipole or a
waveguide horn antenna. This is placed at the focal point of the parabolic reflecting surface. The energy from the
radiating element is arranged so that it illuminates the reflecting surface. Once the energy is reflected it leaves the
antenna system in a narrow beam. As a result considerable levels of gain can be achieved.

Achieving this is not always easy because it is dependent upon the radiator that is used. For lower frequencies a
dipole element is often employed whereas at higher frequencies a circular waveguide may be used. In fact the
circular waveguide provides one of the optimum sources of illumination.
The focal feed system is one of the most widely used feed system for larger parabolic reflector antennas as it is
straightforward. The major disadvantage is that the feed and its supports block some of the beam, and this typically
limits the aperture efficiency to only about 55 to 60%.

2) Cassegrain feed parabolic reflector :

The Cassegrain feed system, although requiring a second reflecting surface has the advantage that the overall
length of the dish antenna between the two reflectors is shorter than the length between the radiating element and
the parabolic reflector. This is because there is a reflection in the focusing of the signal which shortens the physical
length. This can be an advantage in some systems.

Typical efficiency levels of 65 to 70% can be achieved using this form of parabolic reflector feed system.
➢ Feed is connected at the backside
➢ It consists of two reflectors one is parabola(main reflector) and the other is hyperbola(sub reflector).
 ➢ These two must be placed in such a manner that focal point must be same
➢ There is no chance of spill over radiation
➢ EMI is less
➢ Aperture blockage depends on size of sub reflector
3) Gregorian feed parabolic reflector:
The Gregorian parabolic reflector feed technique is very similar to the Cassegrain design. The major difference
is that except that the secondary reflector is concave or more correctly ellipsoidal in shape.

4)Off Axis or offset feed parabolic reflector:

As the name indicates this form of parabolic reflector antenna feed is offset from the centre of the actual antenna
dish used.The reflector used in this type of feed system is an asymmetrical segment of the parabolic shape
normally used. In this way the focus, and the feed antenna are located to one side of the reflector surface. The
advantage of using this approach to the parabolic reflector feed system is to move the feed structure out of the
beam path. In this way it does not block the beam.
This approach is widely used in home satellite television antennas, which are often relatively small and this would
mean that any the feed structure including the low noise box (amplifier, etc) would otherwise block a significant
percentage of the beam and thereby reduce the antenna efficiency and signal level.The offset feed is also used in
multiple reflector designs such as the Cassegrain and Gregorian because the small reflector would also suffer the
same issues.

8)Draw and explain Cassegrain feed antennas

Cassegrain feed antennas are a type of satellite dish antenna used for high-frequency communication, remote
sensing, and radio astronomy applications. This type of antenna consists of a parabolic main reflector and a
smaller secondary reflector mounted in front of the main reflector.

• The secondary reflector is usually a hyperbolic or ellipsoidal shape and is positioned at the focal point of
the main reflector. The feed antenna is located at the center of the secondary reflector and is pointed back
toward the main reflector.
• The Cassegrain feed antenna has several advantages over other types of antennas. It provides a wider field
of view than a standard parabolic dish, which is important for applications such as remote sensing and
radio astronomy. It also allows for a more compact design since the secondary reflector acts as a focal
reducer, resulting in a shorter focal length and a smaller main reflector diameter.
• Another advantage of Cassegrain feed antennas is that they can provide a more efficient and accurate
signal transfer. Since the feed is located at the focal point of the secondary reflector, the signal is reflected
back toward the main reflector and then directed to the receiver with minimal loss or distortion.

Cassegrain feed antennas are a versatile and effective solution for high-frequency communication and remote
sensing applications, offering improved accuracy, efficiency, and compactness compared to other types of
antennas.
Advantages

• The feed radiator is more easily supported, and the antenna is geometrically compact.
• It provides minimum losses as the receiver can be mounted directly near the horn.

Disadvantage:

• The sub-reflectors of a Cassegrain type antenna are fixed by bars. These bars and the secondary reflector
constitute an obstacle for the rays coming from the primary reflector in the most effective direction.

A Cassegrain feed antenna consists of a main reflector (parabolic) and a smaller secondary reflector (hyperbolic
or ellipsoidal) mounted in front of the main reflector.

The process of a Cassegrain feed antenna can be explained as follows:

1. An incoming signal is received by the feed antenna, which is located at the center of the secondary
reflector.
2. The signal is reflected by the secondary reflector and directed back toward the main reflector.
3. The main reflector then collects the reflected signal and reflects it toward the focal point.
4. At the focal point, the signal is focused and directed to the receiver located behind the main reflector.
5. The receiver then processes the signal for use in communication, remote sensing, or other applications.

Overall, the Cassegrain feed antenna uses a combination of two reflectors to focus and direct signals with high
accuracy and efficiency. This makes it an ideal choice for applications that require precise signal transfer, such
as high-frequency communication, remote sensing, and radio astronomy.
 ANTENNAS AND WAVE PROPAGATION
ASSIGNMENT -3 -20331A0490
8MARKS:

9. Explain the operation of corner reflector antenna. Mention its advantages and
disadvantage?
Ans: Corner Reflector antenna: A corner reflector antenna is a type of directional antenna
that operates by reflecting radio waves or other electromagnetic radiation in a particular
direction. It consists of a metal frame that forms a corner reflector, with one or more dipole or
other types of antennas located at the focus of the reflector.
• If two planes connected together at their ends with an angle 𝛼 (edge angle) and length
of two planes 𝑙1 and 𝑙2 with a height ‘h’, then it is called corner reflector.
• Corner Reflector is of two types – 1.Square corner reflector and 2.Grid corner reflector

Top view of corner reflector antenna

The operation of a corner reflector antenna can be explained in several steps:

• Signal transmission: The first step in the operation of a corner reflector antenna is the
transmission of the signal to be sent. This can be done using a variety of methods,
including wired connections or wireless transmission.

• Reception by the dipole antenna: Once the signal has been transmitted, it is received
by the dipole or other type of antenna located at the focus of the reflector. This antenna
is typically a simple, linear element that is designed to capture the signal and convert it
into an electrical signal.

• Reflector reflection: Once the signal has been received by the dipole antenna, it is
reflected by the corner reflector. This reflector is designed to reflect the incoming signal
back in the direction of the transmitter or receiver, depending on the intended use of the
antenna.

• Radiation pattern: The combination of the dipole antenna and the corner reflector
produces a radiation pattern that is highly directional. This means that the antenna is
able to send or receive signals in a particular direction, while rejecting signals from
other directions.

• Gain and efficiency: The corner reflector antenna is designed to have high gain and
efficiency, which means that it is able to capture or transmit signals with a high degree
of accuracy and precision. This makes it ideal for use in applications where high
performance is required, such as in radar systems or satellite communication.

Overall, the corner reflector antenna is a highly effective and efficient directional antenna that
is capable of capturing or transmitting signals over long distances with a high degree of
accuracy and precision. Its unique design makes it ideal for use in a wide range of applications,
from military and aerospace to telecommunications and broadcasting.

Advantages :
• High gain: Corner reflector antennas can achieve high gain because of their directional
characteristics.

• Simple construction: The corner reflector antenna is relatively simple to construct and
can be made from inexpensive materials such as wire mesh or aluminum foil.

• Wide bandwidth: The corner reflector antenna has a wide bandwidth, which means it
can operate over a range of frequencies.

Disadvantages:
• Narrow beamwidth: Corner reflector antennas have a narrow beamwidth, which
means they are highly directional and must be pointed accurately at the target.
• Large physical size: Corner reflector antennas can be physically large, especially for
lower frequency applications, which may limit their use in certain applications.

• Limited polarization: The corner reflector antenna is typically polarized in only one
direction, which may limit its use in certain applications that require circular or
elliptical polarization.
 Unit 3
8marks
10) Explain about different types of reflector antennas?
• A reflector is a passive element which is used to reflect the electromagnetic wave or
energy in a desired direction.
• A reflector is used to avoid back radiation or unwanted radiation.
• It is a passive element which is always placed back side of the source at a distance ‘d’.
• There are different types of reflectors based on frequency of operations and
applications.
(i) Rod reflector
(ii) Plane reflector
(iii) Corner reflector
(iv) Parabolic reflector

ROD REFLECTOR:

If the reflector shape is in the form of rod, then it is known as rod reflector. A rod type of
reflector is the one which is majorly used in Yagi uda antenna. The reflector is located at a
certain distance behind the driven element in that antenna arrangement and has a length
generally more than the length of the driven element i.e., half-wave dipole.

PLANE REFLECTOR:

If the reflector is made up of plane metallic sheet, then it is known as plane reflector antenna.
CORNER REFLECTOR:

• If two planes connected together at their ends with an angle of α ( wedge / corner angle)
and length of two planes 𝑙1 and 𝑙2 with height h.
• A corner reflector is a reflecting object which consists of 2 or 3 mutually intersecting
conducting planes. If the two planes are mutually perpendicular to each other, the it is
called as square corner reflector.

PARABOLIC REFLECTOR:

• It is a reflector antenna which has a shape of parabola and employees the properties of
a parabola.
• It is defined as plane of curve obtained by a locus of a point so that its distance from
focal point + its distance upto directrix is constant.
• A parabolic reflector is a 2D antenna.
• In a parabolic reflector primary antenna is called as feed antenna, which is placed at a
focal point.
• The electromagnetic energy that falls on the parabolic surface, it reflects back towards
the direction of propagation.
• Similarly, in the remaining case the electromagnetic energy falls on the parabolic dish,
it is confined to a point called focal point.
• If the parabolic antenna rotated along its axis in the direction of propagation, we will
get paraboloid.
• Paraboloid is a 3D antenna.
• If the base part of the parabola is trauncated then it is called as microwave dish antenna.
 4marks

10)Derive the Focal to Directrix ratio of a paraboloid when Half-wave dipole

feed is used?

• It is one of the Important parameters for a parabolic reflector

• It simply defined as ratio of focal length to the mouth diameter of parabola.

1 θ₀
F/D = 4 cot ( 2 )

Derivation:
FP+PQ = FS+SR = K

For parabola,

OP+PQ = constant = 2f

PQ = rꞋ cos θꞋ

rꞋ + rꞋ cos θꞋ = 2f
rꞋ (1+ cos θꞋ) = 2f

2𝑓
rꞋ = (1+ cos θꞋ)

𝐷/2
first find, θ₀ = tan−1[ ]
𝑍

D²
where Z₀ = f - 16𝑓

𝐷/2
θ₀ = tan−1[ 𝐷² ]
𝑓−
16𝑓

By simplifying,

𝐷 θ₀
f= cot( 2 )
4

1 θ₀
F/D = cot ( )
4 2
11) Explain the operation of corner reflector antenna with image antenna concept?

Ans)The flat reflecting sheets meeting at an angle or corner form an effective

directional antenna. The corner reflector antenna is a driven antenna, generally a half
wave dipole, associated with a reflector constructed of two flat conducting sheets
which meet at a corner or angle to form a corner. This arrangement with corner
reflector and driven antenna is known as corner reflector antenna.

If corner angle β is equal to 900, then the two flat sheets meeting at a right angle,
forming a square corner reflector. When the driven antenna is used in conjunction with
the corner reflector, the arrangement is a effective directional antenna for a wider
range of corner angle, where as the square corner reflector without the driven antenna
is an effective reflector.

Corner angle of β=1800 may be considered as limiting case of corner reflector which
is eqivalent to a flat sheet.
The corner reflector antenna may be analysed by using the method of images for
corner angle.

β= 1800n where n=an integer=1,2,3……

Thus if n=1,β=1800 ………flat sheet

if n=2,β=900 ………square corner reflector

if n=3,β=600 ………corner angle 60

if n=4,β=450 ………corner angle 45

There fore by method of images corner angles of 180,90,60,45 etc..can only be used
and not the intermediate angle.
Let us now illustrate the method of image for square corner reflector. For consider the
below figure,

In this the driven antenna is shown by D and there are three images (2,3,4)
corresponding to the driven antenna (1)- a half wave dipole. The driven antenna and all
the three images carry equal currents . However , driven antenna (1) and image
element (2) are in the same phase and to the image elements(3) and (4) but there
exist 1800phase shift between the phases of elements (1),(2) and (3) and (4).

Expression for the gain for corner angles 600 and 450 etc can be calculated similarly.
For corner angle 600 corner angle the system requires 6 elements (one driven and 5
imaginary elements)and for 450 it needs 8elements(one driven element and 7 image
elements)
 ANTENNAS AND WAVE PROPAGATION
20331A0486 (Assignment – 3)

8 MARKS:

12. Explain the characteristics of an active square corner reflector with the
help of image principle?

Ans :
 If the corner angle 𝛼 = 90° then it is called square corner reflector.
 In square corner reflector there are 3 image sources and one real source. All these are
seperated with an angle 90° .
 The height of the square corner reflector is 1.5 times the height of the feed element.
 The distance between source and vertex point must be vary from 0.3λ to 0.5 λ.
 The aperture distance (𝐷𝑎 ) between the two planes must vary from 𝐷𝑎 = 1λ to 3λ.

1800
 The corner angle 𝛼 = , where ‘n’ is the number of planes.
𝑛
 If n=2, then the corner angle is 900 then it is called as square corner reflector.
 Number of images depends on the corner angle of the reflector i.e, number of images is
3600
not equal to .
𝛼
 If 𝛼is 900 then the number of sources are 4, in that 1 is real source placed in front of the
reflector and the remaining 3 are the image sources.

𝟑𝟔𝟎𝟎
Therefore, no.of image sources are –1
𝜶
Advantages :

 If offers ease of construction.

 It possesses high directivity by reflecting the electromagnetic wave in the direction of its
source.

Disadvantages :

 Its presence makes the antenna arrangement quite bulky.

 The use of this reflector increases the cost .
 ASSIGNMENT – 3
Regd No: 20331A04C2

8 Marks :

13) Explain various types of feeds used in parabolic reflectors with neat
diagrams?
A) Feed System in Antenna :
The feeder is the cable or other transmission line that connects the antenna
with the transmitter or receiver.
Depending on the types of feed given we have the following feed systems :
1. Dipole feed parabolic reflector.
2. Yagi -uda feed parabolic reflector.
3. Horn Feed parabolic reflector.
4. Casegrain feed parabolic reflector.
5. Offset feed parabolic reflector.
Dipole feed parabolic reflector :
• This is the simplest feed system.
• Dipole antenna is the primary radiator which is fed with a coaxial line.
• It is occasionally used.

1
Yagi-uda feed parabolic reflector :
• The yagi-uda antenna is the primary antenna which is fed with a coaxial
line.
• The main function of the reflector element in the Yagi antenna design is
to reflect the backward radiation to the specific direction of propagation.

Horn Feed parabolic reflector :

• Waveguide horn antenna is the primary radiator which is pointing the
paraboloid.
• For getting maximized beam pattern along the parabolic axis, feed is
placed at the focus point.

2
Casegrain feed parabolic reflector :
• In this feed system two radiators are used.
• The primary feed radiator which is a horn antenna is placed at the
backside of the paraboloid.
• The second feed is a hyperbolic reflector.
• The focal points should coincide with each other.
• To reduce aperture block size of secondary reflector hyperbola must be
small.
• Spill over radiation and interference at the main feed is reduced.

Offset feed parabolic reflector :

• The main feed is placed at a offset distance from the focal point.
• Only half of the surface of the parabolic reflector is covered • The
aperture blockage can be avoided by using an offset reflector.
• They are widely in the fixed point to point microwave communication
and satellite reception and tracking.

3
4
 ANTENNAS & WAVE PROPAGATION
ASSIGNMENT-III (20331A04B0)
Cluster-3(8marks)
14. Explain the effect of Spill over and Aperture blocking in parabolic
reflectors?
Ans: Parabolic reflectors, such as those used in telescopes or satellite dishes, rely on the
reflection of electromagnetic waves off a curved surface to direct and focus the waves onto a
single point or a small area.
Two key factors that can affect the performance of a parabolic reflector are (a) spillover and
(b)aperture blocking.
SPILLOVER RADIATION:
Spillover is a phenomenon that can occur in parabolic reflectors where electromagnetic
waves reflect off the edges of the reflector and spill over the rim, leading to wasted energy
and reduced efficiency. This can occur when the reflector is too small for the wavelength of
the waves being used or when the reflector is not deep enough. The effect of spillover is a
reduction in the efficiency of the reflector, which can result in decreased gain and increased
noise in the signal. The excess radiation from spillover can also cause unwanted interference
with other equipment.
To minimize spillover, designers of parabolic reflectors aim to optimize the size and shape of
the reflector to match the wavelength of the waves being used. Additionally, some reflector
designs incorporate a "skirt" or rim that extends beyond the edge of the reflector to prevent
spillover from occurring. By minimizing spillover, the efficiency of the reflector can be
maximized, resulting in a stronger, clearer signal or image.
APERTURE BLOCKAGE:
Aperture blocking, on the other hand, occurs when the central area of the reflector is
blocked, either partially or completely. This can happen when there is an obstruction, such as
a secondary reflector or a support structure, in the path of the waves. Aperture blocking can
lead to reduced gain, increased side lobes, and reduced resolution in the resulting image or
signal. It is a phenomenon that occurs in parabolic reflectors when the central area of the
reflector is blocked, either partially or completely. This blockage can be caused by
obstructions such as a secondary reflector, a support structure, or even the feed horn itself.
The result of aperture blockage is a reduction in the efficiency of the reflector, which can lead
to decreased gain, increased side lobes, and reduced resolution in the resulting image or
signal. Aperture blockage can also result in increased noise in the signal, as well as a
distorted beam shape.
To minimize the effects of aperture blockage, designers of parabolic reflectors aim to keep
the obstruction to a minimum and position it out of the path of the waves as much as possible.
In addition, some reflector designs incorporate a "chopped" or segmented surface to
minimize blockage and improve the efficiency of the reflector.
To optimize the performance of a parabolic reflector, it is important to minimize spillover
and aperture blocking by choosing the appropriate size and shape of the reflector and
positioning any obstructions out of the path of the waves.
1
 ANTENNAS &WAVE PROPOGATION
UNIT 3
ASSIGNMENT-3(20331A04B3)

7marks
16. What is an electromagnetic horn antenna Write its applications and Explain
about different types of horn antennas?

• Horn antenna is a radiating element which has the shape of horn. It is a waveguide in
which one end is open and other end is flared out, to improve radiation efficiency,
radiation pattern and directivity

• Horn antenna is basically used at microwave frequencies (0.3-30Hz).

• It has high directivity; hence called as high directional antenna.

• It is mainly used as a reference antenna in testing applications, antenna measurements.

• The gain of horn antenna is very high which is used to produce high directional narrow
beam.

• They are used as feed elements.

Types of horn antenna:

• Sectorial horn antenna

• E plane

• H plane

• Pyramidal

• Conical horn antennas

• Corrugated horn antennas

Sectorial horn is a horn in which flaring exists only in one direction.

• If flaring is along the direction of electric field, it is called sectorial E-plane horn.
 • If flaring is along the direction of magnetic field, it is called sectorial H-plane horn.

• If flaring is along E and H, it is called as pyramidal horn.

Conical horn antennas

The figure-3 depicts conical horn antenna type. It is made by flaring out one end of circular
waveguide.
Following are the applications of horn antennas:
• Used as feed elements in reflector antennas
• Used to provide moderate gain
• Used in lab measurements and experiments

Corrugated Horn Antennas

In a corrugated horn the surface of the inner part is corrugated. The corrugations are typically
axial (that is, in the direction of the horn axis) or radial (i.e., perpendicular to the axis). The
depths of the corrugations usually vary between λ/4 and λ/2, where λ is the wavelength of the
electromagnetic field at the design frequency
4 MARKS
16.An 8GHz pyramidal horn is with dimensions 16x8 cms, find its gain and
directivity assuming 90 percent efficiency?
A: The gain and directivity of a pyramidal horn antenna can be calculated using the following
equations:

Gain = (4 * pi * efficiency * Aperture efficiency) / lambda^2 Directivity = (4 * pi * Aperture

efficiency) / lambda^2
Aperture efficiency = ( 1 / (1 + (4 * L / W) ) )^2
L is the length of the horn (16 cm in this case)
W is the width of the horn (8 cm in this case)
First, let's calculate the wavelength lambda of the operating frequency:
lambda = c / f where:

c is the speed of light (3 x 10^8 m/s)

f is the operating frequency (8 GHz = 8 x 10^9 Hz)

lambda = 3 x 10^8 / (8 x 10^9) = 0.0375 meters

Next, let's calculate the aperture efficiency:

Aperture efficiency = ( 1 / (1 + (4 * L / W) ) )^2 = ( 1 / (1 + (4 * 16 / 8) ) )^2 = ( 1 / (1 + 8) )^2 =

0.0123

Now, we can calculate the gain and directivity:

Gain = (4 * pi * efficiency * Aperture efficiency) / lambda^2 = (4 * pi * 0.9 * 0.0123) / (0.0375)^2

= 96.85

Directivity = (4 * pi * Aperture efficiency) / lambda^2 = (4 * pi * 0.0123) / (0.0375)^2 = 110.28