MAHALAKSHMI ENGINEERING COLLEGE-TRICHYmahalakshmiengineeringcollege.com/pdf/ece/VIsem/EC2353/UNIT 4.pdf · Define lens antenna? An antenna, ... dielectric lens or H plane metal plate

MAHALAKSHMI ENGINEERING COLLEGE-TRICHY

K.SRINIVASAN, AP/ECE

ANTENNAS &WAVE PROPAGATION DEPT./SEM.: ECE /V

UNIT-IV

PART-A

1. State Huygen’s Principle?

Huygen‘s principle states that each point on a primary wave front can be considered to be a

new source of a secondary spherical wave that a secondary wave front can be constructed as

the envelope of these secondary waves.

2. What is Slot Antenna?

The slot antenna is an opening cut in a sheet of a conductor, which is energized through a

coaxial cable or wave guide.

3. Which antenna is complementary to the slot dipole?

The dipole antenna is the complementary to the slot antenna. The metal and air regions of the

slot are interchanged for the dipole.

5. What is the relationship between the terminal impedance of slot and dipole antenna?

ZsZd = ho2/4

Where Zs is the terminal impedance of the slot antenna

Zd is the terminal impedance of the dipole antenna

ho is the intrinsic impedance of the free space » 377W\\

6. What is the difference between slot antenna and its complementary dipole antenna?

1. i. Polarization are different

2. ii. The electric field be vertically polarized for the slot and horizontally polarized for its

complementary dipole

3. iii. Radiation form the backside of the conducting plane of the slot antenna has the opposite

polarity from that of the dipole antenna.

7. Define lens antenna?

An antenna, which collimates the incident divergent energy to prevent it from spreading in

undesired directions, is called as lens antenna.

8. What are the different types of lens antenna?

1. i. dielectric lens or H plane metal plate lens

2. ii. E plane metal plate lens antenna

9. What is a dielectric lens antenna?

Dielectric lens antennas are the antennas in which the traveling wave fronts are delayed by lens

media



10. What are the drawbacks of lens antenna?

Lens antennas are used only at higher frequencies (above 3 GHz) because at lower frequencies

they become bulky and heavy. Lens antennas have excessive thickness at low frequencies.

Thickness, t = l/m-1 = C/ f(m-1)

Costlier for the same gain and beam width in comparison with reflectors

11. What are the field components that are radiated from open end of a coaxial line?

Eq = {-hbwKsinq(b2-a2)e-jbro}/8r0

Hf = {-bweKsinq(b2-a2)e-jbro}/8r0

12.What are the advantages of stepped dielectric lens antenna?

1. i. It is mechanically strong

2. ii. Reduces weight

3. iii. Less power dissipation

13.What is biconical antenna?

The biconical antenna is a double cone antenna which is driven by potential , charge or an

alternating magnetic field at the vertex. In this antenna both the cones face in the opposite

direction.

14.What is Lunenburg lens?

The Lunenburg lens is a spherical symmetric delay type lens formed from a dielectric with index

of refraction ‗n‘ which varies as a function of radius given by

.n = Ö[2 –{ r/R}2]

where r = radial distance from the center of the sphere

R = radius of the sphere

15.What are the advantages of lens antenna?

1. i. the lens antenna, feed and feed support do not block the aperture as the rays are

transmitted away from the feed

2. ii. It has greater design tolerance

3. iii. It can be used to feed the optical axis and hence useful in applications where a beam is

required to be moved angularly with respect to the axis.

16.Mention the uses of lens antenna?

1. i. Unstepped dielectric lens is a wide band antenna as its shape does not depend on the

wavelength and hence it can be used over a wide frequency range, however this is not true for

the dielectric lens antenna which is frequency sensitive.

2. ii. Both reflectors and lens antenna are commonly used above 1000 MHz. Lens antenna is a

microwave device. So it is preferred to be usually above 3000 MHz and not below it.



17.How spherical waves are generated?

When a voltage V is supplied at the input terminals of a biconical antenna, it will produce

outgoing spherical waves. The biconical antenna acts as a guide for spherical waves.

18.Define the characteristic impedance of biconical antenna?

The Characteristic impedance Zc of a biconical antenna is the ratio of voltage (r ) and current ( r

) Zc = V( r) / I ( r) = 120ln cot(a/4)

19.Bring out the expressions for voltage across the feed points of the biconical antenna

and current flowing through the surface of the cone?

V(r ) = 2hHmln cot(a/4)

I( r) = 2pHmee-jbr

20.What do you meant by sect oral horn?

If flaring (opened out) is done only in one direction, then it is called as a sectoral horn.

21.What do you meant by pyramidal horn?

If flaring is done along both the walls( E & H), then it is called as a pyramidal horn.

22.What is back lobe radiation?

Some radiation from the primary radiator occurs in the forward direction in addition to the

desired parallel beam. This is known as back lobe radiation.

23.What are the various feeds used in reflectors?

1. i. Dipole antenna

2. ii. Horn feed

3. iii. End fire feed

4. iv. Cassegrain feed

24.What are the different types of horn antennas?

1. i. Sectoral horn

2. ii. Pyramidal horn

3. iii. Conical horn

4. iv. Biconical horn antenna

25.Define refractive index of lens antenna?

Refractive index, m = (Velocity of wave in air)/(velocity of wave in lens medium)



PART-B

1. Explain about the Radiation from an elemental area of a plane wave (Huygen‘s

source)

Radiation from he open end of a coaxial line – Radiation from a rectangular aperture treated as

an array of huygen‘s source – Equivalence of fields of a slot and complementary dipole –

Relation between dipole and slot impedances – Method of feeding slot antennas – Thin slot in

an infinite cylinder – Field on the axis of an E-plane sectoral horn – Radiation from circular

aperture – Beam width and effective area – Reflector type of antennas (dish antennas).

dielectric lens and metal plane lens antennas – Luxemberg lens – Spherical waves and

biconical antenna.





Huygens' Principle

Each point on a wavefront acts as a new source of waves

The 3-dimensional radiation pattern in free space is a fat doughnut with the dipole piercing its

central hole. Notice that unlike an isotropic radiator that radiates equally well in all directions, the

dipole radiates more RF in some directions than others. This means that the dipole has a gain

or directivity over an isotropic radiator of approximately 2.1 dB. That means that the radiation

from the dipole is 2.1 dB stronger in the direction of maximum radiation than the radiation from

an isotropic radiator in the same direction, when both antennas are fed with the same amount of

RF power..

The input impedance of a dipole antenna also depends on its electrical length. When the

antenna is approximately an odd multiple of a half wavelength long, the input impedance is

resistive and lies between 50 and 200 ohms. For antennas that are an even number of half

wavelengths long, the input impedance is resistive and extremely high, between 1000 and

50,000 ohms.



2. With neat sketch Explain about Aperture antenna



3. Write short notes on lens antenna

ANTENNA.—Another antenna that can change spherical waves into flat plane waves is the lens

antenna. This antenna uses a microwave lens, which is similar to an optical lens to straighten

the spherical wavefronts.

Since this type of antenna uses a lens to straighten the wavefronts, its design is based on the

laws of refraction, rather than reflection. Two types of lenses have been developed to provide a

plane-wavefront narrow beam for tracking radars, while avoiding the problems associated with

the feedhorn shadow.

These are the conducting (acceleration) type and the dielectric (delay) type. The lens of an

antenna is substantially transparent to microwave energy that passes through it. It will, however,

cause the waves of energy to be either converged or diverged as they exit the lens. Consider

the action of the two types of lenses.

The conducting type of lens is illustrated in figure 1-10, view A. This type of lens consists of flat

metal strips placed parallel to the electric field of the wave and spaced slightly in excess of one-

half of a wavelength.

To the wave these strips look like parallel waveguides. The velocity of phase propagation of a

wave is greater in a waveguide than in air. Thus, since the lens is concave, the outer portions of

the transmitted spherical waves are accelerated for a longer interval of time than the inner

portion



4.Describe about Horn antenna

The shape of the horn, along with the dimensions of the length and mouth, largely determines

the beam‘s shape. The ratio of the horn‘s length to mouth opening size determines the

beamwidth and thus the directivity. In general, the larger the opening of the horn, the more

directive is the resulting field pattern.

FEEDHORNS.—A waveguide horn may be used to feed into a parabolic dish. The directivity of

this horn, or feedhorn, is then added to that of the parabolic dish. The resulting pattern (fig. 1-9,

view A) is a very narrow and concentrated beam. Such an arrangement is ideally suited for fire

control use.

In most radars, the feedhorn is covered with a window of polystyrene fiberglass to prevent

moisture and dirt from entering the open end of the waveguide. One problem associated with

feedhorns is the shadow introduced by the feedhorn if it is in the path of the beam. (The shadow

is a dead spot directly in front of the feedhorn.) To solve this problem the feedhorn can be offset

from center .This takes it out of the path of the RF beam, thus eliminating the shadow



5. What is meant by zoning? What are their advantages?

Lens, antennas are suitable for frequencies above 3000 MHz .If the frequency is less than

3000 MHz, lens antennas have more thickness.

The thickness of lens antennas can be reduced with the help of zoning. Thickness (t) is given

by,

Where,

t = Thickness

λ= Free space wavelength

μ = Refractive index =(c/v)

Zoning is classified into two types

(i) Curved surface zoning

(ii) Plane surface zoning.



(a) Curved Surface Zoning

1. In curved, surface zoning stepping or zoning is done to the curved surface of lens antenna.

2. Thickness of curved surface zoned lens is t =

3. Curved surface zoned lens is mechanically stronger than the plane surface zoned lens,

4. Curved surface zoning lens antennas have less weight and less power dissipation

(b) Plane Surface Zoning



1. In plane surface zoning stepping or zoning is done to the plane surface.

2 .Thickness of plane surface zoned lens is,

3. Plane surface zoned lens is less strong

4. Here the power dissipation is more.

6. Explain the basic principal of operation of lens antenna and also distinguish between natural dielectric and artificial dielectric lenses.

Lens antenna work on the principle of refraction. Lens antennas are made of a dielectric

material. Figure 2.1(a) illustrates the principle of operation of such an antenna. A point

source of radiation is placed at the focus of the km. The rays arriving at the lens closer

to the edges of the lens encounter a larger curvature as compared to those arming at

the center portion of the lens. The rays closer to the edges are refracted more than the

rays closer to the censor



Similarly, on reception the rays arriving parallel to the lens axis are focused on to the focal

point where the feed antenna is placed. Figure 2.1(b) shows that spherical waves emitted by the

point source are transformed in to plane waves during transmission. The reason for this

is that those portions of the wave front closer to the center are slowed down relatively

more than those portions that are closer to the edges, with result that outgoing waves

are planar. The same way plane waves incident on the lens antenna during reception

emerge as spherical waves travelling towards the feed. The precision with which these

transformations take place depends upon the thickness of the lens in terms of operating

wavelength. This makes the lens antennas less attractive at lower microwave

frequencies.

1. Artificial dielectric lenses have less weight compared to natural dielectric lenses.

2. Artificial dielectric lenses are made up of discrete metal particles whereas natural dielectric

lenses consist of molecular particles.

3. Natural dielectric lenses, does not have any resonant effect

Characteristics

1. Both type of lens can be used to speed up (or) delay the travelling wave front.

2. Artificial and natural dielectric lenses are not much dependent on the wavelength.

3. The variation in thickness front ideal contour and variations in refractive index causes

change in path length.

4. Lenses may be turned frequency sensitive with the help of zoning.

5. The thickness of lens antenna depends upon the refractive index (u). Thickness can be

increased by reducing the refractive index.

6. The design conditions of artificial and natural dielectric lens antennas are same for same

refractive index.

Merits and Demerits

1. Artificial dielectric lenses have less weight compared to the natural dielectric lenses.

2. Disadvantage of artificial dielectric lens is that they may have resonance effects.

3. Power dissipation is more in natural dielectric lenses.



7. Distinguish between sartorial, pyramidal and conical horns .Explain their utility?

Horn Antenna:

A Horn Antenna is similar to the opened out waveguide. It is excited at one end and kept opened

at the other to get the energy radiated out of it. The radiation is more from a waveguide

compared to two wire transmission lines. The amount of energy radiated from (out of) the

waveguide is very less compared to the reflected energy due to impedance mismatch. In order to

overcome the non-directive radiation pattern and poor radiation, we use horn antenna. The horn

antenna is similar to the opened waveguide only difference is abrupt discontinuity is replaced by

a gradual transformation.

Horn antennas are classified as,

1. Sectoral horn antenna

2. Pyramidal horn antenna

3. Conical horn antenna.

Depending upon the flaring, all the above horn antennas are classified.

Sectoral Horn Antenna

When the flaring is done at only one particular direction, it is known as sectoral horn

antenna, depending upon the direction of flaring, sectoral horn antenna is classified as,

(i) H-plane sectoral horn antenna

(ii) E-plane sectoral horn antenna.



If the flaring is done to the walls of circular waveguide it is known as Conical Horn as shown

below. The main function of electromagnetic horn antenna is the impedance matching and to

produce uniform phase front with a larger aperture to provide greater directivity. The general

equation for flare angle (2θ) in horn antenna is given as,



Here,

h = Height of horn antenna

L=Axial length

δ = Permissible phase angle variations expressed as a fraction of 360° and 1

θ = (1/2) of flare angle.

The above equations are design conditions of horn antenna. If the value of flare angle (26) is

very large, the wave front on the mouth of the horn antenna will be curved rather than plane.

General expressions for Half Power Beam Width (HPBW) of optimum flare horn in E and H

directions, is given approximately as,

8. Derive the expression for Frii's transmission formula

Frii's transmission formula is used to evaluate the power received by one antenna under idealized condition given another antenna some distance away transmitting a known amount of power,

Derivation

Consider two antennas in free space separated by distance R.



Assume that PT watts of total power are delivered to transmit antenna. For the moment,

assume

that the transmit antenna is Omni directional, lossless, and the receive antenna is in the far field

of the transmit antenna. Then the power 'P' of the plane wave incident on the receive antenna a

distance 'R" from the transmit antenna is given by,

If the transmitting antenna has a gain 'Gr' in the direction of the receiving antenna, then the

above power equation becomes,

The gain term factors in the directionality and losses a real antenna. Assume now that the

receive

antenna an effective aperture given by Ae. Then the power received by this antenna PR is given

by,

Since the effective aperture for any antenna can also be expressed as,

The resulting received power in expression (3) can be written as,

This is known as frii’s transmission formula. It relates the free space path loss, antenna gains

and wavelength to the received and transmits powers Measurement of Antenna Gain Using

above Formula





9. Write a short note on “Antenna Pattern Measurements”?

The antenna pattern is also termed as radiation pattern of an antenna. The radiation pattern is

nothing but the plot of the intensity of radiation taken at different points that are at equal

distance from the antenna. It is also defined as plot of power density with respect to the

direction. The radiation pattern of an antenna is a 3-D figure. Hence, it needs intensity of

radiation measurement over all spatial angles.

Let us assume a 3-coordinate Cartesian system in which the antenna, whose pattern is to be

measured, is placed at the origin

The plane XY is horizontal plane. For horizontal plane antenna, the two patterns exhibited are,

(a) The ‘θ’ component of E-field (horizontal) is measured as a function of (j) in XY plane (0 =

90°). This is indicated as E Φ (θ = 90°, (j>) and called as E-plane pattern.

(b)The Φ component of E-field is measured as a function of c in XZ plane (Φ = 0°). This is

represented as E Φ = (θ, Φ = 0°) and called as H-plane pattern. These E-plane and H-plane

patterns are mutually perpendicular to the major lobe.

The plane XZ is called vertical plane and the two patterns to be measured in this plane are,

(a) The ' θ ' component of it-field is measured as a function of Φ in XY plane (θ = 90°). This is

represented by Eθ (θ=90°, Φ) and called as H-plane pattern.

(b) The θ component of E-field is measured as a function of θ in XZ plane ((j) = 0°). This is

represented as Eθ(θ, (j) = 90°) and called as E-plane pattern.

For circularly or elliptically polarized antennas, these four patterns should be measured.

The two types of techniques for measuring antenna pattern in any one of the plane are,



1. The primary antenna is fixed, and the secondary antenna is equipped for free rotation around

the primary antenna in circular fashion. The field strength and direction of secondary antenna

with respect to primary is noted at different points on the circular path of secondary antenna.

Then, the required antenna pattern plot is made.

2. In this procedure, the primary antenna is rotated along vertical axis with respect to secondary

antenna. The field strength at primary receiving antenna with respect to transmitting secondary

antenna is recorded at different point on the vertical rotation path of primary antenna. Then, with

recorded values, plot is made.

10. Describe the method of measuring the gain and radiation pattern of an antenna ?

Absolute Method of Gain Measurement:

To measure the absolute gain of an antenna another identical antenna is used. They are arranged

at a distance V from each other as shown in the figure. The directions of both the antennas are

adjusted for maximum signal.

Then the input to the transmitting antenna is adjusted to a specified level and the corresponding

receiver reading is recorded, i.e., the attenuator dial setting and power bridge readings are

recorded as Wr and Pt1 respectively. Then the transmitter is connected to the receiver directly

(through pads).

The attenuator dial is adjusted until the receiver shows the same previous level. Now the

attenuator dial setting and the power bridge readings are recorded as Wr and Pt2 Since the two

antennas are identical pt1 = pt2 and the gain 'G ' is calculated as, Measurement of Field Pattern

The energy radiated by an antenna is not same in all directions it is more in one direction and

less in another direction. The energy radiated by an antenna is measured in terms of field

strength. Consider the general arrangement shown below. Here primary antenna (transmitting

antenna) and receiving antenna are separated by a distance of



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MAHALAKSHMI ENGINEERING COLLEGE-TRICHYmahalakshmiengineeringcollege.com/pdf/ece/VIsem/EC2353/UNIT 4.pdf · Define lens antenna? An antenna, ... dielectric lens or H plane metal plate