LATHA MATHAVAN ENGINEERING COLLEGE … -VI SEM.pdfLATHA MATHAVAN ENGINEERING COLLEGE Kidaripatti, Madurai – 625301 Subject: EC6602 Antennas & Wave Propagation Year & Semester : III

LATHA MATHAVAN ENGINEERING COLLEGE

Kidaripatti, Madurai – 625301

Subject: EC6602 Antennas & Wave Propagation Year & Semester : III year ECE / VI SEM

Faculty: Mr.M.ANANDAN, ASP/ECE Question Bank & University Questions with Answer

UNIT 1 Question Bank PART – A

1. What is the significance of gain of an Antenna? (Nov 2015)

2. Define the brightness temperature of the antenna? (Nov 2015)

3. Write the importance (significance) of radiation resistance of an Antenna? (May 2015, 2011)

4. What is the significance of aperture of the antenna? (May 2015)

5. Compare short dipole from half wave dipole? (May2014)

6. Draw E and H plane radiation pattern of a Dipole? (May2014)

7. Define Isotropic radiator (Nov 2014)

8. Distinguish between power gain and directive gain. (Nov2014)

9. Compare the Short Dipole and half Wave Dipole?(May 2013)

10. Calculate the Rr of /8 wire dipole in free space?(May2012)

11. Define Directivity and effective aperture of an Antenna? (May2012)

12. Define Half Power Beam Width? (May 2012)

13. If the Radiation Resistance of an Antenna is 65 Ω & loss resistance is 10Ω, find its efficiency? (Nov2012)

14. What are the and patterns in Antenna radiation pattern? (Nov/Dec 2012)

15. What are the dBi and dBd? Write their significances? (Nov 2012)

16. What is Hertizian Dipole?(Nov 2011)

PART – B

1. Explain the following parameters of an Antenna: (i) Radiation pattern(ii) Gain (iii) Polarization

(iv) Bandwidth (Nov2015, 2011)

2. Examine the effectiveness of the following parameters of an antenna : (i) Beam Solid Angle

(ii) Directivity (iii) Pattern Lobes (iv) Input impedance (Nov 2015)

3. A thin dipole is /15 long. If it has loss resistance of 1.5 Ω ,Calculate: (i) Directivity (ii) Gain

(iii) Effective Aperture (iv) Beam Solid Angle (v) Radiation Resistance. (Nov 2015)

4. Derive the Electric and Magnetic field components of a Hertzian dipole. (May2015)

5. Derive the field quantities and draw radiation pattern for a half wavelength dipole. (May 2011, 2015)

6. (i) Two space crafts are separated by 3 Km. Each has an antenna with directivity D=200 operating at 2 GHz.

If craft A’s receives 20 db power what is the transmitted power by the craft B? (May2015)

(ii) Explain the following terms with respect to Antenna: (1) Polarization (2) Effective Aperture

(3) Directivity (May2015)

7. Derive the expression for power radiated and find the radiation resistance of a half wave dipole?(May2014)

8. Solve the Wave equation for uniform plane waves in an infinitely extending conducting medium (May2012)

9. Explain the following parameters of an Antenna: (a) Beam Solid Angle(3) (b) Radiation pattern(3)

(c) Gain (3) (d) Polarization(4) (e) Bandwidth(3) (Nov2011)

10. What are Hertizian dipoles? Derive the Electric and Magnetic field quantities of infinitesimal dipole

and radiation pattern? (16 Mark) (Nov2011)

11. Explain the principle operation of Yagi Uda array with neat schematic diagram? ( May ,Nov 2011)

12. Explain the following terms with respect to Antenna: (a) Polarization(4) (b) Effective Aperture (3)

(c) Directivity (3) (d) Antenna Temperature (3) (e) Radiation Pattern (3) (May 2011)

13. (i) Define and explain the polarization and its significance in Antenna analysis? (8 mark ) (Nov2011)

(ii) State and prove Lorentz Reciprocity Theorem for Antennas? (8) (Nov2011)





UNIT II PART – A

1. The impedance of an infinitesimally thin /2 antenna (L= 0.5 and L/D =) is 73+j42.5 Ω.

Calculate the terminal impedance of an infinitesimally thin/2 slot antenna. (Nov 2015)

2. Write any four salient features of microstrip antenna. (Nov 2015)

3. State Huygen’s Principle? (Nov2013,May 2015)

4. What are the applications of microstrip antenna? (May2015)

5. What are the features of pyramidal Horn Antenna? (May 2011,2015)

6. State field equivalence principle? (May2014)

7. What is called method of imaging? (May2014)

8. State Babinet’s principle and how it gives rise to the concept of complementary Antenna? (May2013)

9. The aperture dimensions of a pyramidal horn are 12x6 cm and operating at a frequency of 10 GHz.

Find the beam width and Directivity? (May2013)

10. What are the merits and application of offset feed reflector Antenna? (Nov2013)

11. What are the limitations of lens Antenna? (Nov2011)

12. State Babinet’s Principle? (Nov2011)

PART – B

1. Discuss the construction of Rectangular Horn antenna and draw the measured E and H plane field

Patterns of rectangular horns as a measured function of flare angle and horn length.(Nov2015)

2. A square corner reflector has a driven /2 element /2 from the corner .(i) Calculate and plot the (Nov2015)

Far field pattern in both principal planes. (ii) What are the HPBWs in the two principal planes?

(iii) What is the terminal impedance of the driven element? (iv) Calculate the directivity from

Impedances of driven and image dipoles. Assume perfectly conducting sheet reflectors of infinite extent.

3. Explain the Radiation Mechanism of Horn Antenna with diagram. Draw the different types of

Horn structures.(May2015)

4. Explain the principle of Reflector antenna and various types of feed used with neat diagram. (May 2015)

5. With neat diagram, explain the principle of parabolic reflector antenna and various types of feed used? (16)

6. (i) Explain the principle of rectangular horn Antenna with a neat sketch?(8) (May 2014)

(ii) Explain the salient features of Flat and corner reflector Antenna? (8) (May 2014)

7. Write short notes on: (i) Slot Antenna (ii) Lens Antenna (May 2013)

8. (i) Compare flat reflector and corner reflector Antennas? (2) May2013)

(ii) Explain how a paraboloidal Antenna gives a highly directional pattern? (6)(May2013)

(iii) Explain in detail about the feeding structure of parabolic reflector Antenna? (8) (May2013)

9. (i) Explain Image theory and its application in detail ?(8) (Nov2013)

(ii) Explain the construction and principle of pyramidal horn antenna. A pyramidal horn antenna having

Aperture dimensions of a = 5.2 cm and b = 3.8 cm is used at frequency of 10GHz.Calculate its

Gain and half power beam widths.(8) (Nov2013)

10. (i) Discuss the various feed techniques for Rectangular patch antenna with neat diagrams.(8) (Nov2013)

(ii) Find the diameter of a dish antenna that will form a beam having 0.5 deg, half power beam width

(HPBW) at a frequency of 8.2 GHz.. Assuming an efficiency constant of 0.6, calculate the antenna

gain and effective aperture.(8) (Nov2013)

11. Explain the radiation mechanism of slot Antenna with diagram? (16) (May 2011)

12. With necessary illustrations explain the radiation characteristics of Microstrip antenna an mention

its possible application? (Nov2011)





UNIT III PART – A

1. Define pattern Multiplication.(Nov,May 2015)

2. What are the advantages of antenna arrays?(May2014)

3. What is Binomial array ? (Nov 2014)

4. A uniform linear array contains 50 isotropic radiance with an inter element spacing of N2. Find the directivity

of broadside forms of array? (May2013)

5. Define Pattern Multiplication.(May 2011)

6. What is meant by uniform linear array?

7. What are the types of array?

8. What is Broad side array?

9. Define End fire array?

10. What is collinear array?

PART – B

1. Two identical radiators are spaced d = 3λ /4 meters apart and fed with currents of equal magnitude but

with 180o phase difference. Evaluate the resultant radiation , the direction of maximum and minimum

radiation?(16) (May 2015)

2. (i) Derive the expression for the field produced by a linear array and deduce it for an end fire array.(May2014)

(ii) Compare End fire array and Broadside array. (6) (May2014)

3. (i) Describe the method of Pattern multiplication. (8) (Nov2014)

(ii) A linear broad side array consists of 4 equal isotropic in phase point sources with λ /3 spacing. Find the

Directivity and beam width. (Nov2014)

4. (i) What is binomial array? (2) (May2013)

(ii) Draw the pattern of 10 element binomial array with spacing between the elements 3λ /4 and λ /2.(14)

5. Derive the array factor of a uniform linear array. Explain the significance of array factor? (Nov2013)

6. Derive the expression for electric field of a broadside array of n sources and find the maximum direction,

minimum direction & half power point direction?(May2012)

7. Derive the expression for electric field of a end fire of n sources and also find the maximum direction,

minimum direction & half power point direction? (May2012)

8. For a two element linear antenna array separated by d=3λ /4 , derive the field quantities and draw its

Radiation pattern for the phase difference of 45o .(Nov2012)

9. Two identical vertical radiators are spaced d = λ /4 meters apart and fed with currents of equal

magnitude but with a phase difference ‘ ’. Evaluate the resultant radiation for =0o

and thereby identify

the direction of maximum and minimum radiation?(16) (May 2011)

10. Write short notes on. i. Binomial arrays (8) ii. Phased arrays (8)

UNIT IV PART – A

1. Calculate the radio horizon of a TV antenna placed at a height of 166 m. If the receiver is at a distance of

66 Km, what should be the height of the receiving antenna? (Nov 2015)

2. Differentiate near field and far field (May 2015)

3. What are the applications of loop antenna? (May 2015)

4. Why is a frequency independent antenna are called so? (May2014)

5. Mention two applications of turnstile antenna? (May2014)





6. What is the difference between Yagi Uda Antenna and log periodic dipole? (Nov2013)

7. Mention the requirements of an anechoic chamber? (Nov2013)

8. Mention any two applications of helical antenna?(Nov2012)

9. What are features of anechoic chamber? (Nov2012)

10. What are near and far field measurements?(May 2011)

11. Why is log periodic antenna called so?(Nov 2011)

PART – B

1. (i) Elaborate on Log Periodic Antenna with a neat sketch.(10)

(ii) Design a Log Periodic dipole array with 7 dBi gain and a 4 to 1 bandwidth. Specify apex angle α ,

Scale constant k and the number of elements.(6) (Nov2015)

2. With a neat sketch design a quad helix earth station antenna. Calculate the directivity

and effective aperture. (Nov2015)

3. Explain the principle of operation of Log periodic antenna with neat schematic diagram. (May2015)

4. What is the importance of Helical antenna? Explain the construction and operation of

Helical antenna with neat sketch. (May2015)

5. Describe the construction and operation of a normal and axial mode helical antenna?(Nov 2014)

6. (i) Explain the construction and characteristic features of frequency independent antenna .(Nov 2014)

(ii) Illustrate the reciprocity in Antenna measurement. (Nov 2014)

7. Explain in detail about (i) Directivity Measurement (ii) Gain measurement (16) ( Nov2011,May2014)

8. With neat block diagram explain how radiation pattern and gain of an antenna can be measured? (May2013)

9. With neat block diagram explain how radiation pattern and gain of an antenna can be measured? (May2013)

10. Differentiate V rhombic antenna. Explain their construction and principles in detail? (Nov2013)

11. Discuss briefly on construction and working principle of turnstile antenna? (Nov 2011)

12. With suitable geometry describe the design of a log periodic dipole array? (Nov2011)

UNIT V PART – A

1. Find the maximum distance that can be covered by a space wave, when the

Antenna heights ate 60m and 120m. (Nov2015,2013)

2. A HF radio link is established for a range of 2000 Km. If the reflection region of the ionosphere is

At a height of 200Km and has a fc of 6MHz, calculate MUF. (Nov2015)

3. Define optimum working frequency. (May 2015)

4. What is meant by Faraday rotation? (Nov2011,2015)

5. Differentiate Virtual height from actual height?(May2014)

6. What are the effects of ground on low frequency wave transmission? (May2014)

7. What are the factors that affect radio wave propagation? (Nov2014)

8. Define skip distance. (Nov2014)

9. What is fading? And how it is compensated? (May2013)

10. What is free space loss factor?(Nov2013)

11. What is Gyro frequency? (Nov2013)

12. Define maximum usable frequency?(May 2011)

13. Define Critical frequency?(Nov2011)





PART – B

1. Evaluate the the value of surface impedance if σ = 5x10-5

,r = 15 , = o at

(i) 5 kHz (5) (ii) 50 kHz (5) (iii) 500 kHz (6) (Nov2015)

2. Derive the expressions for phase velocity and group velocity of sky waves.(16) (Nov2015)

3. (i)Draw the structure of ionosphere and explain the mechanism of ionospheric propagation (10) ( May 2015)

(ii) Explain the effects of magnetic fields on EM wave propagation? (6) (May 2015)

4. (i) Explain the terms : (1) MUF (2) Virtual Height (3) Duct propagation

(4) Skip distance (5) Fading (10) (May 2015)

(ii) Explain the mechanism of tropospheric propagation (6) (May 2015)

5. (i) Describe the significant features of ground wave propagation .(8) (Nov2014)

(ii)Obtain an expression for the refractive index of an ionosphere layer.(8)(Nov2014)

6. (i) Explain in detail the mechanism of space wave propagation over ideal flat earth

with a neat sketch. (8) (Nov2014)

(ii)Discuss the factors that give rise to fading in ionosheric radio wave propagation. (8) (Nov2014)

7. (i) What are the effects of earth magnetic field? (6) ( Nov2013)

(ii) Draw a 2 ray model of Sky wave propagation and explain it in detail? (10) (Nov2013)

8. Discuss the factors that are involved in the propagation of radio waves and describe the space wave

propagation and explain the importance of line of sight propagation? (16) (Nov2013)

9. Discuss the effects of earth’s magnetic field on ionosphere radio wave propagation and drive the

Characteristic equations of ionosphere? (May2011)

10. Explain the terms : (1) Optimum working frequency (2) Duct propagation (3) Virtual Height

(4) Whistlers (5) Critical frequency (16)(May 2011)

11. (i) Explain the mechanism of ionospheric propagation with neat diagram? (8) (Nov2011)

(ii) Explain important features of ground wave propagation?( 8) (Nov2011)

12. Explain the terms : (1) Optimum working frequency (2) Skip distance (3) Virtual Height

(4) MUF (5) Multihop propagation (16) (Nov2011)

Unit – 1 University Question with Anwers Part –A

1. Write the importance (significance) of radiation resistance of an antenna? May 2014, 2015

It is defined as the fictitious resistance which when inserted in series with the antenna will consume

the same amount of power as it is actually radiated. The antenna appears to the transmission line as a

resistive component and this is known as the radiation resistance.Radiation resistance varies with

Antenna length. Resistance increases as the increases

2. What is meant by radiation pattern?

Radiation pattern is the relative distribution of radiated power as a function of distance in space .It is a graph

which shows the variation in actual field strength of the EM wave at all points which are at equal distance

from the antenna. The energy radiated in a particular direction by an antenna is measured in terms of field

strength.(E Volts/m)

3. Compare Short Dipole from half wave dipole (May2014)

A half wave antenna is the fundamental radio antenna of metal rod or tubing or thin wire which has a

Physical length of half wavelength in free space at the frequency of operation .The Short dipole has two

Equal charges of opposite sign oscillating up and down in a harmonic motion.





4.Distinguish between Power gain and Directive gain. (Nov2014)

S.

No

Power Gain (Gp) Directive Gain (Gd)

1. It is the ratio of radiation intensity in given

Direction to the total input power.

It is the ratio of the radiation intensity in

that

Direction to the average radiated power.

2. It depends on the power input to the antenna,

power consumed in terminating resistance

It depends on the distribution of radiated

power in space

3. Gp(dB) = 10 log 10 (Gp) Gd(dB) = 10 log 10 (Gd)

4. The power fed to the antenna is considered

For calculation of Power gain.

The radiated power is considered for

calculation of directivity

5. Define an isotropic radiator? (Nov2014)

An isotropic radiator is a fictitious radiator and is defined as a radiator which radiates fields uniformly in all

directions. It is also called as isotropic source or omnidirectional radiator or simply unipole.

6. What is effective area of a half wave dipole operating at 1 GHz? (May2013)

Given, f = 1 GHz , = c/f = 3x108 / 1x10

9 = 0.3 m

Wkt , For a half wave dipole ,Ae = 1.5 2 / 4 = 1.5(0.3)

2 / 4x 3.141

Ae = 0.135/ 12.57 = 0.1074 m2

The effective area of a half wave dipole operating at 1 GHz is , Ae= 0.1074 m2

7. What is elementary dipole and how it is differ from the infinitesimal dipole? (May 2013)

When the length of the short dipole is vanishing small, then such a dipole is called a infinitesimal dipole. If dl be the

infinitesimally small length and I be the current, then Idl is called as the current element.

8. Define Beam solid angle or beam area? (May 2012)

It is the solid angle through which all the power is radiated to free space.

The beam solid angle is defined as the integral of normalized power over a sphere.

ΩA = ∫ ∫ Pn (θ, φ).dΩ steradians, Where, Pn (θ, φ) = Normalized power dΩ = Solid angle

9.How a dipole antenna can be formed from a 2 wire open circuited transmission line? (Nov2013)

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 magnetic field intensity.

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, when the two wires open circuited transmission line is flared,

it will acts as a dipole antenna

10. Define Directivity and Effective Aperture of an Antenna? (May2012)

Directivity: The directivity of an antenna is equal to the ratio of the maximum power density P (,)max

to its average value over a sphere as observed in the far field of an antenna. D = P (,)max / P(,)av.

Directivity from Pattern. D = 4/ A. Directivity from beam area (A).

Effective aperture: Effective aperture is used to describe the power capturing characteristics of the

Antenna when a wave impinges on it.





Part - B

1. Explain the following, i. Beam area ii. Radiation intensity

iii. Beam efficiency iv. Directivity.

i. Beam Area

In polar two-dimensional coordinates an incremental area dA on the surface of

sphere is the product of the length r dθ in the θ direction and r sin θ dΦ in the Φ

direction as shown in figure 1.10.1. Thus, dA = (rdθ) (r sinθ dΦ) = r2 dΩ

Where,dΩ = solid angle expressed in steradians.

The area of the strip of width r dθ extending around the sphere at a constant angle θ is given by

(2πr sin θ) (r dθ). Integrating this for θ values from 0 to π yields the area of the sphere.

Where, 4π = Solid angle subtended by a sphere

The beam area or beam solid angle or ΩA of an antenna is given by the integral of

the normalized power pattern over a sphere

Where, dΩ = sinθ dθ dΦ

ii. Radiation Intensity

The power radiated from an antenna per unit solid angle is called the radiation intensity

U (watts per steradian or per square degree). The normalized power pattern of the previous.

Section can also be expressed in terms of this parameter as the ratio of the radiation intensity

U (θ , Φ ), as a function of angle, to its maximum value. Thus,

Pn(θ,Φ) = U(θ,Φ)/U(θ,Φ)max = S(θ,Φ)/S(θ,Φ)max

Whereas the Poynting vector S depends on the distance from the antenna, the radiation intensity

U is independent of the distance, assuming in both cases that we are in the far field of the antenna.





iii. Beam Efficiency

The beam area QA (or beam solid angle) consists of the main beam area (or solid angle) ΩM

plus the minor-lobe area (or solid angle) Ω m . Thus, ΩA = ΩM + Ω m

The ratio of the main beam area to the (total) beam area is called the (main) beam efficiency εM. Thus,Beam

Efficiency = εM = ΩM/ ΩA (dimensionless)

The ratio of the minor-lobe area ( Ω m ) to the (total) beam area is called the stray factor.

Thus,εm = Ω m / ΩA = stray factor.

iv. Directivity

It is defined as the ratio of maximum radiation intensity of subject or test antenna to the

radiation intensity of an isotropic antenna. (or)

Directivity is defined as the ratio of maximum radiation intensity to the average radiation intensity.

Directivity (D) in terms of total power radiated is,

D = 4π x Maximum radiation intensity/ Total power radiated

2. What is polarization? Explain the types of polarizations used in antenna?

Electromagnetic polarization refers to the orientation of the electric field vector with respect to earth's

surface. In this concept, the electric field whose orientation is varied in regular intervals to retain its strength

along all directions. A polarization vector is a vector whose direction is along the path of the polarization

(i.e., electric field orientation direction).There are three kinds of polarization namely, 1. Linear-polarization

2. Circular polarization 3. Elliptical polarization.

1. Linear Polarization

It is also known as plane polarization. In this electric field is confined to only one particular direction. There

are two forms of linear polarization. They are, .

(i) Horizontal polarization (ii) Vertical polarization.

In horizontal polarization, the electric field propagates parallel to the earth's surface, whereas in vertical

polarization, the electric field propagates perpendicular to the earth's surface.

2. Circular Polarization

The polarization in which polarization vector rotates 360° over one period of the wave is referred

as circular polarization. In the circular polarization, the strength of the field vector has a constant Value in

all directions of polarization.

3. Elliptical Polarization

The elliptical polarization in which also, the polarization vector rotates 360° over one period of

the wave. In elliptical polarization, the strength of the field varies with the changes in polarization.

This polarization is further classified into left handed and right handed elliptical polarization based on the

rotating direction of the wave. If the vector rotates in clockwise direction, it is referred to as right handed

and if the vector rotates in anticlockwise direction, it is referred to as left handed.

3. State reciprocity theorem for antennas. Prove that he self –impedance of an Antenna in

transmitting and receiving antenna are same?

Reciprocity Theorem

Reciprocity theorem states that when current I is applied at the terminals of antenna 1, an e.m.f E21 induces

at terminals of antenna 2 and when current I applied at the terminals of antenna 2, an e.m.f E12 induces at

terminals of antenna 1, then E12 = E21 provided I1 = I2.

Equality of Antenna Impedance

Consider, the two antennas separated with wide separation as shown below figure 3.2.





The current distribution is same in case of transmitting and receiving antenna. Let antenna no. 1 is the

transmitting antenna and antenna no.2 is the receiving antenna. The self impedance (Z11) of transmitting

antenna is given by, E1 = Z11I1 + Z12I2

Here, Z11 = Self impedance of antenna1, Z12 = Mutual impedance between the two antennas.

Since the separation is more, mutual impedance (Z12) is neglected, Z12 = 0

E1 = Z11I1 + Z12I2 , E1 = Z11I1 + 0 (I2) , Z11 = E1/I1

The receiving antenna under open circuit and short circuit conditions are shown below.

Receiving Antenna under Open Circuit Condition

E1 = Z11I1 + Z12I2

When the receiving antenna is open circuited, current I1 is zero

E1 = Z11(0) + Z12I2 ,EOC = Z12I2

Receiving Antenna under Short Circuit Condition

When the receiving antenna is short circuited, the voltage (E) will be zero.

E1 = Z11I1 + Z12I2 , 0 = Z11ISC + Z12I2 , ISC = - Z12 I2 / Z11

From above, the term Z12I2 acts as a voltage source and Z11 as the self-impedance.

Hence, impedance of the antenna is same whether it is used for transmission or reception

Unit – 2 Part –A

1. State Huygen’s Principle? (Nov2009,2013,May 2015)

Huygens’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 are the applications of microstrip antenna? (May2015)

The missile applications of microstrip antenna are such as missile guiding, fuzing, telemetry, satellite

Communication, radars, GPS etc. The space applications are International Sun Earth Explorer (ISEE) ,

Earth Limb Measurement satellite (ELMS) ,Solar Mesospheric Explorer (SME) ,SEASAT, GEOSTAR

and Mars Path finder. Also, the commercial applications are Mobile Communications, Direct Broadcast

Satellite (DBS) services, Global Positioning System (GPS), Aeronautical and Marine Radars and Earth

Remote Sensing.

3. What are the features of pyramidal Horn Antenna? (May 2011, 2015)

An electromagnetic horn basically a flared out waveguide to secure grater directivity. The rectangular

Waveguide flared in one plane only is called sectoral horn, while the rectangular waveguide flared in two

planes is called Pyramidal horn.

4. State field equivalence principle? (May 2014)

The field equivalence principle states that the fields outside surface in volume V2 can be

Calculated using either original sources present inside surface in volume V1 or equivalent surface

Current sources on surface S.

5. What is called method of Imaging? (May 2014)

The image theory states that a given charge configuration above an infinite grounded, perfect

Conducting plane may be replaced by the charge configuration itself, its image and an equipotential

Surface in place of the conducting plane.





6. What are the merits and application of offset feed reflector Antenna? (Nov2013)

To overcome the aperture blocking effect due to the dependence of the secondary reflector

Dimensions on the distance between feed and sub reflector, the offset feed reflector antenna is used.

The offset feed reflector antenna uses all the collimated rays and eliminates blocked rays.

7. State Babinet’s principle and how it gives rise to the concept of complementary antenna? (May2013)

The Babinet’s Principle is stated that the field at any point behind a plane having a screen, if added

to the field at the same point when the complementary field is substituted, is the same as the field

at the point when there is no screen. 8. The aperture dimensions of a pyramidal horn are 12 x 6 cm and operating at a frequency of 10 GHZ.

Find the beam width and directivity. (May2013)

Given, f = 10 GHz , = c/f = 3x108 / 10x10

9 = 0.03 m

Wkt, The Directivity D = (4π/λ2) A emax ,Where A emax = Maximum effective aperture ,λ = Wavelength

9. What is parabolic reflector?(May2012)

It is a parabola shaped reflective devices used to distribute energy entering the reflector at

a particular angle.

10.What are applications loop antennas? ( Nov2012)

i) Radio receivers ii) Aircraft receivers iii) Direction Finding iv) VHF transmitter

Part –B

1. Write short notes on Corner Reflector Antenna?

Comer reflector antenna is an arrangement with a corner reflector i.e., flat reflecting sheets meeting at an

angle or corner, and a driven antenna, generally a half wave dipole as shown in Fig.5.6.1.If corner angle

β = 90° then the two flat sheets meeting at right angles form a square corner reflector shown in Fig.5.6.2.

When the driven antenna is used in conjunction with the corner reflector, the arrangement is an

effective or active directional antenna for a wider range or corner, i.e. 0 < β < Π. The square

corner reflector without the driven antenna is an effective passive reflector over a wide range of

angles of incidence (0 < i < ± Π/4) Applications

Corner reflectors are extensively used in applications like (a) Television (b) Point to point communication

(c) Radio Astronomy. The advantage of using corner reflector for these applications lies in the increased

power gain of 10 to 13 db (10 to 20 times that of an isolated half wave dipole antenna with reasonable

radiation resistance).





2. Explain the cassegrain mechanism in transmission mode. List out the advantages and disadvantages

of cassegrain feed?

Cassegrain principle is an optical technique widely used in telescope construction. The same principle is

used to build cassegrain antenna used in microwave region.

Cassegrain feed is a dual reflector system with the larger primary reflector having a parabolic surface and a

secondary sub-reflector with a hyperbolic contour in order to achieve uniform illumination of the primary

reflector. This arrangement is shown in the above figure 5.10.1.

One of the two foci of the hyperbola is the real focal point of the system. The feeder is located at this point

Which corresponds, to the vertex of the parabola. The other focus of the hyperbola is a virtual focal point

and is located at the locus of the parabolic surface. A parallel beam of rays are reflected by the parabola as

a convergent beam width which is re-reflected by the hyperbolic sub-reflector to make the beam convergent

at the position of the feed.

Advantages of Cassegrain Feed Arrangement

1. The paraboloid surface is more uniformly illuminated. 2. There is less spill over reducing interface.

3. Shorter wave guide feed of the horn results in lower feed losses. 4. There are lesser polarization errors.

5. There is a greater reduction in side-lobe power. 6. The noise level is lower due to small cross-section.

Disadvantages

1. Design of the entire antenna system is with critical mechanical alignment and is complex.

Application

1. Cassegrain feed arrangement is extensively used in Monopulse Radar.

2. Low noise temperature makes this antenna very important for radio astronomy, microwave

communication and satellite tracking.

Unit – III Part –A

1.Define pattern Multiplication.(May 2015)

The total field pattern of an array of non isotropic but similar sources is the product of the individual source pattern

and the array pattern of isotropic point sources each located at the phase center of the individual source having the

same amplitude and phase.

2. What are the advantages of antenna arrays?(May2014)

The advantage of Antenna arrays is as follows: 1.Increases the overall gain. 2. Provide diversity receptions.

3. Cancel out interference from a particular set of directions.4."Steer" the array so that it is more sensitive in

a particular direction.5. Determines the direction of arrival of the incoming signals.6. It maximize the Signal

to Interference plus Noise ratio





3.What is Binomial array? (Nov 2014)

It is an array in which the amplitudes of the antenna elements in the array are arranged according to the

coefficients of the binomial series.

4. Write the principle of pattern multiplication? (May2005 ,2011) The total field pattern of an array of non isotropic but similar sources is the multiplication of the individual

source pattern and the pattern of an array of isotropic point sources each located at the center of individual

sources. Whereas the total phase pattern is the addition of the phase pattern of the individual sources and

that of the array of isotropic point sources.

5. Differentiate broadside array and end fire array? (May2008, 2011)

S.

No

Broadside array End fire array

1

In this number of identical antennas

are set up along a line drawn

perpendicular to their respective axis.

In this number of identical antennas are

spaced equally along a line parallel to

their respective axis.

2

Individual elements are fed with

current of equal amplitude and phase

Individual elements are fed with current

of equal amplitude and out of phase

3 Radiates perpendicular to the axis Radiates parallel or along the axis.

4 Narrow beam width Wide Beamwidth

6. Define array factor? (Dec2007) The normalized value of the total field is given by, E = 1/n [sin (nψ/2)/sin (ψ/2)]

The field as given by the expression E will be referred to as array factor.

7. What is parasitic array?

In this array the elements are fed parasitically to reduce the problem of feed line. The power is given to one

element from that other elements get by electromagnetic coupling. Eg. Yagi uda antenna.

8. What is tapering of arrays?

Tapering of array is a technique used for reduction of unwanted side lobes .The amplitude of

Currents in the linear array source is non-uniform; hence the central source radiates more energy than the

ends. Tapering is done from center to end.

9. What are the advantages of binomial array?

Advantage: No minor lobes

Disadvantages: Increased beam width

Maintaining the large ratio of current amplitude in large arrays is difficult

10. List the arrays used for array tapering

Binomial Array: Tapering follows the coefficient of binomial series

Dolph Tchebycheff Array: Tapering follows the coefficient of Tchebycheff polynomial





Part –B

1. What is the necessity of an array? Explain the three different types of array with a neat sketch.

Antenna Array

This is one of the common methods of combining the radiations from a group of similar antennas

in which the wave-interference phenomenon is involved. The field strength can be increased in preferred

Directions by properly exciting group or array of antennas simultaneously, such as arrangement is known

as antenna array. Array of antenna is an arrangement, of several individual antennas so spaced and

phased that their individual contributions coming in one preferred direction and cancel in all other

Directions, which will be going to increase the directivity of the system.The different types of arrays with

regard to beam pointing direction are as follows,1. Broadside array 2. End fire array 3. Collinear array.

Broadside Array

Broadside array is one of the most commonly used antenna array in practice. The array in which a

number of identical parallel antennas are arranged along a line perpendicular to the line of array axis is

known as broadside array, which is shown in figure (2.1). In this, the individual antennas are equally

spaced along a line and each element is fed with current of equal magnitude, all in the same phase.

The radiation pattern of broadside array is bidirectional, which radiates equally well in either direction of

maximum radiation.

.

End Fire Array

The array in which a number of identical antennas are spaced equally along a line and individual elements

are fed with currents of unequal phases (i.e., with a phase shift of 180°) is known as end fire array .This

array is similar to that of broadside array except that individual elements are fed in with, a phase shift of 180

In this, the direction of radiation is coincides with the direction of the array axis, which is shown in

figure (2.2) The radiation pattern of end fire array is unidirectional. But, the end fire array may be

bidirectional also. One such example is a two element array, fed with equal current, 180° out of phase.





Collinear Array

The array in which antennas are arranged end to end in a single line is known as collinear array.

Figure (2.3), shows the arrangement of collinear array, in which one antenna is stacked over another

antenna. Similar to that of broadside array, the individual elements of the collinear array are fed with equal

in phase currents. A collinear array is a broadside radiator, in which the direction of maximum radiation is

perpendicular to the line of antenna. The collinear array is sometimes called as broadcast or Omni

directional arrays because its radiation pattern has circular symmetry with its main to be everywhere

perpendicular to the principal axis.

2. What is uniform linear array? Discuss the application of linear array? and also Explain

the advantages and disadvantage of linear array?

In general single element antennas having non uniform radiation pattern are used in several broadcast

services. But this type of radiation pattern is not useful in point-to-point communication and services that

require to radiate most of the energy in one particular direction i.e., there are applications where we need

high directive antennas. This type of radiation pattern is achieved by a mechanism called antenna array.

An antenna array consists of identical antenna elements with identical orientation distributed in space. The

individual antennas radiate and their radiation is coherently added in space to form the antenna beam.

In a linear array, the individual antennas of the array are equally spaced along a straight line. This individual

antenna of an array are also known as elements. A linear array is said to be uniform linear array, if each

element in the array is fed with a current of equal magnitude with progressive phase shift (phase shift

between adjacent antenna elements).

Application of Linear Array

1. Adaptive linear arrays are used extensively in wireless communication to reduce interference

Between desired users and interfering signals.

2. Many linear arrays spaced parallel on the common plane create a planar array antenna.

These are used in mobile radar equipment.

3. The linear array is most often used to generate-a fan beam and is useful where broad coverage in

One plane and narrow beam width in the orthogonal plane are desired.

4. Linear arrays can be made extremely compact and .are therefore very attractive for shipboard

Applications. The advantages and disadvantages of linear arrays are as follows.

Advantages

1. Increases the overall gain. 2. Provide diversity receptions.

3. Cancel out interference from a particular set of directions.

4."Steer" the array so that it is more sensitive in a particular direction.

5. Determines the direction of arrival of the incoming signals.

6. It maximize the Signal to Interference plus Noise ratio

Disadvantages

1. Ray deflection only in a single plane possible.

2. Complicated arrangement and more electronically controlled phase shifter needed. ;

3. Field view is restricted. 4. Considerable minor lobes are formed.

5. Large power loss due to current flowing in all elements.

6. Overall efficiency decreases. 7. Costly to implement.





Unit -4 PART A

1. Differentiate near field and far field (May 2015)

The induction field will predominate at points close to the current element ,where the distance from the

center of the dipole to the particular point is less.This field is more effective in the vicinity of the current

element only. It represents the energy stored in the magnetic field surrounding the current element or

conductor. This field is also known as near field.

The radiation field will be produced at a larger distance from the current element, where the distance from

the center of the dipole to the particular point is very large. It is also called as distant field or far field.

2. What are the applications of loop antenna? (May 2015)

AM broadcast receiver, VHF, Direction Finding

3. Why a frequency independent antenna is are called so? (May2014)

A frequency independent antenna is defined as the antenna for which the impedance pattern and directivity

remains constant as function of frequency.

4. Mention two applications of turnstile antenna? (May2014)

Used for Omnidirectional VHF and UHF communication,

5.Mention the requirements of an anechoic chamber? (Nov2012, 2013)

An anechoic chamber is used to simulate a reflectionless free space and can carry out all whether

measurement of any antenna under controlled environment conditions in laboratory.In an anechoic chamber,

a coating of absorbing materials is made over walls, celing, floor completely.To improve isolation of test

area, many times shielding is done which also allows Electro Magnetic Compatibility (EMC) measurements.

The absorbing material is Pyramidal or wedge shaped made up of Carbon loaded Polyurethane foam.

6. What are the three different regions in log periodic antenna and how they are differentiated?

1) In active region-L < (λ/2) 2) Active region -L = (λ/2) 3) In active reflective region L > (λ/2)

7. Mention any two applications of helical antenna?(Nov2012)

i) Used for satellite and space communication ii) Used in radio astronomy

iii) In the ballistic missiles and satellites used as telemetry links.

8.What is LPDA? (May2012) LPDA is log periodic dipole array. It is unidirectional broadband, multi element, narrow beam, frequency

independent antenna that has impedance and radiation characteristics that are regularly repetitive as a

logarithmic function of frequency.

9.What are the applications of rhombic antenna? (Apr2012) Used as a broad band antenna in microwave applications.ii)Used in radio communications.

10.Why is log periodic antenna called so?(Nov 2011)

A Logperiodic antenna consists of dipoles and other basic elements which are arranged in such form that it

Forms constant angle array. The radii of the arm which define location of successive teeth are with constant

Ratio given by τ = Rn+1 / Rn . This is energized at the vertex , the properties exhibited by the structure at

frequency f will be repeated at all frequencies given by τn.f .When these frequencies are plotted on a

logarithmic scale , it is observed that all are spaced equally with period equal to logarithm of τ .Because of

such a unique property , the antenna is named as Log periodic Antenna.





Part – B

1. Write short notes on Long wire or Harmonic antenna?

An antenna will be resonant so long as its length is integral multiple of half wave length. When an antenna is

more than a half-wave long it is called as a long-wire or a harmonic antenna. Thus the long wire antenna is a

single long wire, generally two or more wave length (i.e. 4 λ/2 or more λ/2) long at the operating frequency.

The higher the number of λ/2 the better its directivity. Since the wire is made longer in terms of the number

of half wave lengths

(λ/2), the directional effect changes. The directional characteristics split up into various lobes at different

angles from wire axis as against "doughnut shape" of a single λ/2 antenna. It radiates a horizontally

polarized wave at low angles from about 17° to 24" relative to the earth surface.

In long wire antenna, the currents in adjacent half wave section must be out of phase and hence any feeder

system can be used that does that disturb this condition. This condition can be satisfied if long wire antenna

is fed at either end or at any current loop. A long wire antenna is generally made a half wave length at the

lowest frequency of operation and fed at the end. Long wire antennas are shown in Fig. 4.2.1 in which n is

the number of half wavelength.

The long wire may assume two forms e.g. resonant (unterminated) and non-resonant (terminated at

characteristics impedance). In resonant long wire antenna standing wave exists along its length and the

pattern is bidirectional corresponding to incident waves and reflected waves. However, in case of non-

resonant long wire antenna all the incident waves are absorbed in terminating impedance and there is no

reflected wave. This is why the pattern is only due to incident wave’s i.e. unidirectional only and uniform

current and voltage exist along the axis of the wire.

The directional patterns of resonant and non-resonant types of Antennas are shown in Fig. 4.2.2. The angle

of radiation with reference to wire axis depends on number of wavelength i.e. even or odd. For n = 3 and n =

4 directional pattern is shown in figure.For example maximum radiation from along wire antenna of 8λ long

w.r.t. wire axis is at 17.5° with many small minor lobes.

The physical length of a long wire antenna can be extended from the physical length of λ/2 antenna as

follow from equation λ/2 = (492 x 0.95)/f (MHz) feet if one half wavelength in wavelength. Hence for n half

wavelength long wire antenna. Resonant and non-resonant long wire antennas are used for transmission and

reception i.e. from 500 kHz to 30 MHz. They provide a simple and effective method of obtaining directional

pattern and power gain. These properties are utilized when long wire antennas are used as circuit element in

an array such as V antennas or rhombic antennas. Hence a long wire antenna has practical value because of

its structural simplicity and relatively low cost, irrespective of theoretical complications.

2. What is a V- antenna? Explain its characteristics?

The V antenna is an extension of long wire antennas. Two long wire antennas (called legs) are arranged in

the form of a horizontal V, fed at the apex as shown in Fig. 4.3.1 If the angle between the two sides of the

V, is equal to twice the angle that the cone of maximum radiation of each wire makes with the axis of that

wire, then the two cones will add up in the direction of the line bisecting the apex angle of V, and there

produce a maximum lobe of radiation. The two wires are fed 1800 out of phase with each other.





This provides gain and directivity. The higher the length of legs, the greater the directivity and gain. These

are achieved by cancelling oppositely directed corresponding radiation lobes in each leg and by adding the

similarly directed corresponding lobes in each leg. The resultant is bi-directional patterns which

are sharper than the same length single long wire. The gain achieved with the V an antenna is

nearly twice in comparison to the single long wire antenna, which has a length equal to that of

the legs of the V antenna. For example, nearly 12 db gain is achieved over a λ /2 dipole if the

each leg is 16 x λ/2 i.e. 8 λ long. The apex angle for a particular V antenna structure is also

important. This apex angle varies according to length of the leg. It varies between 36 0 to 72 0for

a V antenna structure of 8 λ to 2 λ long. If the V antenna is to be operated over a wide range, the

apex angle is made the average between the optimum for the highest and lowest frequencies in

terms of the number of λ/2 in each leg. V antenna provides multiband operation so it can conveniently be

fed by tuned feeders. If non resonant lines are to be used, probably a better matching system is to use a λ/4

matching section or stub. The resonant V antenna is perhaps the one of the cheapest forms of receiving or

transmitting antenna for providing a low angle beam for fixed frequency operation in HF band. One of the

serious drawback of V antenna is that it provides strong minor lobes too.

Unit – V PART – A

1. Define Optimum Working Frequency (OWF) (May 2015)

Optimum Working Frequency is the frequency normally used for ionospheric transmission .It is chosen

about 15% less than MUF. It is also defined as frequency lying between 50% to 85% of predicted MUF

Between the transmission and reception points.

2. What is meant by Faraday rotation? (Nov2011,2015)

Due to the earth’s magnetic fields, the ionosphere medium becomes anisotropic and the incident plane wave

entering the ionosphere will split into ordinary and extra ordinary waves/modes. When these modes re-

emerge from the ionosphere they recombine into a single plane wave again. Finally the plane of polarization

will usually have changed, this phenomenon is known as Faradays rotation.

3. Differentiate Virtual height from actual height?(May2014)

Virtual height is defined as the height that would be reached by a short pulse of energy showing the same

delay, as does the actual pulse reflected from the layer but travelling with the velocity of light. Since the

interchange of energy takes place between the wave and the electrons of the ionosphere, the velocity of

propagation is reduced. Hence the virtual height is always less than the actual height of reflection

4. What are the effects of ground on low frequency wave transmission? (May2014)

i. Curvature of earth. ii. Earth’s magnetic field. iii. Frequency of the signal. iv. Plane earth reflection





5. What are the factors that affect radio wave propagation? (Nov2014)

The factors that determine the absorption of energy by electrons from radio waves in Ionosphere

1. Geo pressure. 2. Velocity of the electrons. 3. Number of electrons.

6. What is skip distance? (May2007, Nov2012, 2014) The skip distance is defined as the minimum distance from the transmitter at which a sky wave of given

frequency will be returned to earth by the ionosphere.

7. Define critical frequency. (May 2011)

Critical frequency is the highest frequency, which can be reflected by a particular layer at vertical incidence.

It is the limiting frequency at which a radio wave is reflected by and above which it penetrates through an

ionosphere layer fc = 9√Nm where, fc is the critical frequency, Nm is the Max.electron density.

8. Define Maximum Usable Frequency?(May 2011)

MUF (Maximum Usable Frequency) is the frequency, which makes a given receiving point correspond to a

distance from a transmitter equal to the skip distance for that frequency. The maximum possible frequency

for which reflection takes place for a given distance of propagation is known as MUF.

MUF= fc/ Cosθ , where, θ is the angle of incidence.

9. What is fading? And how it is compensated? (May2013)

Fading is defined as the fluctuation in the received signal strength caused due to variations in height and

density of ionization in different layers. It can be minimized by diversity techniques.

10. What is Gyro frequency? (May2005, 2010, Nov2013)

Frequency whose period is equal to the period of an electron in its orbit under the influence of the

Earth’s magnetic flux density B. The frequency of rotation of an electron or other charged particle

as it spirals in a magnetic field is called as gyro frequency. fg = Be/2πm

Part – B

1.What are the different mechanisms of propagation of electromagnetic waves? Explain?

Electromagnetic waves may travel from transmitting antenna to the receiving antenna in a number

of ways. Different propagations of electromagnetic waves are as follows, (i) Ground wave propagation

(ii) Sky wave propagation (iii) Space wave propagation (iv) Tropospheric scatter propagation.

This classification is based upon the frequency range, distance and several other factors.

(i) Ground Wave Propagation :Ground wave propagation is also known as surface wave propagation. This

propagation is practically important at frequencies up to 2 MHz. Ground wave propagation exists when

transmitting and receiving antenna are very close to the earth's curvature. Ground wave propagation suffers

attenuation while propagating along the surface of the earth. This propagation can be subdivided into two

types which are space wave and surface wave propagation. Applications: Ground wave propagation is

generally used in TV, radio broadcasting etc.

(ii) Sky Wave Propagation :Sky wave propagation is practically important at frequencies between 2 to 30

MHz Here the electromagnetic waves reach the receiving point after reflection from an atmospheric layer

known as ionosphere. Hence, sky wave propagation is also known as 'ionospheric wave propagation'. It can

provide communication over long distances. Hence, it is also known as point-to-point propagation or point-

to-point communication. Sky wave propagation suffers, from fading due to reflections from earth surface;

fading can be reduced with the help of diversity reception.





Applications: 1. It can provide communication over long distances.2. Global communication is possible.

(iii) Space Wave Propagation : Space wave propagation is practically important at frequencies above 30

MHz It is also known as tropospheric wave propagation because the waves reach the receiving point after

reflections from tropospheric region. In space wave propagation, signal at the receiving point is a

combination of direct and indirect rays. It provides communication over long distances with VHF ,UHF

and microwave frequencies. Space wave propagation is also known as "line of sight propagation".

Applications :1. Space wave propagation is used in satellite communication.2. It controls radio traffic

between a ground station and a satellite.

(iv) Troposcatter Propagation

Troposcatter propagation is also known as forward 1 scatter propagation, it is practically important

at frequencies above 300 MHz.This propagation covers long distances in the range of 160 to 1600 km.

2. Write short notes on Duct propagation? The higher frequencies or microwaves are continuously reflected in the duct and reflected by

the ground. So that they propagate around the curvature for beyond the line of sight. This special refraction

of electromagnetic waves is called super refraction and the process is called Duct propagation. Duct

propagation is also known as super refraction.

Consider the figure,

Here, two boundary surfaces between layers of air

form a duct or a sort of wave guide which guides

the electromagnetic waves between the walls.

Temperature inversion is one of the important

factors for the formation of duct. For proper value

of curvature, the refractive index (n) must be replaced by a modified refractive index (N). N = n + (h/r)

The term modified index of refractive modules (m) is related to N as

N = n + (h/r)

(N-1) = n-1+ h/r

(N-1) x 106 = [n-1+ h/r] x 10

6

m = (N-1) x 106 = [n-1+ h/r] x 10

6

Where, n = Refractive index, h = Height above ground, r = Radius of the earth = 6370 km

Duct can be used at VHF, UHF and microwave frequencies. Because, these waves are neither reflected nor

propagated along earth surface. So, the only possible way to transmit such signal is to utilize the

phenomenon of refraction in the troposphere.

3. Briefly the describe the terms related to the sky wave propagation: virtual heights,

Critical frequency, maximum usable frequency, skip distance and fading?

Sky wave propagation:

It is also called as Ionosphere wave propagation. The ionosphere

acts like a reflecting surface and is able to Reflect back the

electromagnetic waves of frequencies between 2 MHz to 30MHz

.Since, long distance point to point communication is possible

with sky propagation, it is also called as point to point propagation.

This mode of propagation is also known as short wave propagation.





Virtual heights:The virtual height (h) has the great advantage of being easily measured, and it is very useful

in transmission path calculations. For flat earth approximation and assuming that ionosphere conditions are

symmetrical for incident and refracted waves. The transmission path distance, TR=2h/tanβ

Where β=Angle of elevation ,h =Virtual height

Critical frequency:When the refractive index, n has decreased to the point where n = sin φi the angle of

refraction φ will be 90°and wave will be travelling horizontally. The higher point reached by the wave is

free. The electron density N at the point satisfies the relation.

If the electron density at some level in a layer is sufficient great to satisfy the above condition. Then the

wave will be returned to earth from that level. If maximum electron density in a layer is less than n', the

wave will penetrate the layer(Though it may be reflected back from a higher layer for which N is

greater).The largest electron density required for reflection occurs when the angle of incident φi is zero,

i.e., for vertical incidence. For any given layer the highest frequency that will be reflected back for vertical

incidence will be

The characteristics of the ionospheric layers are usually described in terms of their virtual heights and

critical frequencies, as these quantities can be readily measured. The virtual height is the height that would

be reached by a short pulse of energy showing the same time delay as the actual pulse reflected from the

layer travelling with the speed of light. The virtual height is always greater than the true height of reflection,

because the interchange of energy taking place between the wave and electrons of the ionosphere causes the

velocity of propagation to be reduced. The extent of this difference is influenced, by the electron

distributions in the regions below the level of reflection. Itis usually very small, but on occasions may be as

large as 100 Kms or so.The critical frequency is the highest frequency that is returned by a layer at vertical

incidence.

The critical frequencies of the E and F1 layers primarily depend on the zenith angle of the sun.

It, therefore, follows a regular diurnal cycle, being maximum at noon and tapering off an either side. The fc

of the F2 layer shows much larger seasonal variation and also changes more from day to day. It can be seen

that the critical frequencies of the regular layers decrease greatly during night as a result of recombination in

the absence of solar radiation. But the fc of sporadic E shows regular variation throughout the day and night

suggesting that sporadic E is affected strongly by factors other than solar radiation.

There is a long term variation in all ionospheric characteristics closely associated with the 11 year sunspot

cycle. From the minimum to maximum of the cycle, fc of F2 layer varies from about 6 to 11 MHz (ratio of

1:1.8), fc of E layer varies from 3.1 to 3.8 MHz (a ratio of mere 1 to 1.2). Long term predictions of

ionospheric characteristics are based on predictions of the sunspot number. Reliable estimates can be made,

for as much as a year, in advance.

Maximum Usable Frequency:

Although the critical frequency for any layer represents the highest frequency that will be reflected back

from that layer at vertical incidence, it is not the highest frequency that can be reflected from the layer.

The highest frequency that can be reflected depends also upon the angle of incidence, and hence, for a given

layer height, upon the distance between the transmitting and receiving points. The maximum, frequency that

can be reflected back for a given distance of transmission is called the maximum usable frequency (MUF)

for that distance. It is seen that the MUF is related to the critical frequency and the angle of incidence by





the simple expression MUF = f c r secφi

The MUF for a layer is greater than the critical frequency by the

factor secφi the largest angle of incidence φ i that can be obtained

in F-layer reflection is of the order of 74°.This occurs for a ray that

leaves the earth at the grazing angle. The geometry for this case is

shown by Fig. 1.2 The MUF at this limiting angle is related to the

critical frequency of the layer by MUFmax = f c r / cos74o = 3.6 f c r

4. Explain the structure of ionosphere on the surface of the earth?

As the medium between the transmitting and receiving antennas plays a significant role, it is essential to

study the medium above the earth, through which the radio waves propagate. The portion of the atmosphere,

extending up to a height (average of 15 Km) of about 16 to 18 Kms from the earth's surface, at the equator is

termed as troposphere or region of change. Tropopause starts at the top of the troposphere and ends at the

beginning of or region of calm. Above the stratosphere, the upper stratosphere parts of the earth’s

atmosphere absorb large quantities of radiant energy from the sun. This not only heats up the atmosphere,

but also produces some ionization in the form of free electrons, positive and negative ions. This part of the

atmosphere where the ionization is appreciable is known as the ionosphere.

The most important ionizing agents are ultraviolet UV radiation, α , β and Cosmic rays and Meteors.

The ionization tends to be stratified due to the differences in the physical properties of the atmosphere

at different heights and also because various kinds of radiation are involved.

The levels, at which the electron density reaches maximum are called as layers. The three principal day time

maxima are called E, F1, and F2 layers. In addition to these three regular layers, there is a region (below E)

responsible for much of the day time attenuations of HF radio waves, called D region (ref. Fig.4a). It lies

between the heights of 50 and 90 Km (ref. Fig. 3). The heights of maximum density of regular layers E and

F1are relatively constant at about 110 Km and 220Km respectively. These have little or no diurnal variation,

whereas the F2 layer is more variable, with heights in the range of 250 to 350 Km. At Night F1 and F2

layers combine to form a single night time F2 layer (Fig. 4b). The E layer is governed closely by the amount

of UV light from the sun and at night tends to decay uniformly with time. The D layer ionization is largely

absent during night. A sporadic E layer is not a thick layer. It is formed without any cause. The ionization is

often present in the region, in addition to the regular E ionization. Sporadic E exhibits the characteristics of

a very thin layer appearing at a height of about 90 to 130 Kms. Often, it occurs in the form of clouds,

varying in size from 1 Km to several 100 Kms across and its occurrence is quite unpredictable. It may be

observed both day and night and its cause is still uncertain.

Documents

LATHA MATHAVAN ENGINEERING COLLEGE … -VI SEM.pdfLATHA MATHAVAN ENGINEERING COLLEGE Kidaripatti, Madurai – 625301 Subject: EC6602 Antennas & Wave Propagation Year & Semester : III