antenna & wave propagation

ANTENNA & WAVE PROPAGATION

SCT-ECE DEPARTMENT

SELVAM COLLEGE OF TECHNOLOGY,NAMAKKAL DEPARTMENT OF ECE

TWO MARK

SUBJECT NAME : ANTENNA & WAVE PROPAGATION.

STAFF INCHARGE :Ms.M.MAHAMUDHA


SCT-ECE DEPARTMENT

SELVAM COLLEGE OF TECHNOLOGY

NAMAKKAL -637 003

DEPARTMENT OF ECE UNIT 1

RADIADION PROPERTIES 1) What is magnetic vector potential?

The source for magnetic vector potential is current element. The current having both magnitude and direction. Hence the potential in the case of magnetic field is magnetic vector potential. The expressions for magnetic vector potential (A) A = µ/4π∫ J/r dV (volts /metre) 2) What is a monopole antenna? A monopole antenna consists of one half of a dipole (usually a short vertical) antenna mounted above the earth or group (reflecting plane). 3) What is a quarter wave monopole? One-quarter wavelength long .it is also called as Marconi antenna. 4) What is a hertz antenna? Effective length or height of the antenna becomes half of the actual length. 5) Define electric scalar potential. The source for producing a electric potential is charge. The charge is having magnitude only. Hence the potential in the case of electric charge is electric scalar potential. E= -∆V Unit is weber/m²


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6) Write down the expressions for magnetic vector potential using three standard current distributions. i) for volume current A = µ/4π ∫ J/r dV (volts /metre) ii) for line current A = µ/4π ∫ I/r dl (volts /metre) iii) for surface A = µ/4π ∫ K/r dS (volts /metre) 7) What is retarded current?

Since current flowing through a short dipole is assumed to be constant throughout its length, the effect of this current is not felt instantaneously at a Distant point P, but only after an interval equal to the time required for the wave to propagate over a distance r. This current is called as retarded current. Expressions for retarded current [I] = Im e jw(t- r/c) : r/c – retardation current. 8) Define hertzian dipole? Hertzian dipole is defined as a short linear conductor or short electric dipole , whose length is very short compared to wavelength(l <<λ ), and current is assumed to be constant throughout its length. 9) Write down the expressions for radiated fields of a half wave dipole antenna(λ/2) . E θ =60 Im /r [( cos(π/2 cosθ)/sinθ] H φ = Im /2πr [( cos(π/2 cosθ)/sinθ]

10. State reciprocity principle. If a current I1 at the terminals of antenna no.1 induces an emf E21 at the open terminals of antenna no.2 and a current I2 at the terminals of antenna no.2 induces an emf E12 at the open terminals of antenna no.1 then E12= E21 provided I1= I2


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11. Define antenna . An antenna is a structure that has been designed to radiate or receive electromagnetic waves. 12. Define Radiation pattern An antenna radiation pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antennas a function of space coordinates. The radiation properties include power flux density, radiation intensity, field strength,directivity, phase or polarization. 13). What are the two types of radiation pattern. Field pattern If the radiation from the antenna is expressed in terms of the field strength(either E or H ) then the radiation pattern is called as field pattern. Power pattern If the radiation from the antenna is expressed in terms of power per unit area, then the resultant pattern is power pattern.

14) Define Induction field(near field) The field which predominates at the points closer to the current element where r is small is known as induction field. The near field is inversely proportional to square of the distance(1/ r²). 15) Write down expression for retarded vector potential. Retarded vector potential [A] = µ/4π ∫v ( Jm e jw(t- r/c) ) / r. 16) Write down the expression for radiation resistance of a hertzian dipole Radiation resistance Rr = 80 π² ( L / λ)² ohms 17) Define retardation time It is the time required for the wave to propagate over the distance r. It is given by r/c where c is 3*108 m/s. 18) What is the radiation resistance of a current element whose overall length is λ/50.


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Radiation resistance Rr = 80 π² ( L / λ)² ohms Given L = λ /50. Ans = 0.316 ohms 19) Calculate the physical height of a halfwave dipole (λ/2) having antenna Q of 30 and bandwidth of 10 MHz. f = Q. BW λ= c / f ; ans: 0.5 m 20) Calculate the bandwidth of a 50 cm long half wave diploe having a Q of 15. Bandwidth = f / Q; Height of a half wave dipole (λ/2) = 50 cm. f = C/ λ ans : 20 MHz. 21) If the electric field strength(E) of a plane wave is 5 volts/metre,calculate the strength of a magnetic field(H)in free space. E θ = 5 v η = 120 π = E θ / H φ

22). What is the Special feature of folded dipole antennas? Folded dipole has got a versatile quality of providing any desired input impedance by increasing the number of elements and selecting suitable lengths of dipole.


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23). What are the Advantages of folded dipole? AN.U.2003

The radiation pattern of a folded dipole and a conventional half wave

is same but the input impedance of the folded dipole is higher. It has low

directivity and bandwidth than simple dipole.

24). What is Application of folded dipole?

Folded dipole is used as a driven element in yagi-uda antenna. It has

built in impedance transforming properties, which makes it easier to match a

transmission line that feeds the antenna.

25. Compare half wave dipole & folded dipole? AN.U.2005 s.no Half wave dipole Folded dipole 1. Directivity is unidirectional Directivity is bi-directional, but

because of the distribution of currents in the parts of folded dipole, the input impendence is higher.

� Used as impedance matching devices.

2 Broadness in bandwidth is less than folded dipole.

Broadness in bandwidth

26). Define radiation efficiency Radiation efficiency=Max power gain/directivity

UNIT II


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ANTENNA ARRAYS

1.What is a Loop antenna? AN.U.2003

The loop antenna is a radiating coil of any convenient cross section of one or more turns, carrying radio frequency current. It may assume any shape likes rectangular, square, triangular, hexagonal and circular loop, most popular out of these are square and circle. 2.What are the Applications of loop antenna?

1.They are seldom employed for transmission in radio communicating they are used in such application; it is usually in receiving mode where antenna efficiency is not important 2.They also use it for finding the direction. 3. What is meant by Broadband antenna?

Antenna which maintain certain required characteristics like gain, front to back ratio, SWR, polarization, input impedance and radiation pattern over wide range of frequencies are called wide band or broad band antenna. EG: Log-periodic 5. What is the Special feature of folded dipole antennas?

Folded dipole has got a versatile quality of providing any desired input impedance by increasing the number of elements and selecting suitable lengths of dipole. 6. What are the Advantages of folded dipole? AN.U.2003

The radiation pattern of a folded dipole and a conventional half wave is same but the input impedance of the folded dipole is higher. It has low directivity and bandwidth than simple dipole. 7. What is Application of folded dipole?

Folded dipole is used as a driven element in yagi-uda antenna. It has built in impedance transforming properties, which makes it easier to match a transmission line that feeds the antenna.


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8. What are the disadvantages of loop antenna?

� � Only suitable for LF & MF except for a very small distance. � Transmission efficiency is poor & hence cannot be used for UHF

unless design is changed. � Spurious induced voltages are produced, if nearby loop, wires &

conductors are present. � It is subjected to antenna effect & night effect, which are minimized

by balancing & using Adcock antenna for direction finding. �

9. What is Adcock antenna? & how it eliminates the effects of loop antenna? AN.U.2003

It consists of 2 spaced vertical antenna.In vertical polarization the action is same as loop antenna. The horizontally polarized down coming sky waves have no effect on this antenna due to its circuit arrangement. If the horizontal wires are placed inside a metal tube shielded from the radio waves, this antenna system becomes insensitive to horizontally polarize down coming sky waves & thus it is free from polarization effect. 10. compare half wave dipole & folded dipole? AN.U.2005 s.no Half wave dipole Folded dipole 1. Directivity is unidirectional Directivity is bi-directional, but

because of the distribution of currents in the parts of folded dipole, the input impendence is higher.

� Used as impedance matching devices.

2 Broadness in bandwidth is less than folded dipole.

Broadness in bandwidth

11.What is a normal mode of helix antenna?

Radiation field is maximum in the direction normal to the helix axis & wave is nearly or exactly circularly polarized wave . Mode is said to be normal if the dimensions of helix is small compared with the wavelength.


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12. What is a axial mode of helix antenna?

Radiation field is maximum in the end fire direction.(ie) along the helix axis & polarization is circular or nearly circular. 13. How bandwidth & efficiency is increased? AN.U.2005

Efficiency is increased by increasing the dimension & current feed in phase with helix axis. Bandwidth is increased by providing phase shifter at regular intervals. 14.How the axial mode is improved?

When the helix circumference D & spacing S of order of one wavelegth. Axial mode of radiation is produced by raising helix circunference (c/λ) of order of one wavelength & spacing is approximately of λ/a. 15.Define radiation efficiency

Radiation efficiency=Max power gain/directivity 16.What is the error introduced in direction finding of loop antenna?

Antenna or Vertical effect Night effect or Aeroplane effect or polarization error. 17. What is the radiation resistance of multi turn loop antenna? & how it change the o/p power?

Rr = 31,200 (NA/ λ 2 ) 2 N= number of turns , λ= wavelength, A= effective area. 18. What is the principle of pattern multiplication?


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In case of isotropic antenna arrays the total field of the antenna array is simply the vector sum of those of individual radiating sources. In case of non isotropic, but similar sources I s the multiplication of the individual source patterns and the pattern of an array of isotropic point sources, each located at the phase centre of individual sources and having relative amplitude and phase. Whereas the total phase pattern is the addition of the phase patterns of the individual sources and tat of the array of isotropic point sources 19. What is the general effect of earth on antennas?

Effect of earth modifies the following: 1.Currents flowing in reflecting surface. 2.Radiation pattern. 20. What is an “array factor”?

It is apparent that the total field of an array is equal to equal field of a single element positioned at the origin multiplied by a factor which is widely referred as array factor. 21. What are the disadvantages of binomial arrays?

1. As HPBW increases and hence directivity decreases. 2. For design of a large array, larger amp ratio of sources is required. 22. What is the disadvantage of a binomial array?

A major practical disadvantage of binomial array is the wide variations between the amplitudes of the different elements of an array, especially for an array with a large no of elements. This leads to very low efficiencies, and makes this method not very desirable in practice. 23. What is a binomial array?

The coefficients of binomial expansion represent relative amplitudes of the elements. Since the coefficients are determined from the binomial series, expansion the array is known as binomial array.


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24. What are the types of antenna arrays?

1.Broad side array 2.End fire array 3.Collinear array 4.Parasitic array 25. Define the length of an array

The length of an array is the length measured along the axis of the array from the first element to the last element. 26. What is array factor or space factor?

The factor by which the antenna array increases the field strength over that of a single element radiating the same total power is called array factor or space factor. 27. Define antenna array. AN.U.2003

Antenna array is an arrangement of several individual antennas so spaced and phased that their individual electric field contributions combine in one preferred direction and cancel in all other directions to get greater directivity. 28. How to convert broad side array radiation pattern into unidirectional? AN.U.2004

Bidirectional pattern of a broad side array can be converted into unidirectional by installing an indentical array behind this array at a distance λ/4 and exciting it by current leading in phase by 90*. 29. What are the applications of arrays? AN.U.2005


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Because of their physical size, their use is generally limited to HF bands and higher but they are used in LF bands for point to point communications. They are also used in overseas broadcasting stations in MF and HF bands for repeated broad casting to some distant area. 30.Give the formula for beam width between first nulls(BWFN) of a broad side array.

BWFN=115/(L/λ) L=length of array λ=wave length 31.Give the formula for BWFN of an end-fire array.

BWFN=115/√(L/λ) 32.Give the formula for half power beam width of a broad side array.

BWFN=51/(L/λ) L=length of array λ=wave length. 33.Give the formula for HPBW of an end-fire array.

HPBW=52/√(L/λ) 34.Give the formula for calculating directive gain of broad side array.

Directive gain=2L/λ(when the elements along with axis parallel to array axis) Directive gain=4L/λ(when the elements at right angles to array axis) 35.What is the formula for directive gain when the elements right angels to array axis? Directive gain=4L/λ 36. What is a collinear array? AN.U.2004

In this array the antennas are arranged co-axially and the elements are fed with equal inphase currents. The radiation pattern of a collinear array


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has circularly symmetry with its main lobe every where perpendicular to the principle axis. 37. When will be the gain of the collinear array be maximum?

When the spacing between elements is of the order of 0.3λ to 0.5λ. 38. Define antenna .

An antenna is a structure that has been designed to radiate or receive electromagnetic waves. 39. Define Radiation pattern

An antenna radiation pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antennas a function of space coordinates. The radiation properties include power flux density, radiation intensity, field strength,directivity, phase or polarization. 40. What are the two types of radiation pattern.

1) Field pattern

If the radiation from the antenna is expressed in terms of the field strength(either E or H ) then the radiation pattern is called as field pattern.

2) Power pattern If the radiation from the antenna is expressed in terms of power per unit area, then the resultant pattern is power pattern.

41. What is broadside array?

The simplest array consist of a number of dipoles of equal size,equally spaced along a straight line with the dipole fed in the same phase from the same sources. Such an arrangement is called broad side array. 42.What is an end-fire array?


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In the end-fire array each element is fed with same magnitude but

different phase and this phase is progressive from left to right. 43.What is the direction of maximum radiation is maximum only in one direction that is in the direction of array axis.

(180 & 360 (or) 0 degree) 44.What are the characteristics of an end-fire array and broad side array? 1.linear 2.Resonant 3.Narrow bandwidth 45.What is a collinear array? AN.U.2004

In this array the antennas are arranged co-axially and the elements are fed with equal inphase currents. The radiation pattern of a collinear array has circularly symmetry with its main lobe every where perpendicular to the principle axis. 46. When will be the gain of the collinear array be maximum?

When the spacing between elements is of the order of 0.3λ to 0.5λ. 47. What are stacked arrays?

When a number of BSAs of linear types, are arranged (stacked) one above the other, a rectangular stack is formed. The directivity is given by 4πhl 12.56* area of aperture D = ____ = __________________ λ2 λ2

48. Define HPBW.

HPBW is the angular width measured on the major lobe of radiation pattern between points where the radiated power has fallen to half on its maximum value. It is otherwise called as 3 db beamwidth.


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49. State reciprocity principle.

If a current I1 at the terminals of antenna no.1 induces an emf E21 at the open terminals of antenna no.2 and a current I2 at the terminals of antenna no.2 induces an emf E12 at the open terminals of antenna no.1 then E12= E21 provided I1= I2 50.State image theory.

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.

UNIT III

ANTENNA TYPES

1). What are the Application of paraboloidal or parabolic reflector?

Parabolic reflector is used mainly for point to point microwave communication system. 2). What are the Various types of feeds used for parabolic reflector.

1.end fire array feed. 2.horn antenna as feed 3.cassegrain feed 3). What are the types of horn antenna?

1.E-plane antenna 2.H-plane antenna 3.Pyramidal antenna. 4.Sectoral horn. 5.Conical horn. 6.Biconical horn. 4). What are the Advantages & disadvantages of cassegrain feed?


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Adv:Reduction in spill over & minor lobe radiation Ability to place the feed in a convenient lacation. Capability for scanning or broadening of the beam by moving one of

the reflecting surface.

Disadv: 1.Some of the reflected wave is obstructed problematic

Especially with small dimension. 2.Dimension of the secondary reflector depends on the distancee beween the horn feed & sub reflector, mouth of

the horn inturn on frequency. This can be avoided by using an offset reflector.

5). What is primary & secondary radiator?

The antenna placed at the focus of a paraboloid is called feed radiator or feed or primary radiator & its radiation is called primary pattern..The paraboloid reflector is known as secondary radiator & pattern as secondary pattern or antenna pattern. 6). What are the cassegrain feed mechanism?

In this feed system, primary radiator is put at the opening of vertex of paraboloid reflector B. the secondary feed of the cssegrain system is a hyperboloid reflector whose focus F coincides with focus of the paraboiloid reflector . 7). What is biconical antenna?

The biconical antenna is a double cone antenna which is dtiven by potential, charge or an alternating magnetic field at the vertex. In this antenna , both the cones face in opposite direction

8). What is backlobe radiation?

Some radiation from the primary radiator occur in the forward direction in addition to the desired parallel beam. This is known as backlobe radiation.


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9). Define spill over.

Some of the desired rays are not captured by the reflector antenna and this constitute spill over.

10). How spherical waves are generated?

When a voltage V is applied at the input terminals of a biconical antenna. It will produce outgoing spherical waves . the biconical antenna acts as a guide for the spherical waves.

11). What is the difference between slot antenna and its complementry dipole antenna?

1) Polarization are different ie., the electric fields associated with the

slot antenna are identical with the magnetic field of the complementry dipole antenna.

2) The electric field be vertically polarized for the slot and horizontally polarized for the dipole.

3) Radiation from the back side of the conducting plane of the slot antenna has the opposite polarity from that of the complementary antenna.

12). State hygenes principle.

Hygenes principle states that each point ona primary wave front can be considered to be a new source of asecondary spherical wave and tbat a secondary wave front can be constructed as the envelop this secondary waves.

13). What is yagi uda antenna?

It is an array of a driven element, a reflector and one or more directors. 14). What do u meant by parasitic elements?


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The passive elements which are not connected directly to the transmission line but are electrically coupled are called as parasitic elements. 15). What do you meant by driven elements?

Driven element is an active element where the power from the transmitter is fed or which feeds the received power to the receiver. 16). What is the purpose of using more directors in yagi uda antenna?

The purpose of using more directors in yagi uda antenna is to increase the gain.

17). Why folded dipole antenna is used in yagi uda antenna?

The folded dipole has high input impedance.If the distance between the driven and the parasitic element is decreased, it will load the driven element , so input impedance of driven element reduces.But this will be compensated i.e., raised by the folded dipole. 18). What is beam antenna?

If three elements array i.e., one reflector,one driven element and one director are used, then such type of yagi – uda antenna is referred to as beam antenna. 19). Write short notes on horn antenna.

A wave guide is capable of radiating radiation into open space provided the same is excited at one end and opened at the other end.This type of antenna is called flared out or opened out waveguide. 20). What are the uses of horn antenna?

a) They are used at microwave frequencies. b) Power gain is high. 21). What is aperture?

In parabolic reflectors,the open mouth of the parabola is known as the aperture.


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22). What is a Loop antenna? AN.U.2003

The loop antenna is a radiating coil of any convenient cross section of one or more turns, carrying radio frequency current. It may assume any shape likes rectangular, square, triangular, hexagonal and circular loop, most popular out of these are square and circle. 23). What are the Applications of loop antenna?

1.They are seldom employed for transmission in radio communicating they are used in such application; it is usually in receiving mode where antenna efficiency is not important 2.They also use it for finding the direction. 24). What is meant by Broadband antenna?

Antenna which maintain certain required characteristics like gain, front to back ratio, SWR, polarization, input impedance and radiation pattern over wide range of frequencies are called wide band or broad band antenna. EG: Log-periodic 25). What are the disadvantages of loop antenna?

� Only suitable for LF & MF except for a very small distance. � Transmission efficiency is poor & hence cannot be used for UHF

unless design is changed. � Spurious induced voltages are produced, if nearby loop, wires &

conductors are present. � It is subjected to antenna effect & night effect, which are minimized

by balancing & using Adcock antenna for direction finding. 26). What is a normal mode of helix antenna?

Radiation field is maximum in the direction normal to the helix axis & wave is nearly or exactly circularly polarized wave . Mode is said to be normal if the dimensions of helix is small compared with the wavelength. 27). What is a axial mode of helix antenna?


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Radiation field is maximum in the end fire direction.(ie) along the helix axis & polarization is circular or nearly circular. 28). How bandwidth & efficiency is increased? AN.U.2005

Efficiency is increased by increasing the dimension & current feed in phase with helix axis. Bandwidth is increased by providing phase shifter at regular intervals. 29). How the axial mode is improved?

When the helix circumference D & spacing S of order of one wavelegth. Axial mode of radiation is produced by raising helix circunference (c/λ) of order of one wavelength & spacing is approximately of λ/a. 30). What is the error introduced in direction finding of loop antenna?

Antenna or Vertical effect Night effect or Aeroplane effect or polarization error. 31). What is the radiation resistance of multi turn loop antenna? & how it change the o/p power?

Rr = 31,200 (NA/ λ 2 ) 2 N= number of turns , λ= wavelength, A= effective area. 32). What are the Advantages &disadvantages of rhombic antenna?

Adv: 1. An end to end receiving array of a number of rhombics can be

designed to form a multi unit steer able antenna (MUSA) system which is used for long distance short-wave reception of horizontal polarized down coming waves

2. The input impedance & radiation pattern do not change over a considerable frequencies range as compared to resonant dipole.

3. Highly directional 4. Vertical angle of radiation is low & hence these are suitable for

long distance ionosphere propagation (F layer).

Disadv: large space for installation Half the power is wasted in the terminal resistors.


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Transmission efficiency is poor. (Lager number of Minor lobes) It is overcome by using one or more rhombics antenna 33). What are the types of horn antenna?

1.E-plane antenna 2.H-plane antenna 3.Pyramidal antenna. 34). What are the Applications of rhombic antenna?

1.HF transmission 2.HF reception 3.Point to point communication 35). What are the Disadvantages of limiting factor of traveling wave antenna?

The limiting factor of traveling wave antenna is the misalignment of the lobes that takes place as the frequency is changed. 36) What are the Advantages of traveling wave antenna?

Traveling wave antennas input impedance is mostly resistive and relatively independent of frequency. Thus these antennas can operate over fairly broad frequency band. 37). What are the Type of long wire antenna 1.Horizontal V 2.Horizontal rhombic 3.Vertical V 4.Vertical rhombic 5.Vertical inverted V 6.Half rhombic 7.Single horizontal wire 38). What are the Applications of log-periodic antenna

1.used mainly in HF communications 2.used for television reception where only one log-periodic design will suffice for all the channels even upto UHF band. 39). What is the Applications of yagi-uda antenna 1. High frequency communications 2. Television reception


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40). What is an Log-periodic antenna?

Log-periodic antenna is a broadband antenna. It has a number of dipoles of different lengths. The performance of the system is periodic as a function of frequency that results the name. It is also known as frequency independent antenna. 41). What is meant by Frequency independent antennas?

A frequency independent antenna is defined as the antenna for which the impedance pattern and directivity remains constant as function of frequency. 42). Define rhombic antenna.

An antenna which consists of four straight wires, arranged in the shape of diamond , suspended horizontally above the surface of earth is called as rhombic antenna. It is otherwise called as diamond antenna or traveling wave antenna 43). What are the three different regions in log periodic antenna and how they are differenciated?

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

44). What is the Applications of yagi-uda antenna

1. High frequency communications 2. Television reception 45). What is an Log-periodic antenna?

Log-periodic antenna is a broadband antenna. It has a number of dipoles of different lengths. The performance of the system is periodic as a function of frequency that results the name. It is also known as frequency independent antenna. 46). What are the Applications of log-periodic antenna?


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1.used mainly in HF communications

2.used for television reception where only one log-periodic design will suffice for all the channels even upto UHF band. 47). What are the types of long wire antenna?

1.Horizontal V 2.Horizontal rhombic 3.Vertical V 4.Vertical rhombic 5.Vertical inverted V 6.Half rhombic 7.Single horizontal wire 48). What are the Advantages of V antenna?

1.Its directivity is high 2.Side lobe level is small 49). What are the Advantages of traveling wave antenna?

Traveling wave antennas input impedance is mostly resistive and relatively independent of frequency. Thus these antennas can operate over fairly broad frequency band. 50). What are the Disadvantages of limiting factor of traveling wave antenna?

The limiting factor of traveling wave antenna is the misalignment of the lobes that takes place as the frequency is changed. 51). What are the Applications of rhombic antenna 1.HF transmission 2.HF reception 52). What is meant by Frequency independent antennas?

A frequency independent antenna is defined as the antenna for which the impedance pattern and directivity remains constant as function of frequency.


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UNIT IV

WAVE PROPAGATION

1. Define ground wave and its properties. The ground wave or surface wave or Norton’s surface wave is the wave that is guided along the surface of the earth. It is of importance at the broadcast of lower frequencies. Surface wave permits the propagation around the curvature of the earth. 2. What are the conditions for the existence of ground wave propagation? Ground wave propagation exist when the transmitting and receiving antennas are close to the surface of earth and are supported at its lower edge by the presence of earth. 3. How the ground waves are produced? Vertical antennas usually produce the ground waves and it is vertically polarised. Any horizontal component of electric field in contact with the earth is short circuited by the earth. The ground wave propagation along the surface of the wave, induce charges in the earth, which travel with the wave and hence constitute a current. While carrying this current, the earth behaves just like a leaky capacitor and therefore the earth can be represented as a shunt resistance with a capacitor. 4. What are the reasons for ground attenuation? 1.Diffraction 2.Tlit in the wave front.


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5. What are sky waves? Sky waves are those that reach the receiving point after reflection from the ionised region in the upper region known as ionosphere-situated between 50Km to 400Km above the earth’s surface. Sky wave propagation is practically important at medium and higher frequencies. 6. Define space wave propagation. In this mode of propagation, the electro magnetic waves from the transmitting antenna reach the receiving point either directly or after reflections from ground in the earth’s troposphere region. Troposphere is that region of the atmosphere, which extends up to 16Km from the earth’s surface. Space waves consist of 2 components, the direct and the reflected one.7. What are the various layers of ionosphere? 1.D layer (vanishes during night) 2.E layer. 3.F1 and F2 (these layers combine together as a single layer, F during night) 8. Describe D region of ionosphere.

• It is the lower most region and is located in the height range of 50-90 Km.

• This layer is present only during daytime and vanishes during night.

• It is ionised by photo- ionisation NO (Nitrogen oxide), which prevails in that height.

• Ionisation density is maximum at noon and its electron density is ranging from 1014 to 1016per cubic centimetre.

• Its critical frequency is about 100KHz. • It is also known as absorbing layer for short wave signals.

9. Why the D region vanishes during night?

D region vanishes during night because at night, the recombination rate

is highest. This is due to the fact that the degree of ionisation depends on the altitude of the sun and on sunset the recombination increases resulting in vanishing of the region all together. 10. What is sporadic E-region?


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Sporadic E layer is an anomalous ionisation layer, whose presence is unpredictable .It may appear anywhere in the height range of 90Km to 130Km with normal E layer. Occurrence of this layer increases with geomagnetic latitude. 11. Define virtual height. 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 true height of reflection 12. Define critical frequency. Critical frequency is the highest frequency, which can be reflected by a particular layer at vertical incidence. fc = 9√Nm

where, fc is the critical frequency,Nm is the max. electron density. 13. What are the factors that determine the absorption of energy by electrons from radio waves in ionosphere? 1.Gec pressure. 2.Velocity of the electrons. 3.Number of electrons. 14. Define skip distance. 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. 15. Define MUF. 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. 16. What are the types of fading in ionosphere?


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1.Interference fading. 2.Selective fading. 3.Absorption fading. 4.Skip fading. 5.Polarisation fading. 17. Define gyro-frequency. The average velocity of electrons is inversely proportional to frequency, hence at higher frequencies, the electrons vibrate in paths that are narrow ellipses but as the frequency is lowered the amplitude of vibration increases and the minor axis of the ellipse becomes larger. This continues until a frequency is lowered to a point where the cyclotron resonance occurs and he electrons follow a spiral path and the velocity increases. This cyclotron resonance occurs at a frequency known as gyro frequency,1400 KHz (approx).

18. What is diversity reception and what are its types? Diversity reception is used for reducing fading. There are two types of diversity reception-Space diversity and frequency diversity. 19. What is frequency diversity reception? The signals at different frequencies do not fade out at the same time. This fact is used in frequency diversity system. The same antenna is used for the receiver that works with simultaneous transmission at two or more frequencies. 20. What are the applications of diversity reception system? It is used in data transmission such as,

1.telegraph transmission. 2.voice transmission. using PCM techniques. 21. What are the disadvantages of diversity reception system? 1.The audio output will have a slight phase difference when compared to other receiving systems. 2.The diversity reception is restricted in voice communication. 22. Define OTF or OWF (Optimum Working Frequency)


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OTF-Optimum traffic frequency is the frequency used for ionospheric transmission .It is chosen about 1.5% less than MUF. 23. What is duct propagation? When the frequency is high, the region where the variation of dielectric constant or refractive index is usually high, traps the energy and causes it to travel along the earth’s surface as happens in a wave guide. This happens near the ground often within the 50 m of the troposphere. The higher frequencies or microwaves are thus continually refracted in the duct and reflected by the ground so that they propagate around the curvature for beyond the line of sight, even up to 1000Km.This special refraction of EM waves is called super refraction and the process is known as duct propagation. 24.What is fading? Intensity variation with time is defined as fading. The space wave signals received at a considerable distance from a transmitter will often fade. This is due to changing tropospheric conditions. 25. Name some ionospheric abnormalities. 1.Ionospheric storms. 2.Sun spot cycle. 3.Tides and waves in ionosphere. 4.Fadings. 5.Sudden ionospheric disturbances(SID) 26. Calculate the basic transmission loss in free space for d=10 km λ=20000. Ls= (4Πd)/λ = 39.44 27. A pulse of a given frequency transmitted upward is received back after a period of 5ms. Find the virtual height of the reflecting layer. h=CT/2= 750km

UNIT V

ANTENNA MEASUREMENTS

1. What do you meant by antenna measurements?


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The process of analyzing and calculating all the charecteristics of the

antenna. 2. What are the classifications of antenna measurements? 1.Impedance measurement 2.Pattern measurement 3. Define Impedance measurement. Impedance measurements are done only for the signals whose radio frequencies are below 30 MHZ & above 1000 MHZ. 4. Define Gain Measurement. Gain Measurement is defined as the ratio of the total power of output received to the total power of input transmitted. Gp = W2 / W1 Gp(db) = W2(db) - W1(db) 5. What is Phase measurement? The phase of an antenna is a relative quantity & is periodically defined in multiples of 360 degree.The phase for the shortest distance can be easily determined. 6. What is antenna efficiency measurement? Antenna efficiency measurement is defined as the ratio of directivity(D) to the aperture area(A) & they are named as antenna radiation efficiency or antenna aperture efficiency. 7. What is polarization ratio? The ratio of maximum to minimum field intensity is called as polarization ratio or ellipticity. 8. What are the types of fields in antenna measurements? 1.Near Field. 2.Far Field. 9. Define Near Field.


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When the antenna is placed in a particular location then the field gets

radiated due to the input power received from the transmitter antenna within the limit then it is called as near field. 10. Define Far Field. When the antenna is placed in a particular location then the field gets radiated due to the input power received from the transmitter antenna beyond the limit then it is called as near field. 11. Define Polarization measurement. The polarization of electromagnetic field may be measured at one frequency as tha function of the space angles i.e. polarangle & azimuth angle angle at one angular position as a function of frequency. Measurement of polarization characteristics are specially desired where the dominant radiation is circularly or elliptically polarized. 12. What are the methods of polarization? 1.Linear component method. 2.Circular component. 3. Polarization pattern method. 13. What are the methods of directivity measurements? 1.Orange slice method. 2.Conical cut method. 14.Define directivity Measurement. The ratio of maximum radiation intensity to the particular radiation intensity. 15.Define group velocity. The ratio of square of the velocity of light(C2) to the wave velocity(Vp). Vg = C2 / Vp 16.What is antenna aperture efficiency? It is related to the directive gain of large aperture antennas of all types. D = ηA [4πA/ λ2] ηA = D λ2 / 4πA


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17. What do you meant by orange slice method? For discrete values of φ the radiation pattern is measured by radiation intensity Ө & each pattern is multiplied continuously by sin Ө weighting factor and then integrated.then all the integrated values are added. 18. What do you meant by Conical cut method? For discrete values of φ the radiation pattern is measured by radiation intensity Ө & each pattern is multiplied continuously by sin Ө weighting factor and then integrated.then all the integrated values are multiplied.


SCT-ECE DEPARTMENT

antenna & wave propagation

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