TWO MARKS QUESTIONS WITH ANSWERS UNIT-1 ELECTROMAGNETIC RADIATION AND ANTENNA FUNDAMENTALS
1)What do you understand by retarded current?
Since, the short electric dipole is so short, the current which is flowing through the dipole is assumed to be constant throughout its length. The effect of this current is not felt instantaneous at a distance point only after an interval equal to the time required for the wave to propagate over the distance r is called the retardation time.The retarded current is given as,
Where, r-Distance ሾܫሿ ൌ travelled
c- velocity of propagation.
2)Define induction field
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.
3)Define Radiation 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.
4)At what distance from the dipole is the induction field equal to the
As the distance from the current element or the short dipole increases,both induction and radiation fields emerge and start decreasing. However, a distance reaches from the conductor at which both the induction and radiation field becomes equal and the particular distance depends upon the wavelength. The two fields will thus have equal amplitude at that particular distance. This distance is given by r = 0.159λ.
5)Define Radiation Resistance
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.
6)What is capacitance hat?
The capacitance hat is circular in shape with mast at the center of the circle. There are number of horizontal conducting wires with their ends Moined together by means of a ring. The capacitance hat is used to increase the electrical length of low frequency antennas.
7)Define top loading
Top loading is a method to increase the effective capacitance at the top of the antenna. This is accomplished by mounting one or more horizontal conductors at the top of the antenna.
8)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 the velocity of light which is given as 3x108m/s.
9)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) is given as,
Where, J- current Density, r- Distance.
10)Define electric scalar potential.
The source for producing an electric potential is charge. The charge is having magnitude only. Hence the potential in the case of electric chargeiselectricscalar potential,E=V,Unit is weber/m2.
11) Write down the expressions for magnetic vector potential using
threestandard current distributions.
Volume current,ܣ ൌ
ii) Line current,ܣ ൌ
iii) Surface current,ܣ ൌ
12)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 aDistant point P, but only after an interval equal to the time required for thewave to propagate over a distance r. This current is called as retardedcurrent.Expressions for retarded current,ሾܫሿ ൌ ܫ
Where r/c ± retardation time.
13)State reciprocity principle. If a current I
14)Define Radiation pattern
An antenna radiation pattern is defined as a mathematical function or agraphical representation of the radiation properties of the antennas afunction of space coordinates.The radiation properties include power flux density, radiation intensity, Field strength,directivity, phase or polarization.
15) 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 fieldpattern.
• Power pattern
If the radiation from the antenna is expressed in terms of power per unitarea, then the resultant pattern is power pattern.
16) 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).
17)Define an antenna.
Antenna is a transition device or a transducer between a guided wave and a free space wave or vice versa. Antenna is also said to be an impedance transforming device.
18)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 EΜ 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.
19)Define 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 radiation intensity is independent of distance.
20)Define Beam efficiency?
The total beam area (Ω
) consists of the main beam area (Ω
Μ) plus the minor lobe area (Ω
) . Thus Ω
m.The ratio of the main beam area to the total beam area is called beam efficiency.
Beam efficiency = ε
The directivity of an antenna is equal to the ratio of the maximum power density P(θ,φ)max to its average value over a sphere as observed in the far field of an antenna.
av .Directivity from Pattern. D=4 π / Ω
Directivity from beam area(Ω
22)What is meant by effective height?
The effective height h of an antenna is the parameter related to the aperture. It may be defined as the ratio of the induced voltage to the incident field. i.e, H= V / E.
23)What are the field zone?
The fields around an antenna ay be divided into two principal regions.
1ear field zone (Fresnel zone) Far field zone (Fraunhofer zone)
24)What is meant by Polarization?
The polarization of the radio wave can be defined by direction in which the electric vector E is aligned during the passage of at least one full cycle. Also polarization can also be defined the physical orientation of the radiated electromagnetic waves in space.
Polarization can be classified into three types. They are
• Elliptical polarization
• circular polarization
• linear polarization.
25)What is meant by front to back ratio?
It is defined as the ratio of the power radiated in desired direction to the power radiated in the opposite direction. i.e
26)Define antenna efficiency
The efficiency of an antenna is defined as the ratio of power radiated to the total input power supplied to the antenna.
27)What is radiation resistance?
The antenna is a radiating device in which power is radiated into space in the form of electromagnetic wave.
28)What is meant by antenna beam width?
Antenna beam width is a measure of directivity of an antenna. Antenna beam width is an angular width in degrees, measured on the radiation pattern (maMor lobe) between points where the radiated power has fallen to half its maximum value .This is called as “beam width” between half power points or half power beam width(HPBW).
29)What is meant by Antenna Temperature?
The antenna temperature T A is defined as the temperature of far field region of space and near surroundings which are coupled to the antenna through radiation resistance.
30)What is meant by isotropic radiator?
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 omni directional radiator or simply unipole.
The ratio of maximum radiation intensity in given direction to the maximum radiation intensity from a reference antenna produced in the same direction with same input power. i.e
32)Define self impedance
Self impedance of an antenna is defined as the ratio of voltage to current at a pair of terminals.
-Radiation -Self Reactance
33)Define mutual impedance
The presence of near by antenna no.2 induces a current in the antenna no.1 indicates that presence of antenna no.2 changes the impedance of the antenna no.1.This effect is called mutual coupling and results in mutual impedance.
34)What is meant by cross field?
1ormally the electric field E is perpendicular to the direction of wave propagation. In some situation the electric field E is parallel to the wave propagation that condition is called Cross field.
35)Define axial ratio
The ratio of the maMor to the minor axes of the polarization ellipse is called the Axial Ratio. (AR)
36)What is meant by Beam Area?
The beam area or beam solid angle or W
A of an antenna is given by the normalized power pattern over a sphere.
UNIT-2 WIRE ANTENNAS AND ANTENNA ARRAYS
1)What is a Short Dipole?
A short dipole is one in which the field is oscillating because of the oscillating voltage and current. It is called so because, the length of the dipole is short and the current is almost constant throughout the entire length of the dipole. It is also called as Hertzian Dipole, which is a hypothetical antenna and is defined as a short isolated conductor carrying uniform alternating current.
2)How radiations are created from a short Dipole?
The dipole has two equal charges of opposite sign oscillating up and down in a harmonic motion. The charges will move towards each other and electric filed lines were created. When the charges meet at the midpoint, the field lines cut each other and new field are created. This process is spontaneous and so more fields are created around the antenna. This is how radiations are obtained from a short dipole.
3)Why a short dipole is also called an elemental dipole?
A short dipole that does have a uniform current will be known as the elemental dipole. Such a dipole will generally be considerably shorter than the tenth wavelength maximum specified for a short dipole. Elemental dipole is also called as elementary dipole, elementary doublet and hertzian dipole.
4)What is an Infinitesimal Dipole?
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.
5)Why a short dipole is called an oscillating dipole?
A short dipole is initially in neutral condition and the moment a current starts to flow in one direction, one half of the dipole require an excess of charge and the other a deficit because a current is a flow of electrical charge. Then,there will be a voltage between the two halves of the dipole. When the current changes its direction this charge unbalance will cause oscillations. Hence an oscillating current will result in an oscillating voltage. Since, in such dipole, electric charge oscillates,it may be called as Oscillating electric dipole.
6)Give the expression for the effective aperture of a short dipole
The effective aperture of a short dipole is given by A
7)What is a dipole antenna?
A dipole antenna may be defined as a symmetrical antenna in which the two ends are at equal potential relative to the midpoint.
8)What is a half wave dipole?
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.
9)Give the expression for the effective aperture of a Half wave Dipole
The effective aperture of a half wave dipole is given by A
10)What is the radiation resistance of a half wave dipole
The radiation resistance of a half wave dipole is given by R
11)What is a loop antenna?
A 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 (e.g. rectangular, square, triangular and hexagonal).
12)Give an expression of radiation resistance of a small loop
Radiation resistance of a small loop is given by
=31,200 (1A/λ2) 2Ω ,Where 1- 1umber of turns.
13)How to increase the radiation resistance of a loop antenna
The radiation resistance of a loop antenna can be increased by:
• Increasing the number of turns
• Inserting a ferrite core of very high permeability with loop antenna’ s circumference which will rise the magnetic field intensity called ferrite loop.
14)What are the types of loop antennas? Loop antennas are classified into:
A.Electrically small (circumference <l/10) B. Electrically large (dimension comparable to l)
15)What are Electrically Small loop antennas?
Electrically Small loop antennas is one in which the overall length of the loop is less than one-tenth of the wavelength. Electrically Small loop antennas have small radiation resistances that are usually smaller than their loop resistances. They are very poor radiators and seldom employed for transmission in radio communication.
16)What are electrically large loop antennas?
Electrically Large loop antennas is one in which the overall lengthof the loop approaches the wavelength.
17)List out the uses of loop antenna
Various uses of loop antenna are:
• It is used as receiving antenna in portable radio and pagers
• It is used as probes for field measurements and asdirectional
antennas for radio wave navigation.
• It is used to estimate the direction of radio wave Propagation. 18)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).
19)What is a quarter wave monopole?
One-quarter wavelength long .it is also called as Marconi antenna.
20)What is a hertz antenna?
Effective length or height of the antenna becomes half of the actual length.
21)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 <
current is assumed to be constant throughout its length.
22) Write down the expressions for radiated fields of a half wave dipole
23)Calculate the physical height of a halfwave dipole (G/2) havingantenna
Q of 30 and bandwidth of 10 MHz.
f = Q X BW=30 X10MHz=300MHz. G= c / f ;ans: 0.5 m
24)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.
25)What are the Advantages of folded dipole?
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.
26)What is Application of folded dipole?
Folded dipole is used as a driven element in yagi-uda antenna. It hasbuilt in impedance transforming properties, which makes it easier to match atransmission line that feeds the antenna.
27)What is point source?
It is the waves originate at a fictitious volume less emitter source at the center ‘O ’of the observation circle.
28)What is meant by array?
An antenna is a system of similar antennas oriented similarly to get greater directivity in a desired direction.
29)What is meant by uniform linear array?
An array is linear when the elements of the array are spaced equally along the straight line. If the elements are fed with currents of equal magnitude and having a uniform progressive phase shift along the line, then it is called uniform linear array.
30)What are the types of array?
• Broad side array.
• End fire array
• Collinear array.
• Parasitic array.
31)What is Broad side array?
Broad side array is defined as an arrangement in which the principal direction of radiation is perpendicular to the array axis and also the plane containing the array element. For Broad side array the phase difference adMacent element is d = 0.
32)Define End fire array
End fire array is defined as an arrangement in which the principal direction of radiation is coincides with the array axis
33)What is collinear array?
In this array the antenna elements are arranged coaxially by mounting the elements end to end in straight line or stacking them one over the other with radiation pattern circular symmetry. Eg.Omni directional antenna.
34)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 electro magnetic coupling. Eg.Yagi-uda antenna.
35)Define beam width of maMor lobe?
It is defined the angle between the first nulls (or) it is defined as twice the angle between the first null and the maMor lobe maximum direction.
36)Differentiate broad side and End fire array.
Sl.1o Broad side array End fire array
Antenna is fed in phase d = 0
Antenna elements are fed out of phase d = -bd
Maximum radiation is perpendicular along the of array axis
radiation is along the array
Beam width of maMor lobe is 3.
twice the reciprocal of array axis,േ
Beam width is greater than that for that of a broad side array for same భ lengthേʹቀ
37)What is the need for the Binomial array?
The need for a binomial array is
i. In uniform linear array as the array length is increased to
increase the directivity, the secondary lobes also occurs. ii. For certain applications, it is highly desirable that secondary lobes should beeliminated completely or reduced to minimum desirable level compared to main lobes.
38)Define power pattern.
Graphical representation of the radial component of the pointing vector with constant radius as a function of angle is called power density pattern or power pattern.
39)What is meant by similar Point sources?
Whenever the variation of the amplitude and the phase of the field with respect to the absolute angle for any two sources are same then they are called similar point sources. The maximum amplitudes of the individual sources may be unequal.
40)What is meant by identical Point sources?
Similar point sources with equal maximum amplitudes are called identical point sources.
41)What is the principle of the pattern multiplication?
The total field pattern of an array of non isotropic but similar sources is the product of the
i) individual source pattern and ii) The array pattern of isotropic point sources each located at the phase center of the individual source having the same amplitude and phase.While the total phase pattern is the sum of the phase patterns of the individual source pattern and array pattern.
42)What is the advantage of pattern multiplication?
1.Useful tool in designing antenna 2.It approximates the pattern of a complicated array without making lengthy computations.
43)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.
44)What is a binomial array?
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.
45)What are the advantages of binomial array?
a) 1o minor lobes Disadvantages:
a) Increased beam width b) Maintaining the large ratio of current amplitude in large arrays is difficult.
46)What is the difference between isotropic and non-isotropic source
• Isotropic source radiates energy in all directions but non-
isotropicsource radiates energy only in some desired directions.
• Isotropic source is not physically realizable but non-isotropic
source is physically realizable.
47)What is a Loop antenna?
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.
48)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.
49)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.
50)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.
51)What are the Advantages of folded dipole?
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.
52) 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.
53) What are the disadvantages of loop antenna?
1.Only suitable for LF & MF except for a very small distance. 2.Transmission efficiency is poor & hence cannot be used for UHF unless design is changed. 3.Spurious induced voltages are produced, if nearby loop, wires& conductors are present. 4.It is subMected to antenna effect & night effect, which are minimized by balancing & using Adcock antenna for direction finding.
UNIT-3 APERTURE ANTENNAS
1)What are the different types of aperture?
• Effective aperture.
• Scattering aperture
• Loss aperture.
• collecting aperture
• Physical aperture.
2)Define different types of aperture?
Effective aperture(Ae) It is the area over which the power is extracted from the incident wave and delivered to the load is called effective aperture. Scattering aperture(As) It is the ratio of the reradiated power to the power density of the incident wave. Loss aperture It is the area (A l)of the antenna which dissipates power as heat. Collecting aperture (Ac) It is the addition of above three apertures. Physical This aperture aperture (A is p a ) measure of the physical size of the antenna.
3)Define Aperture efficiency?
The ratio of the effective aperture to the physical aperture is the aperture efficiency. i.e
Aperture efficiency = Ω
4)What is duality of antenna?
It is defined as an antenna is a circuit device with a resistance and temperature on the one hand and the space device on the other with radiation patterns, beam angle,directivity gain and aperture.
5)Define Side Lobe Ratio
Side Lobe Ratio is defined as the ratio of power density in the principal or main lobe to the power density of the longest minor lobe.
6)List the arrays used for array tapering
Binomial Array: Tapering follows the coefficient of binomial seriesDolphTchebycheff Array: Tapering follows the coefficient of Tchebycheff polynomial.
7)Define a traveling wave antenna?
Traveling wave or non resonant antenna are those in which there is no reflected wave, i.e., only incident traveling wave travel in the antenna.
8)What is beverage or wave antenna?
A single wire antenna terminated in its characteristic impedance may have essentially a uniform traveling wave. This type of antenna is referred to as beverage antenna.
9)State Huygen’s Principle?
Huygen’s principle states that each point on a primary wave front can be considered to be a new source of a secondary spherical wave that a secondary wave front can be constructed as the envelope of these secondary waves.
10)What is Slot Antenna?
The slot antenna is an opening cut in a sheet of a conductor, which is energized through a coaxial cable or wave guide.
11)Which antenna is complementary to the slot dipole?
The dipole antenna is the complementary to the slot antenna. The metal and air regions of the slot are interchanged for the dipole.
12)What is the relationship between the terminal impedance of slot and
o2/4 Where Z
sis the terminal impedance of the slot antenna Z
is the terminal impedance of the dipole antenna
is the intrinsic impedance of the free space.
13)What is the difference between slot antenna and its complementary
• Polarization are different
• The electric field be vertically polarized for the slot and
horizontally polarized for its complementary dipole
• Radiation form the backside of the conducting plane of the slot antenna has the opposite polarity from that of the dipole antenna.
• 14)How spherical waves are generated?
When a voltage V is supplied at the input terminals of a biconical antenna, it will produce outgoing spherical waves. The biconical antenna acts as a guide for spherical waves.
15)What do you meant by sectoral horn?
If flaring (opened out) is done only in one direction, then it is called as a sectoral horn.
16)What do you meant by pyramidal horn?
If flaring is done along both the walls( E & H), then it is called as a pyramidal horn.
17)What is back lobe radiation?
Some radiation from the primary radiator occurs in the forward direction in addition to the desired parallel beam. This is known as back lobe radiation.
18)What are the various feeds used in reflectors?
• Dipole antenna
• Horn feed
• End fire feed
• Cassegrain feed
• 19)What are secondary antennas? Give examples?
Antennas that are not radiators by themselves are called secondary antennas. For example ,Cassegrain, Hyperbolic antennas.
It states that the vector product of electric field intensity vector E and the magnetic filed intensity vector H at any point is a measure of the rate of energy flow per unit area at that point.The direction of power flow is perpendicular to both the electric field and magnetic field components.
21)Define beam width of maMor lobe?
It is defined the angle between the first nulls (or) itis defined as twice the angle between the first null and themaMor lobe maximum direction.
22)State Uniqueness theorem?
It states that for a given set of sources and boundary conditions in a lossy medium the solution to Maxwell’s equation is unique.
23)Define 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 plane of the conducting plane.
24)State the method of feeding slot antennas?
The slot antenna can be energized with a coaxial transmission line or by a waveguide.
25)What are the applications of corner reflector?
• In Television
• Point to point communication
• Radio Astronomy applications.
26)What is Flat Reflector?
It has a large flat sheet near a linear dipole antenna. It provides backward radiation. It provides substantial gain in the forward direction by reducing the space between the antenna and the sheet.
27)What is Corner Reflector?
It is a retro reflector consisting of 2 or 3 mutually perpendicular, intersecting flat surfaces which reflects the EM waves back towards the source.
28)What is Active Corner Reflector?
When the driven element is used in conMunction with the corner reflector, it is called as active corner reflector.
29)What is Passive Corner Reflector?
When thecorner reflector without driven element is called as passive corner reflector.
30)Uses of Corner Reflector?
• Used on the vessel’s mask at a height of atleast 4.6m above sea
• Used on ships as Radar Reflectors.
• Used in automobile and bicycle tail lights.
31)Define V antenna?
An antenna having a V shaped arrangement of conductors fed by a balanced line at the apex is called V-antenna.
32)Types of V-antenna?
• Resonant V antenna
• 1on-Resonant V antenna
UNIT-4 SPECIAL ANTENNAS AND ANTENNA MEASUREMENTS
1)What are the parameters to be considered for the design of an helical
The parameters to be considered for the design of an helical antenna are:
1. Bandwidth 2. Gain 3. Impedance 4. Axial Ratio
2)What are the types of radiation modes of operation for an helical
The two types of radiation modes of operation possible for an helical antenna are:
1. 1ormal mode of operation 2. Axial mode of operation
3)List the applications of helical antenna
The applications of helical antenna are:
• It became the workhouse of space communications for telephone, television anddata, being employed both on satellites and at ground stations
• Many satellites including weather satellites, data relay satellites all
• It is on many other probes of planets and comets, including moon and mars, being used alone, in arrays or as feeds for parabolic reflectors, its circular polarization and highgain and simplicity making it effective for space application.
4)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 benormal if the dimensions of helix is small compared with the wavelength.
5)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.
6)How the axial mode is improved?
When the helix circumference D & spacing S of order of one wavelength. Axial mode of radiation is produced by raising helix circumference (c/G) of order of one wavelength & spacing is approximately of G/a.
7)What is the error introduced in direction finding of loop antenna?
Antenna or Vertical effect, 1ight effect or Aeroplane effect or polarization error.
8)What is an “array factor”?
It is apparent that the total field of an array is equal to equal field of asingle element positioned at the origin multiplied by a factor which is widelyreferred as array factor.
9)What are the advantages of binomial arrays?
• As HPBW increases and hence directivity decreases.
• For design of a large array, larger amp ratio of sources is required.
10)What is the disadvantage of a binomial array?
A maMor 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.
11)What is a binomial array?
The coefficients of binomial expansion represent relative amplitudes of the elements. Since the coefficients are determined from the binomialseries, expansion the array is known as binomial array.
12)Name and draw a frequency independent antenna
Log periodic antenna is a frequency independent antenna.It includes active region and reflective region.
13)What is YagiUda antenna?
It is an array of a driven element, a reflector and one or more directors.
14)What do you mean by parasitic element?
The passive elements which are not connected directly connected to the transmission line but are electrically coupled are called as parasitic elements.
15)What do you mean by driven elements?
Driven elements are an active element where the power from the transmitter is fed or which feeds the received power to the receiver.
16)Why folded dipole antenna is used in yagi antenna?
The folded dipole has high input impedance. If the distance between the driven and parasitic element is decreased, it will load the driven element so input impedance of driven element reduces. But this will be compensated.
17)What is beam antenna?
If three-element array are used then such a type of yagiuda is referred to as beam antenna.
18)What is a frequency independent antenna?
An antenna in which the impedance, radiation pattern and directivity remain constant as a function of frequency is called as frequency independent antenna. Eg, Log periodic antenna.
19)What is LPDA?
LPDA means log periodic dipole array. It is defined as an antenna whose electrical properties repeat periodically with logarithm of the frequency.
20)What are the different regions in log periodic antenna and how are they
• Inactive region ± L< l
• Active region ± L» l
• Inactive reflective region ± L>l
21)What are the applications of log periodic antenna?
• HF communication
• Television reception
• All round monitoring
22)What are the applications of Rhombic antenna?
• HF transmission and reception
• point to point communication.
23)Define rhombic antenna.
An antenna which consists of four straight wires arranged in the shape of diamond, suspended horizontally above the surface of the earth is called as a rhombic antenna. It is otherwise called as diamond antenna or traveling wave antenna.
24)What are the two types of rhombic antenna design?
• Maximum field intensity design
25)What are the limitations of rhombic antenna?
• It needs a larger space for installation
• Due to minor lobes transmission efficiency is low. 26)Define lens antenna?
An antenna, which collimates the incident divergent energy to prevent it from spreading in undesired directions, is called as lens antenna.
27)What are the different types of lens antenna?
• dielectric lens or H plane metal plate lens
• E plane metal plate lens antenna
28)What is a dielectric lens antenna?
Dielectric lens antennas are the antennas in which the traveling wave fronts are delayed by lens media
29)What are the drawbacks of lens antenna?
• Lens antennas are used only at higher frequencies (above 3 GHz) because at lower frequencies they become bulky and heavy. Lens antennas have excessive thickness at low frequencies.
• Costlier for the same gain and beam width in comparison with
30)What is biconical antenna?
The biconical antenna is a double cone antenna which is driven by potential, charge or an alternating magnetic field at the vertex. In this antenna both the cones face in the opposite direction.
31)What is Lunenburg lens?
The Lunenburg lens is a spherical symmetric delay type lens formed from a dielectric with index of refraction ‘n’ which varies as a function of radius .
32)What are the advantages of lens antenna?
• the lens antenna, feed and feed support do not block the aperture as
the rays are transmitted away from the feed
• It has greater design tolerance
• It can be used to feed the optical axis and hence useful in applications where a beam is required to be moved angularly with respect to the axis
33)Mention the uses of lens antenna?
• Unstepped dielectric lens is a wide band antenna as its shape does not depend on the wavelength and hence it can be used over a wide frequency range, however this is not true for the dielectric lens antenna which is frequency sensitive.
• Both reflectors and lens antenna are commonly used above 1000 MHz. Lens antenna is a microwave device. So it is preferred to be usually above 3000 MHz and not below it.
UNIT-5 RADIO WAVE PROPAGATION
1)Define Sky wave.
Waves that arrive at the receiver after reflection in the ionosphere is called sky wave.
2)Define Tropospheric wave.
Waves that arrive at the receiver after reflection from the troposphere region is called Tropospheric wave.(ie 10 Km from Earth surface).
3)Define Ground wave.
Waves propagated over other paths near the earth surface is called ground wave propagation.
4)What are the type of Ground wave.
Ground wave classified into two types.
• Space wave • Surface wave.
5)What is meant by Space Wave?
It is made up of direct wave and ground reflected wave. Also includes the portion of energy received as a result of diffraction around the earth surface and the reflection from the upper atmosphere.
6)What is meant by Surface Wave?
Wave that is guided along the earth’s surface like an EM wave is guided by a transmission is called surface wave. Attenuation of this wave is directly affected by the constant of earth along which it travels.
7)What is meant by fading?
Variation of signal strength occur on line of sight paths as a result of the atmospheric conditions and it is called .It can not be predicted properly.
8)What is meant by diversity reception?
To minimize the fading and to avoid the multi path interference the technique used are diversity reception. It is obtained by two ways.
• Space diversity reception.
• Frequency diversity reception.
• Polarization diversity.
9)Define Space diversity Reception.
This method exploits the fact that signals received at different locations do not fade together. It requires antenna spaced at least 100 l apart are referred and the antenna which high signal strength at the moment dominates.
10)Define frequency diversity Reception.
This method takes advantage of the fact that signals of slightly different frequencies do not fade synchronously. This fact is utilized to minimize fading in radio telegraph circuits.
11)Define polarization diversity reception.
It is used in normally in microwave links, and it is found that signal transmitted over the same path in two polarizations have independent fading patterns. In broad band dish antenna system, Polarization diversity combined with frequency diversity reception achieve excellent results.
12)What is meant by Faraday’s rotation?
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 Faraday’s rotation.
13)What are the factors that affect the propagation of radio waves?
• Curvature of earth.
• Earth’ s magnetic field.
• Frequency of the signal.
• Plane earth reflection.
14)Define gyro frequency.
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.
15)Define Magneto-Ions Splitting.
The phenomenon of splitting the wave into two different components (ordinary and extra-ordinary) by the earth’s magnetic field is called Magneto- Ions Splitting.
The lowest useful HF for a given distance and transmitter power is defined as the lowest frequency that will give satisfactory reception for that distance and power.It depends on
• The effective radiated power
• Absorption character of ionosphere for the paths between transmitter
• The required field strength which in turn depends upon the radio
noise at the receiving location and type of service involved.
17)Define Refractive index?
18)Define maximum Usable Frequency.
The maximum Frequency that can be reflected back for a given distance of transmission is called the maximum usable frequency (MUF) for that distance.
MUF = f
19)Define skip distance.
The distance with in which a signal of given frequency fails to be reflected back is the skip distance for that frequency.The higher the frequency the greater the skip distance.
20)Define Optimum frequency?
Optimum frequency for transmitting between any two points is therefore selected as some frequency lying between about 50 and 85 percent of the predicted maximum usable frequency between those points.
21)Define critical frequency?
For any layer, the highest frequency that will be reflected back for vertical incidence is݂
22)Name the possible modes of propagation?
• Ground wave propagation
• Sky wave propagation
• Space wave propagation.
23)What is Free space?
Free space is the space which does not interfere with the normal radiation and propagation of radio waves. In free space no magnetic or gravitational fields or solid bodies or ionized particles are assumed to exist.
24)What is diffraction?
The bending of the path of Electromagnetic waves around the sharp edges and corners of obstacles appearing in their path is known as diffraction.
25)What is Tropospheric Scatter?
Troposcatter is a mechanism by which propagation is possible by the scatter and diffracted rays. This scattering takes place in the tropospheric region.
• It is a whistling tone with gradually falling pitch.
• It occurs due to transient EM disturbances.
27)What are the types of Whistlers?
• Long Whistlers
• Short Whistlers
• 1oise Whistlers
The measurement of virtual height is normally carried out by means of an instrument called as ionosonde.
29)Define secant law?
is greater than F
c by a factor seci.This is known as secant law, which gives the maximumfrequency used for sky wave communication.
30)Define point to point propagation?
Electromagnetic waves of frequency more than 300MHz are reflected back from ionosphere but they penetrate it.This is otherwise called as Short wave propagation or point to point propagation.
31)Define cutoff frequency?
The frequency at which the attenuation of a waveguide increases sharply and below which a traveling wave in a given mode cannot be maintained. A frequency with a half wavelength that is greater than the wide dimension of a waveguide.