An antenna is a system of similar antennas oriented similarly to get greater directivity in a desired direction.
28.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 .
29.What are the types of array.?
a. Broad side array. b. End fire array c. Collinear array. d. Parasitic array.
30.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
31.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.
32. 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. Omnidirectional antenna.
33. 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.
36. Define beam width of major lobe?
It is defined the angle between the first nulls (or) it is defined as twice the angle between the first null and the major lobe maximum direction.
39.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 be eliminated completely or reduced to minimum desirable level compared to main lobes.
40. Define power pattern.?
Graphical representation of the radial component of the pointing vector Sr at a constant radius as a function of angle is called power density pattern or power pattern.
41. 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.
42. What is meant by identical Point sources.?
Similar point sources with equal maximum amplitudes are called identical point sources.
43. 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.0
44.What is the advantage of pattern multiplication? ._
Useful tool in designing antenna ._It approximates the pattern of a complicated array without making lengthy computations
45.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.
46.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.
47.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
48.What is the difference between isotropic and nonisotropic source ._ Isotropic source radiates energy in all directions but non-isotropic source radiates energy only in some desired directions. ._Isotropic source is not physically realizable but non-isotropic source is physically realizable.
49.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.
50. 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
51.What is a Short Dipole?
Ans: 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 throughtout 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.
52.How radiations are created from a short Dipole?
Ans: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.(See Figure from John. D .Kraus Book)
53.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 wave length maximum specified for a short dipole.Elemental dipole is also called as elementary dipole,elementary doublet and hertzian dipole.
54.What is a Infinitesimal Dipole? When the length of the short dipole is vanishingly 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.
55.Why a short dipole is called a 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 Oscilllating electric dipole.
56.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 throughtout 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.
57.Define induction field
The induction field will predominate at points close to the current element ,where the distance from the centre 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.
58.Define Radiation field
The radiation field will be produced at a larger distance from the the current element ,where the distance from the centre of the dipole to the particular point is very large.It is also called as distant field or far field.
59.At what distance from the dipole is the induction field equal to the radiation field?
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.
60.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.
62.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.
63.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
65.What is the radiation resistance of a half wave dipole
The radiation resistance of a half wave dipole is given by Rr=73 ohm
66.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)
67.Give an expression of radiation resistance of a small loop Radiation resistance of a small loop is given by Rr=31,200 (A/_ 2)2
68.How to increase the radiation resistance of a loop antenna
The radiation resistance of a loop antenna can be increased by:
