ElectroMagnetic Theory Concepts & Solved Numericals

Consider a plane wave incident on a short dipole. The wave is assumed to be linearly polarized with electric field in the y direction. The current in the dipole is assumed constant and in the same phase over its entire length, and the terminating resistance is assumed equal to the dipole radiation resistance. The effective aperture of this dipole is given by A e = 0.119 λ The directivity is found to be D = 4ΠA e / sq (λ)

This page contains basic questions and short notes on some of the keywords on antenna theory: 96) What is Antenna matching? When the antenna is receiving with a load resistance matched to the antenna radiation resistance, maximum power is transferred to the load and the power is also re-radiated from the dipole. This is called antenna matching. 97) 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 throughtout the entire length of the dipole. 98) How are fields created from short dipole / oscillating 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 spontaneou

This page contains basic questions and short notes on some of the keywords on antenna theory: Aperture represents the area of the antenna confining the effective radiations. The various types of antenna apertures are: Effective aperture. Scattering aperture. Loss aperture. Collecting aperture. Physical aperture. Effective aperture (A e ): It is the area over which the power is extrated from the incident wave and delivered to the load is called effective aperture. Scattering aperture (A s ): It is the ratio of the re radiated power to the power density of the incident wave. Loss aperture (A l ): It is the area of the antenna which dissipates power as heat. Collecting aperture (A c ): It is the addition of above three apertures. Physical aperture (A p ): This aperture is a measure of the physical size of the antenna. The ratio of the effective aperture to the physical aperture is the aperture efficiency. i.e. Aperture efficiency = η ap = A e / A p

This page contains basic questions and short notes on some of the keywords on antenna theory: 91) Define Refractive index? It is defined as n = c / V p where n = √ε r 92)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 cr sec φ i 93) 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. 94) 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. 95) What is wave impedance? η = η o / √ 1 - (f c - f) η = 377 / √ 1 - (f c - f)

This page contains basic questions and short notes on some of the keywords on antenna theory: 86) 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. 87) Define gyro frequency? Frequency whose period is equal to the period of an electron in its orbit under the influence of the earths magnetic flux density B. 88) Define critical frequency? For any layer, the highest frequency that will be reflected back for vertical incidence is f cr = 9 √N max 89) Define Magneto-Ions Splitting? The phenomenon of splitting the wave into two different components (ordinary and extra-ordinary) by the earths magnetic field is called Magneto-Ions Splitting. 90) Define LUHF? 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 radi

This page contains basic questions and short notes on some of the keywords on antenna theory: 81) 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 three ways: Space diversity reception. Frequency diversity reception. Polarization diversity. 82) Define Space diversity Reception? This method exploits the fact that signals received at different locations do not fade together. It requires antennas spaced at least 100λ apart are preferred and the antenna which high signal strength at the moment dominates. 83) 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. 84) Define polarization diversity reception? It is used in normally in microwave links, and it is found that signal transmitted

This page contains basic questions and short notes on some of the keywords on antenna theory: 76) 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. 77) 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. 78) What is meant by fading? Fading is variation of signal strength occur on line of sight paths as a result of the atmospheric conditions. It can not be predicted properly. 79) What are the type of fading? Two types: Inverse fading. Multi path fading. 80) What is inverse and multi path fading? Inverse bending may transform line of sight path into an obstructed one. Multi path fading is caused by

This page contains basic questions and short notes on some of the keywords on antenna theory: 71) List the applications of helical antenna? The applications of helical antenna are: It became the workhouse of space communications for telephone, television and data, being employed both on satellites and at ground stations. Many satellites including weather satellites, data relay satellites all have helical antennas. 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 high gain and simplicity making it effective for space application. 72) Define Sky wave? Waves that arrive at the receiver after reflection in the ionosphere is called sky wave. 73) Define Tropospheric wave? Waves that arrive at the receiver after reflection from the troposphere region is called Tropospheric wave (i.e. 10 Km from Earth surface). 74) Define Ground wave? Waves propa

This page contains basic questions and short notes on some of the keywords on antenna theory: 66) What are Electrically large loop antennas? Electrically Large loop antennas is one in which the overall length of the loop approaches the wavelength. 67) 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 as directional antennas for radio wave navigation. ™ It is used to estimate the direction of radio wave propagation. 68) What are the parameters to be considered for the design of an helical antenna? The parameters to be considered for the design of an helical antenna are: Bandwidth. Gain. Impedance. Axial Ratio. 69) What are the types of radiation modes of operation for an helical antenna? The two types of radiation modes of operation possible for an helical antenna are: Normal mode of operation. Axial mode of operation. 7

This page contains basic questions and short notes on some of the keywords on antenna theory: 61) 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). 62) Give an expression of radiation resistance of a small loop? Radiation resistance of a small loop is given by R r = 31,200 ( A / λ 2 ) 2 63) 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. 64) What are the types of loop antennas? Loop antennas are classified into: Electrically Small (Circumference < λ / 10 ) Electrically Small (Dimension comparable to λ ) 65) What are Electr

This page contains basic questions and short notes on some of the keywords on antenna theory: 56) Give the expression for the effective aperture of a short dipole? The effective aperture of a short dipole is given by A e = 0.119 λ 2 . 57) 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. 58) 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. 59) Give the expression for the effective aperture of a Half wave Dipole? The effective aperture of a half wave dipole is given by A e = 0.13 λ 2 . 60) What is the radiation resistance of a half wave dipole? The radiation resistance of a half wave dipole is given by R r = 73 ohm.

This page contains basic questions and short notes on some of the keywords on antenna theory: 51) 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 [I]=I o exp( jw '  ( t - r / c ) ) Where w ' r / c is the phase retardation. 52)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

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. 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. A short dipole that does have a uniform current is known as elemental dipole. Such a dipole will generally be considerably shorter than the tenth wave length maximum specified for a

This page contains basic questions and short notes on some of the keywords on antenna theory: 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 non-isotropic 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

This page contains basic questions and short notes on some of the keywords on antenna theory: 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 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. While the total phase pattern is the sum of the phase patterns of the individual source pattern and array pattern. 44) What is the advanta

This page contains basic questions and short notes on some of the keywords on antenna theory: 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. 37) List out the expression of beam width for broad side array and end fire array? For broad side array the expression for beam width between the first nulls is given by, BWFN = ( ( + / -) 2 λ / n d ) For End fire array the expression for beam width between the first nulls is given by, BWFN = ( ( + / -) 2 ( 2 λ / n d ) ) 1/2 38) Differentiate broad side and End fire array? In Broad side array antennas are fed in phase δ = 0, where as in end fire arrays the antenna elements are fed out of phase i.e. δ = - β d. In broad side array the maximum radiation is perpendicular to the direction of array axis, where as in case of end fire array the maximum radiation is directed along the array axis.

This page contains basic questions and short notes on some of the keywords on antenna theory: 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. For end fire array δ = - β d where β = 2Π / λ and d = distance between the elements. 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. 34) What is the condition on phase for the end fire array with increased directivity? When δ = - β d, produces maximum field in the direction φ = 0 but dies not give th

This page contains basic questions and short notes on some of the keywords on antenna theory: 26) What is point source? It is the waves originate at a fictitious volume less emitter source at the center ‘O’ of the observation circle. 27) What is meant by array? 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? Broad side array. End fire array Collinear array. 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

This page contains basic questions and short notes on some of the keywords on antenna theory: 21) What is meant by cross field? Normally 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. 22) Define axial ratio? The ratio of the major to the minor axes of the polarization ellipse is called the Axial Ratio (AR). 23) What is meant by Beam Area? The beam area or beam solid angle or WA of an antenna is given by the normalized power pattern over a sphere. WA = ò ò4p Pn ( q,f ) dW Where dW = Sin q dq.df 24) 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. 25) State Poynting theorem? It states that the vector product of electric field intensity vector

This page contains basic questions and short notes on some of the keywords on antenna theory: 16) What is meant by reciprocity Theorem? If an e.m.f is applied to the terminals of an antenna no.1 and the current measured at the terminals of the another antenna no.2, then an equal current both in amplitude and phase will be obtained at the terminal of the antenna no.1 if the same emf is applied to the terminals of antenna no.2. 17) What is meant by isotropic radiator? A 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. 18) Define gain? 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 Maximum radiation intensity from test antenna (G)= Maximum radiation intensity from the reference antenna

This page contains basic questions and short notes on some of the keywords on antenna theory: 11) 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 atleast one full cycle.Also polarization can also be defined the physical orientation of the radiated electromagnetic waves in space. The polarization are three types. They are Elliptical polarization ,circular polarization and linear polarization. 12) 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 FBR = Power radiated in desired direction / power radiated in the opposite direction. 13) 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. Antenna efficiency = Power radiated / Total input Power 14) What is radi

This page contains basic questions and short notes on some of the keywords on antenna theory: 6) What are the different types of aperture? Effective aperture. Scattering aperture. Loss aperture. Collecting aperture. Physical aperture. 7) 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 (Ae): It is the area of the antenna which dissipates power as heat. Collecting aperture (Ae): It is the addition of above three apertures. Physical aperture (Ap): This aperture is a measure of the physical size of the antenna. 8) Define Aperture efficiency? The ratio of the effective aperture to the physical aperture is the aperture efficiency. i.e Aperture efficiency = hap = Ae / Ap (dimensionless). 9)

This page contains basic questions and short notes on some of the keywords on antenna theory: 1) 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. 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) 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. 4) Define Beam efficiency? The total beam area (WA) consists of the main beam area (