EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 PDF

EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Unit-1 - ANTENNA FUNDAMENTALS 1.1 Antenna An antenna is an electrical conductor or system of conductors or a combination of a dielectric and conducting system which can radiate electromagnetic energy...

EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Unit-1 - ANTENNA FUNDAMENTALS 1.1 Antenna An antenna is an electrical conductor or system of conductors or a combination of a dielectric and conducting system which can radiate electromagnetic energy into space and vice versa. An antenna or aerial or radiator is a system of elevator which couples or matches the transmitter or receiver to free space. An antenna is a structure has been designed to radiate or receive electromagnetic wave. [Antenna at transmitting end] [Antenna at receiving end] 1.2 Power Density EM waves are transmitted through a guiding medium frame one point to another and also the power and energy are associated with EM fields. The quality used to describe the power along with the EM waves is instantaneous poynting vector 1  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Where – instantaneous poyting vector – instantaneous electric field intensity – instantaneous magnetic field intensity For time varying fields, Average power density is obtained by integrating the ( ) over one period and dividing by the period. For time harmonic variations of the form are related by, The equation can also be written as, Average pointing vector or average power density From equations, First term of real part = average power density. The imaginary part represents reactive (stored) power density. The power density associated with EM fields are predominantly real and is referred to as radiation density. Average power radiated by an antenna is 1.3 Directivity Directivity (D) of an antenna is defined as the ratio of maximum radiation intensity of that antenna to the average radiation intensity. 2  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Generally directivity is expressed as Where 1 Total radiated power The directivity and gain are related by, K → efficiency factor = 1 1.4 Gain Gain of an antenna is the ratio of maximum radiation intensity in a given direction to the maximum radiation intensity from a reference antenna produced in the same direction with same input power. For isotropic antenna gain ( ) is, 3  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 1.4.1 Directive Gain In decibels, Both are radiating the same total power. 1.4.2 Power Gain ( It compares the radiated power density of the actual antenna and that of isotropic antenna on the basic of same input power to both. 1.5 Radiation Resistance (RR) OR (RA) It is defined as that fictitious resistance which when substituted in series with the antenna will consume the same power as the antenna radiated. 4  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 The total power loss is equal to the sum of ohmic and radiation loss, Where Radiation resistance depends on, Configuration of antenna Location of antenna with respect to ground Length and diameter of the conductor used 1.6 Input impedance OR Feed Point OR Driving Point Impedance: The antenna impedance at any particular reference point normally contains a reactive as well as a resistive component. The impedance of an antenna at a point where transmission line carrying R.F. power from the transmitter is connected is called as input impedance. 1.6.1 Self Impedance: Antenna impedance decides the maximum available power from transmitter to antenna and vice versa. Self impedance of antenna is, → Self resistance → Self reactance For lossless antenna Self impedance of an antenna is defined as its input impedance when all other antennas are completely removed from it. It is always positive. 1.6.2 Mutual Impedance: When two antennas are kept nearby mutual impedance comes into effect. Consider 2 coupled antennas and these two antennas are separated by a fraction of wavelength and they are parallel to each other. Let the current I1 in antenna 1 induced voltage V21 at the open terminals of antenna 2. Mutual Impedance = 5  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 If the source is shifted to antenna 2 and current of antenna 2 is inducing , Mutual Impedance is Using reciprocity theorem, two mutual impedances are equal, In mutual impedance two antennas are present. Actual impedance = self impedance + Mutual Impedance 1.7 Radiation Pattern Radiation patterns defined as a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space Co-ordinates. Two types of radiation pattern. Filed pattern Power pattern The figure below shows a symmetrical radiation pattern with number of radiation lobes. [Symmetrical radiation pattern] 1.7.1 Field Pattern If the radiation from the antenna is expressed in terms of field strength is called field pattern. The normalized field pattern is, 6  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 [Normalized field pattern] 1.7.2 Power Pattern If the radiation from the antenna is expressed in terms of power per unit area then it is called as power pattern. The normalized power pattern is defined as the ratio between radiation intensity to their maximum values. [Power pattern] 1.7.3 Lobes A radiation lobe is a portion of radiation pattern bounded by regions of relatively weak radiation intensity.. It includes 7  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Major lobes Minor lobes The region of higher radiation pattern is surroundings by the regions of lower radiation pattern are called lobes. Major lobe or main beam: The lobe along the direction of maximum radiation is called main lobe 1.7.3.1 Major Lobes Major lobe is also called as main lobes or main beams. It is the radiation lobe contained in the direction of maximum radiation. Some antennas have more than one main lobe. 1.7.3.2 Minor Lobes All the lobes except major lobes are called as minor lobes. Side lobe occupies the hemisphere in the direction of maximum radiation. Back lobe occupies the hemisphere to the opposite direction of maximum radiation. Side lobe Back lobe Side Lobe It is adjacent to the main lobe and occupies the hemisphere in the direction of the main lobe. Back Lobe It occupies the hemisphere in the direction opposite to the main lobe. 1.8 Beam Width (or) Beam Solid Angle (or) Beam Area Beam width is defined as the angle measured on the radiation pattern between the points where the radiated power has fallen to half of its maximum value (or) 0.707 time its maximum value. P1 and P2 are the half power points and the angle between P1 and P2 is known as beam width or half power beam width (HPBW) It can also be defined as the angular width of major lobe between the two directions at which the radiated power is one half of the maximum power. It is also expressed as the angle between the first two null lobes. BWFN = HPBW/2 Beam width is obtained by the product of the beam widths (HPBW) in E and H plane. 8  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Relation between D and In degrees, 1 radian = 57.3 Beam width and gain (or) directivity are inversely proportional, Factors affecting beam width are, Shape of the radiation pattern Wavelength Dimension of antenna. 1.9 Bandwidth It is defined as the range of frequencies over which the antenna maintains certain required characteristics like gain, radiation, polarization etc. The BW mainly depends on its impedance and pattern. At low frequency (λ/2 or less) the BW determine by impedance variation because pattern characteristics is incentive to frequency. Now under this condition BW is Where 1.10 Other Definitions Radian: One radian is defined as the plane angle with its vertex at the centre of the circle of radius r that is subtended by an arc whose length is r. Steradian: One steradian is defined as solid angle with its vertex at the centre of the sphere of radius r that is subtended by a spherical surface area equal to that of square with reach side of length r. 9  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Broadband Antenna: It is defined as the upper to lower cut-off frequency of acceptable range. Narrowband Antenna: It is defined as the percentage frequency difference l/n upper and lower frequency over the center frequency. Front to Back Ratio (FBR) HPBW: It is the angular width measured on the major lobe of radiation pattern between points where the radiated power has fallen to half of its maximum value. It is otherwise called as 3db bandwidth. FNBW: There are nulls present in the radiation pattern. Nulls are direction of zero radiation or minimum radiation. FNBW is defined as the beamwidth between first null in major lobe and first null in side lobes. 1.11 Adaptive Arrays Adaptive antenna arrays allow the beam to be continuously steered to direction to allow for maximum signal to be received and/ or the nulling of any interference. This is mainly used in smart antennas. In smart antenna beam direction can be estimated using direction of arrival (DOA)> and infinite number of patterns are adjusted in real time. 10  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 It provides optimal gain while simultaneously identifying, tracking and minimizing interfering signals. D/C → converts from RF to IF ADC → Converts analog to digital for further processing W’s → Contains amplification and phase factors. Polarization Polarization (or) plane of polarization of a radio wave can be defined as the direction in which the electric vector E is aligned diving the propagation of EM wave for one full cycle. magnitude vector → and the resultant EM wave are mutually perpendicular. An EM wave is said to be polarized (linearly polarized) if they have the same alignment in space. Similarly, if E is in horizontal plane, the wave is said to be horizontally polarized. If an antenna is vertical it will radiate vertically polarized waves and a horizontal antenna will radiated horizontally polarized waves. Polarization is same in transmitting and receiving antennas. If the amplitudes of the two linearly polarized waves are equal with a phase difference of 90 degree, then circularly polarized wave occurs. Amplitudes are not equal, elliptically polarized waves are produced. The undesired radiation from an antenna is called cross polarization. For linearly polarized antenna cross polarization is perpendicular to the desired radiation. 1.12 Retarded Potential The fields obtained after retardation time is known as retarded fields. The current carrying conductor consists of number of current elements connected in series. The vector potential can be obtained by the superposition of potentials due to various current element (IDL) and this potential is found at a distant point ‘P’ which is at a distance of ‘r’. Effect of current flow due to all current elements cannot be realized at the same time at a distance point ‘P’ because they travel at different varying distances. But it can be realized after a time interval equal to the time needed for the disturbance to propagate over the distance ‘r’. due to this a retardation concept is involved. 11  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Let I = Im sin t I = Instantaneous current (or) I at any instant of time. Im = maximum current Retardation (delay) is the current effective in producing a field at an earlier time from the given current element. It can be written as, → Retarded time R → distance travelled [I] → retarded current Retarded vector potential is, In general, For sinusoidal current element the retarded vector potential is, dv = ds.dl and J.ds = I Scalar potential in the form of r retarded scalar potential Where [ ] = = Retarded charge density. 12  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 1.13 Half Wave Dipole ( Antenna) or Half Wave Doublet The length of the antenna is half wavelength. Mainly used as an element of antenna array. It is made up of mental rod or thin wire. The physical length is It is symmetrical antenna in which 2 ends are at equal voltage with respect to mid point. Dipole is fed at the center having maximum current at the center. Overall length is 2h = L. Radiation pattern of half wave dipole: The current distribution is maximum at the center and zero at ends. Where The vector potential due to current element Idz is, Where R = Distance between Idz to dist point ’P’. Total vector potential is, Sub R = r-z cos R = r distance. For ant → L = 2h Z=h= = 13  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Consider only radiation term, The electric field expression for radiation field is, 14  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Power radiated by half wave dipole and its radiate resistance. The total radiated power, 15  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Directivity of half wave dipole is given below Directivity D is defined as the ratio of radiation intensity U (,) in a given direction (,) to the average value of the radiation intensity. Directivity is defined as the ratio of the maximum radiation intensity to the average radiation intensity. Directivity of half wave dipole antenna is D(,) = U (,)/ Uav = 4 U (,) / Prad D= (4 /λ2) Aem = (4 /λ2) 0.13 λ2 =1.63 1.14 Monopole ( It is also called Marconi antenna or quarter wave monopole. radiated power in all directions. antenna radiates power in a hemisphere surface. antenna consists of one half of a half wave dipole antenna located at a conducting ground plane. Length of the leg of monopole is. Along with it image, total length is dipole Total Radiated Power: Monopole radiates only half of power emitted by half wave dipole. 16  EC T64 / ANTENNAS AND WAVE PROPOGATION UNIT 1 Near and Fair Fields: Near fields or induction field or Fresnel field. It varies as square of distance.|r|