Antenna and Wave Propagation Dr. A .Bharathi Electronics and Communication Engineering Department, UCE(A), Osmania University. AM Transmitter Tower (The tower is the antenn Antennas in Wireless Communication Systems Modulating Signal Transmitter Modulator Amplifier Carrier Signal Impedance Matching Network** Lossless Receiver RF Amplifier *BPF Mixer IF Filter and Amplifier Demod ulator LO Figure 2 Radio Signal Transmission and Reception Display device/ speaker •To understand the various antenna parameters give insight of the radiation phenomena •To have thorough understanding of radiation characteristics of different types of antennas. •To study the characteristics of array antennas having directional radiation characteristics. •To give insight on aperture antennas and modern antennas. •To understand the concepts of wave propagation and create awareness about the different types of propagation of radio waves at different frequencies. Outcomes •The student acquires knowledge about the basic antenna parameters and radiation concepts. •The student learns to analyze wire antennas in detail. •The student attains engineering fundamentals to analyze and design antenna arrays. •The student can classify, analyze and design aperture and modern antennas. •The student gains ability to identify and explain different modes of Unit I Fundamentals of Antenna theory: Principle of radiation, Basic Antenna Parameters – Patterns, Beam Area, Radiation Intensity, Beam Efficiency, Directivity, Gain, Antenna Apertures, Effective Height, Illustrative Problems. Retarded Potentials – Helmholtz Theorem Thin Linear Wire Antennas – Radiation from Small Electric Dipole, Quarter wave Monopole and Half Wave Dipole – Current Distributions, Near field and far field Components, Radiated Power, Radiation Resistance, Beamwidth, Directivity, Effective Area and Effective Height. Loop Antennas – Introduction, Small Loop, Comparison of Far Fields of Small Loop and Short Dipole. Unit II Antenna Arrays: Basic two element array, N element uniform linear array, Pattern multiplication, Broadside and End fire array, Planar array, Concept of Phased arrays, Adaptive array, Basic principle of antenna Synthesis- Binomial array, Tschebysev array. Unit III Practical Antennas: Yagi-uda antenna, V- Antenna, Rhombic antenna, Travelling wave antennas, Microstrip antennas – Introduction, Features, Advantages and Limitations, Rectangular Patch Antennas – Geometry, Design equations and Characteristics. Unit IV Aperture and Modern Antennas: Reflector Antennas – Introduction, Flat Sheet and Corner Reflectors, Paraboloidal Reflectors – Geometry, Pattern Characteristics, Feed Methods, Reflector Types – Related Features, Illustrative Problems. Horn Antennas – Types, Fermat’s Principle, Radiation from sectoral and pyramidal horns, Design Considerations of Pyramidal Horns, Reconfigurable antenna, Active antenna, Dielectric antennas, Electronic band gap structure and applications. Unit V Wave propagation: Ground wave propagation. Space and surface waves, Tropospheric refraction and reflection. Sky wave propagation – Virtual height, Suggested Reading: Constantine A. Balanis, Modern Antenna Handbook, A John Wiley & Sons, Inc., Publication, 2008. John D. Kraus, Ronald J. Marhefka and Ahmed S.Khan, “Antennas for All Applications” 3rd Edition, Tata McGraw- Hill publishing company Limited, New Delhi, 2006. K.D.Prasad, “Antennas and Wave Propagation”, Khanna or Satya Publications. Unit I Fundamentals of Antenna theory: Principle of radiation, Basic Antenna Parameters – Patterns, Beam Area, Radiation Intensity, Beam Efficiency, Directivity, Gain, Antenna Apertures, Effective Height, Illustrative Problems. Retarded Potentials – Helmholtz Theorem Thin Linear Wire Antennas – Radiation from Small Electric Dipole, Quarter wave Monopole and Half Wave Dipole – Current Distributions, Near field and far field Components, Radiated Power, Radiation Resistance, Beamwidth, Directivity, Effective Area and Effective Height. Loop Antennas – Introduction, Small Loop, Comparison of Far Fields of Small Basic Types of Antennas Physical structure of the antenna. Wire antennas, Aperture antennas, Reflector antennas, Lens antennas, Microstrip antennas, Array antennas Frequency of operation Very Low Frequency (VLF), Low Frequency (LF),Medium Frequency (MF), High Frequency (HF), Very High Frequency (VHF), Ultra High Frequency (UHF), Super High Frequency (SHF), Microwave and mm wave Application Point-to-point communications, Broadcasting applications, Radar communications, Satellite communications Physical structure Paraboloid Parabolic cylinder Shaped Stacked beam Convex Lens Concave Lens Antennas for Various Applications MW Radio – Frequency: 530 to 1620 kHz (use λ/4 monopole antenna) Cell Phones – CDMA, GSM900(890-960), GSM1800(1710-1880), 3G(1920-1980,2110-2170), 4G, Wi-Fi/Bluetooth (use monopole, normal mode helical, microstrip antenna, etc.) Cell Towers (use monopole, dipole, microstrip antenna arrays, etc.) Omni or Sectoral coverage Satellite and Defense Communications (use microstrip, horn, spiral, helical, reflector, Yagi-Uda, log-periodic antennas, etc.) What is an Antenna? An antenna is a transition device or transducer between a guided wave and a free space wave, or vice versa. Reciprocity • Antenna characteristics are essentially the same regardless of antenna is sending or receiving electromagnetic energy. • An antenna ability to transfer energy form the atmosphere to its receiver with the same efficiency with which it transfers energy from the transmitter into the atmosphere. Conditions for Radiation 1. Time varying current (or) Acceleration and Deceleration of charge 2. Large separation between conductors 3. It is a high frequency phenomenon and the radiation is always perpendicular to the direction of current flow E and H fields in dipole antenna Current distribution Thin wire transmission line Linear dipole Flared transmission Terminology of Antenna Input Impedance and VSWR of Antenna ZA Radiation Pattern Mathematical or graphical representation of radiation properties of antenna as a function of space coordinates. E(,) or P(,) Properties of Radiation Pattern Plotted with normalized magnitude values (dB/Linear scale) Shape is independent of distance r Depends on antenna polarization 3D Radiation Pattern Side Lobes Transmit mode: Wastage of Radiated power Receive mode : Receive from undesired directions The optimum tradeoff between side lobes, gain, and beam width is an important consideration for choosing or designing radar antennas 2 D Radiation pattern Polar Plot Plot Rectangular/Linear Antenna Beamwidth Beam width of a pattern is the angular separation between two identical points on opposite side of the pattern maximum Half-Power Beamwidth (HPBW ) is defined as: “In a plane containing the direction of the maximum of a beam, the angle between the two directions in which the radiation intensity is one-half value of the beam.” The angular separation between the first nulls of the pattern is referred to as the First-Null Beamwidth (FNBW ) HPBW Calculations Example 1 Find the (HPBW) of an antenna having E() = cos2 for 0o < < 90o Solution E() at half power 0.707 = cos2 = 33o BW = 66o Example 2 Find the (HPBW) of an antenna having P() = cos Example 3 Find the (HPBW) of an antenna having P() = sin 2 D Radiation pattern Horizontal Pattern E() E() Horizontal dipole radiation pattern ???? Vertical Pattern 3D Pattern Principal Plane patterns E Plane : Plane containing E field and the direction of maximum radiation. H plane : Plane containing H field and the direction of maximum radiation. E plane and H plane Patterns for a vertical dipole antenna?? 3-D Radiation Pa` ttern of Antenna Isotropic Radiation Pattern D = 1 = 0dB Omni-Directional Radiation Pattern of λ/2 Dipole Antenna D = 1.64 = 2.1dB Directional Radiation Pattern of Microstrip Antenna Array D = 500 = 27dB Field Regions Near Antenna R1=0.63sqrt(D3/λ) 2 Plane Angle and Solid Angle One radian is defined as the plane angle with its vertex at the centre of a circle of radius r that is subtended by an arc whose length is r. One steradian is defined as the solid angle with its vertex at the centre of a sphere of radius r that is subtended by a spherical surface area equal to that of a square with each side of length r. Radiation Power Density Power radiated per unit surface area from the antenna surface is called radiation power density (w/m2). Instantaneous Poynting vector Average poynting vector or Average power density Real (calculated over one time period): Imaginary part is eliminated Analogous to Ohm’s law : P=1/2 VI* Instantaneous Total Power = Integration of normal component of poynting vector (power density) over the entire surface Average (total) radiated power Radiation Intensity Radiation intensity in a given direction is defined as the power radiated from the antenna per unit solid angle. Radiated Power Calculations Beam Solid angle or Beam Area Asymmetric pattern Directivity`of Antenna Directivity of an antenna is the ratio of radiation intensity in the direction of maximum radiation to the radiation intensity averaged over all the directions. Um DUo Uo Theta in radians Example: For Infinitesimal Aperture Concept Effective Aperture Area, Loss Aperture , Aperture Effective Length Scattering Aperture, Collecting Directivity and G ain of DirectivityAntenna of Large Antenna Directivity of Small Antenna ` D 41253 D 1/ E H Directivity is proportional to the Effective Aperture Area of Antenna Gain = η Directivity Where η is Radiation Efficiency of Antenna η = Prad/ Pin Wave Polarization At z=0 Equation of the curve which the tip of E field traces at specific point Case 1: Linear polarization 0 ratio of fields can be anything Ex0=0 , Ey0=0, Ex0=Ey0 Animation makes it clear Slant linear polarization Case 2: Circular polarization /2 Both field components have equal amplitude Eo Case 3 : Elliptical polarization /2 Both field components have unequal amplitude LHC P LHCP RHC P Clockwis e Anti Clockwise Partial Directivity and Partial Gain Calibration may be in dB relative to max. for that antenna, or relative to isotropic (dBi) or half wave dipole (dBd). Gain (dBi) = Gain (dBd) + 2.14 dB Polarization of Antenna Orientation of radiated electric field vector in the main beam of the antenna Microstrip Antenna in Different Polarizations Wave is Linearly Polarized Wave is Circularly Polarized Phase shift realized with delay line Co-Polarization The desired polarization (the main polarization) (CO-POL) Cross-Polarization The undesired orthogonal polarization (CROSS-POL). 55 Co-polarized antenna pattern Relative Power Single feed Phase shift realized with 900 hybrid (branch line coupler) XPD X-polarized patttern Azimuth Angle Antenna Bandwidth Bandwidth: Range of frequencies within which performance of antenna, where the characteristics (ZA, SLL, G, Polarization, efficiency, beamwidth) conforms to accepted value. Notation: fH:fL , (fH-fL )/fo Impedance Bandwidth, Pattern BW, Axial ratio BW, Gain BW Axial Ratio of Antenna , circular polarization , elliptical polarization , linear polarization Axial Ratio Bandwidth: Frequency range over which AR < 3 dB Axial Ratio Plot of Circularly Polarized MSA Bandwidth for AR < 3dB = 380MHz (13%) Procedure for Antenna Analysis 1. 2. 3. 4. Knowledge of current distribution Find A Find H using H= 1/(∇xA) Find E using Maxwell’s curl equation jE= ∇x H Retarded Vector I = I oej t Potential I = Io e j (t-td) td = r/c j( t- r) Since r/c = r I = I e o If I is considered in the expression of vector potential it is called retarded vector Step 2: Find A Infinitesimal small dipole Step 3: Find H using H= 1/(∇xA) Step 4 Find E using Maxwell’s curl equation jE= ∇x H Fields in the far field region 1. What is the phase difference between fields and current 2. What is the ratio between E and H 3. Does the fields radiated by antenna has identical properties as uniform plane wave (TEM wave) 4. What is the polarization of the antenna. 5. What is power radiated by antenna Power radiated by Hertizian dipole Directivity Effective Aperture Area and Effective length Analysis of Short Dipole R=r CALCULATE POWER, RADIATION RESISTANCE. DIRECTIVITY AND Analysis of Half wave Dipole Step 1 : Current Distribution Step 3: Find H using H= 1/(∇xA) Far field term Step 4: Find E using E/ H Power radiated by half wavelength dipole antenna CALCULATE RADIATION RESISTANCE, DIRECTIVITY, EFFECTIVE APERTURE AREA AND EFFECTIVE LENGTH D= 1.64 (2.15dB) 0.62l Ae = 0.13λ2 le = MONOPOLE ANTENNA All practical antenna at low frequency are monopole antenna Fields radiated by monopole antenna are same as dipole antenna. It radiates only in hemisphere. Image Theory Image Theory Same current distribution as half wavelength dipole CALCULATE POWER RADIATED, RADIATION RESISTANCE, DIRECTIVITY, EFFECTIVE APERTURE AREA AND EFFECTIVE Unit II Antenna Arrays Basic two element array, N element uniform linear array, Pattern multiplication, Broadside and End fire array, Planar array, Concept of Phased arrays, Adaptive array, Basic principle of antenna Synthesis Binomial array, Tschebysev array. Array of Point Sources Usually the radiation pattern of a single element is relatively wide, and provides low values of relative gain. In many applications it is necessary to design antennas with very directive characteristics (very high gain) to meet the demands of long-distance communication. This can only be accomplished by increasing the electrical size of the antenna. Another way is to form an assembly of radiating elements in an electrical and geometrical configuration: ARRAY. In an array of identical elements there are five factors that can be used to shape the overall pattern of the antenna, viz.: Five factors that can be used to shape the overall pattern of the antenna 1. 2. 3. 4. 5. The geometrical configuration of the overall array (linear, circular, rectangular, spherical etc. ) The relative displacement between the elements The excitation amplitude of the individual elements The excitation phase of the individual elements The relative pattern of the individual elements In most cases, the elements of an array are identical (though this is not necessary). The elements may be of any form e.g. wires, apertures etc. The total field of the array is determined by the vector addition of the fields radiated by the individual elements. This assumes that the current in each element is the same as that of the isolated elements. To provide very directive patterns, it is necessary that the fields from individual elements interfere constructively (add) in the desired direction and interfere destructively (cancel each other) in the remaining space. The simplest and most practical array is formed by placing the elements along a line: Two-Element Array Electric field of horizontal dipole in the far-zone Let us represent the electric fields in the far-zone of the array elements in the form The far-field approximation of the two-element array problem: Assumptions: The array elements are • Identical, i.e., • Oriented in the same way in space (they have identical polarization), i.e., • excitation is of the same amplitude, i.e., Therefore for two element array of horizontal hertz antennas the total field is Then, the total field is: The normalized AF, The normalized field pattern of the array is expressed as: PATTERN MULTIPLICATION Since, the array factor does not depend on the directional characteristics of the individual elements, it can be formulated by replacing the actual elements with isotropic (point) sources assuming that each point source has amplitude, phase and location of the corresponding element it is replacing. The field pattern of an array of non-isotropic but similar point sources is the product of the pattern of the individual source and the pattern of an array of isotropic point sources having the same locations, relative amplitudes and phase as the non-isotropic sources. Example 1: An array consists of two horizontal infinitesimal dipoles located at a distance d = λ / 4 from each other. Find the nulls of the total field, if the excitation magnitudes are the same and the phase difference is: a) β = 0; b) β =/2; c) β = −/2 The element factor En(θ,φ) does not depend on β, and it produces the same null in all three cases. Since En(θ,φ) =|cosθ|, the null is at θ1 = / 2. The AF depends on β and produces different results in the 3 cases: a) β = 0 Array factor for various values of d (0): d = λ/2 A solution with a real-valued angle does not exist. In this case, the total field pattern has only 1 null at θ =90°. d=λ b) β = /2 The equation does not have a solution. The total field pattern has 2 nulls: θ1 = 90° and θ2 = 0° c) β = −/2 The total field pattern has 2 nulls: θ1 = 90° and at θ2 =180°. N-Element Linear Array: Uniform Amplitude and Spacing • An array of identical elements all of identical magnitude and each with a progressive phase referred to as a Uniform Array The Array Factor (AF) is given by: AF 1 e j kd co s N AF e e j 2 kd co s j n 1 kd co s N n 1 j n 1 e j N 1 kd co s kd co s n 1 N e is AF e j n 1 …..(1) n 1 kd co s Multiply both sides of (1) bye j , subtract the original equation from the resulting equation , e j N 1 AF j e 1 j N 1 AF e 2 N si n 2 si n 1 2 …..(2) • The phase factor exp[ j(N −1)ψ / 2] represents the phase shift of the array’s phase centre relative to the origin, and it would be one if the origin coincides with the array centre. • Neglecting the phase factor gives N si n 2 AF 1 si n 2 For small values of ,: …..(3) N si n 2 AF 2 …..(4) To normalize equation (3) or (4), we need the maximum of the AF. Re-write equation (3) as: A F m ax N f x Equations 3 and 4 are written in normalized form as: N si n 1 2 A F n N 1 si n 2 si n N x N si n x The function f(x) has its maximum at x = 0, , …, and the value of this maximum is fmax =1. …..(5) N si n 1 2 A F n N 2 …..(6) Nulls of the Array: Equations 5 and 6 are set equal to zero. That is, N si n 2 N 2 0 n 2n …..(7) n co s 1 n 1, 2 , 3, .... N 2 d n N , 2 N , 3 N , .... because for these values of n, equation 5 attains its maximum value as it reduces si n 0 0 to form. n determine the order of the nulls (first, second, etc.). The values of For a zero to exist, the argument of arccosine must be between –1 and +1. nulls depend on d and .The maximum of equation 5 occurs when, 2 1 2 kd co s | m m m co s 1 2 m 2 d AF has only one maximum and occurs when, m 0 0 That is, the observation angle that makes …..(8) m 0 , 1, 2 , m co s 1 2 d …..(9) Secondary maxima/ Side lobe maxima Maxima of first sidelobe The 3-dB point for the array factor of equation 6: N 2 N 2 kd co s | 1 .3 9 1 h h co s 1 2 d 2 .7 8 2 N which can also be written as h 2 si n 1 2 d 2 .7 8 2 N For large values of d it reduces to: …..(10) The half power beamwidth is: h 2 m h …..(11) Broadside Array • Maximum radiation of an array directed normal to the axis of the array. 90 • Maximum of the array factor occurs when (equations 5 and 6): • For broadside array, kd co s 0 kd co s | 9 0 0 0 • For broadside pattern, all elements should have same phase and amplitude excitation. • To ensure that there are no maxima in other directions, which are referred to as grating lobes, the separation between the elements should not be equal to multiples of a wavelength 0 when d=/4,N10 d=,N10 End-Fire Array • An end-fire array is an array, which has its maximum radiation along the axis of the array (θ =0°, 180°). • To direct the maximum toward θ =0°: kd co s | 0 kd • To direct the maximum toward θ =180°: kd co s | 1 8 0 kd If the element separation is a multiple of a wavelength (d=n, n1,2,3,…), then there exists a maxima in the broadside directions. Phased (Scanning) Array The 0th order maximum (m=0) of AFn occurs when kd co s 0 • This gives the relation between the direction of the main beam θ0 and the phase difference β . The direction of the main beam can be controlled by the phase shift β . This is the basic principle of electronic scanning for phased arrays. • The scanning must be continuous. That is why the feeding system should be capable of continuously varying the progressive phase β between the elements. This is accomplished by ferrite or diode shifters (varactors). Example: Values of the progressive phase shift β as dependent on the direction of the main beam θ0 for a uniform linear array with d = λ/4. kd co s 0 2 co s 0 0 0 60 120 -90 -45 45 180 90 The HPBW of a scanning array is obtained with β = −kdcosθ0: 2 .7 8 2 h 1, 2 co s 1 2 d N The total beamwidth is H PBW H PBW co s 1 2 d kd h1 h 2 co s 0 2 .7 8 2 N 1 co s 2 d kd co s 0 2 .7 8 2 N k 2 H P B W co s 1 co s 0 2 .7 8 2 2 .7 8 2 1 co s co s 0 N kd N kd If L is length of the array: N H P B W co s 1 co s 0 L d d 1 0 .4 4 3 co s co s 0 0 .4 4 3 L d L d These equations can be used to calculate the HPBW of a broadside array, too (θ0 =90°=const ). However, they are not valid for end-fire arrays. N-Element Linear Array: Directivity 1. Broadside Array: 0 The radiation intensity can be written as: U A F n 2 N si n kd co s 2 N kd co s 2 Z The directivity is D 0 4 U m ax Pr a d U N 2 2 si n Z Z kd co s m ax U 0 Since the array factor is normalized, the numerator is unity and occurs at9 0 . 2 The Average Radiation Intensity can be written as: U 0 1 Pr a d 4 1 2 0 1 2 U 4 0 0 N si n kd co s 2 N kd co s 2 si n d d 2 si n d changing the variable Z N 2 dZ U 0 1 N kd N kd 2 N kd 2 kd co s N 2 kd si n d si n Z Z 2 dZ U 0 for a large array N kd infinity. 1 N kd N kd 2 N kd 2 2 L ar g e si n Z Z 2 dZ the above equation can be approximated by extending the limits to since si n Z Z 2 dZ The directivity is then, For End fire array Do = ? overall length of the array L N 1 d For large L ( L much larger than d ) N-Element Linear Array: Uniform Spacing, Non-uniform Amplitude • The most often used Broad-Side Arrays (BSAs), are classified according to the type of their excitation amplitude: 1. The uniform BSA – relatively high directivity, but the side-lobe levels are high; 2. Dolph–Tschebyscheff BSA – for a given number of elements maximum directivity is next after that of the uniform BSA; side-lobe levels are the lowest in comparison with the other two types of arrays for a given directivity; 3. Binomial BSA – does not have good directivity but has very low side-lobe levels (when d = λ/2, there are no side lobes at all). Even number (2M) of elements, located symmetrically along the z-axis, with excitation symmetrical with respect to z = 0. For a broadside array (β =0), Odd number (2M+1) of elements, located symmetrically along the z-axis, For a broadside array (β =0), Binomial Array 1 x m 1 1 m 1 x m 1 m 2 2! x 2 m 1 m 2 m 3 3! The positive coefficients of the series expansion: 1 m=1 1 m=2 1 m=3 1 m=4 1 m=5 m=6 1 2 3 4 5 1 1 3 6 10 1 4 10 Binomial BSA – when d = λ/2, there are no side lobes at all 1 5 1 x 3 ... An approximate closed-form expression for the HPBW with d = λ/2: H PBW 1 .0 6 N 1 1 .0 6 2L 1 .7 5 L The directivity with spacing d = λ/2 is D 0 1 .7 7 N 1 .7 7 2L 1 For a 10 element binomial array with a spacing of λ/2 between elements, determine the half power beam width and the maximum directivity(in dB). Dolph-Tschebyscheff Array A compromise between uniform and binomial arrays. Excitations coefficients are related to Tschebyscheff polynomials. A Dolph-Tschebyscheff array with no sidelobes reduces to the binomial design. The excitation coefficients for this case, as obtained by both methods would be identical. Array Factor for symmetric amplitude excitation: • Summation of M or (M+1) cosine terms. • Largest harmonic of the cosine terms is one less than the total no. of elements of the array. • Each cosine term, whose argument is an integer times a fundamental frequency, can be rewritten as a series of cosine functions with the fundamental frequency as the argument. If we let z co s uabove equations can be rewritten as: And each is related to a Tschebyscheff (Chebyshev) polynomial Tm z T m z 2 zT m 1 z T m 2 z 124 These relations between cosine functions and Tschebyscheff polynomials are valid only in the range: 1 z 1 co s m u 1 Tm z 1 for 1 z 1 The recursion formula for Tschebyscheff polynomials is: T m z 2 zT m 1 z T m 2 z The polynomials can also be computed using: T m z co s m co s 1 z T m z co sh m co sh 1 z 1 z 1 z 1, z 1 The first seven Tschebyscheff polynomials have been plotted below: Properties • All polynomials pass through • For (1, 1) 1 z 1 1 Tm z 1 • All roots occur within 1 z 1 • All maxima and minima within this range have values 1 126 PLANAR ARRAYS • Planar arrays provide directional beams, symmetrical patterns with low side lobes, much higher directivity (narrow main beam) than that of their individual element. • In principle, they can point the main beam toward any direction. Can scan the beam in both the planes. • Applications – tracking radars, remote sensing, communications, etc. If the axis of array has arbitrary orientation then array factor is given by If the axis of array is along z axis The AF of a linear array of M elements along the x-axis is M A F x1 m 1 I me j m 1 kd x si n co s x si n co s co s x directional cosine with respect to x-axis. • All elements are equispaced with an interval of dx and a progressive shift βx. • Im denotes the excitation amplitude of the element at the point with coordinates: x=(m-1)dx, y=0. •• This is thearrays element the mnext -th row and the 1stincolumn of the array matrix. array is If N such are of placed to each other the y direction, a rectangular formed. • We assume again that they are equispaced at a distance dy and there is a progressive phase shift along each row of βy. Then, the AF of the entire M×N array is • The pattern of a rectangular array is the product of the array factors of the linear arrays in the x and y directions. • In the case of a uniform planar (rectangular) array, all elements have the same excitation I m 1 I 1n I 0 amplitudes: M AF I 0 e j m 1 kd x si n co s x N m 1 e j n 1 kd n 1 • The normalized array factor is obtained as: A F n , 1 M M si n 2 1 si n 2 N si n x 1 2 N 1 si n x 2 x kd x si n co s y kd y si n si n x y y y y si n si n y 3-D PATTERN OF A 5-ELEMENT SQUARE PLANAR UNIFORM ARRAY WITHOUT GRATING LOBES (d=λ/4, βx= βy=0 ) 3-D PATTERN OF A 5-ELEMENT SQUARE PLANAR UNIFORM ARRAY WITHOUT GRATING LOBES (d=λ/2, βx= βy=0 ): Unit III Practical Antennas Yagi-uda antenna, Travelling wave antennas, V- Antenna, Rhombic antenna, Microstrip antennas – Introduction, Features, Advantages and Limitations, Rectangular Patch Antennas – Geometry, Design equation and Characteristics. YAGI UDA ANTENNA It is HF, VHF, UHF Antenna It overcomes the drawback of non uniform array antenna. 1. Driven Element Active element A resonant half wave dipole’ Power supplied from source through transmission line 2. Reflector Passive element Not directly fed. Derives power by EM coupling Adds up fields of driven element in the direction from reflector towards the driven elemen Length 5% more than driven element 3. Director Passive element Not directly fed. Derives power by EM coupling Adds up fields of driven element in the direction away from driven element past the director Length 5% less than driven element 3 Element Yagi Uda Antenna Endfire Array Current lags Current leads One reflector and many driven elements Z = 73+ j45 ohms Space adjustment Balun Stubs Quarter wave transformer Folded dipole PH = Id2RH Pf=If2Rf Id=I If=I/2 Increased impedance, Large bandwidth Current flowing is unequal Zin =73x r Zin =73x (1+d2/d1)2 Long Wire Antenna/ Travelling wave Antenna/Harmonic Antenna K determines the orientation of main beam Two types of Long wire antenna Non Resonant antenna Resonant antenna EFA radiation pattern with a sharp null V Antenna Apex angle controls the direction of main beam. I I /180 V antenna array EFA Distance between two elements is λ/4. The phase difference in currents is 90 deg. The fields cancel in backward direction and reinforce in forward direction. Do not need a reflector to cancel in back radiation. Gain doubles or triples depending on no. of elemetns. V antenna array BSA Distance between two elements is λ/4. Maximum radiation is in the direction of termination. Rhombic Antenna Each leg is made of two or three wires to achieve constant current along the line. Only P/8 power is wasted. Microstrip Antenna Microstrip antenna--G. A. Deschamps. In 1950s Radiating patch on grounded substrate Geometry of Microstrip Patch Photograph of Microstrip patch Microstrip line Feed Fed by quarter wavelength transmission line Different factors to be Inset feed Offset feed feeding techniques: Co-axial Probe feed Proximity Coupled microstrip feed Aperture Coupled microstrip feed considered for selecting Impedance matching Radiating structure and feed structure Minimization of spurious radiation Suitability of feed for array applications Advantages: Simple Allows for planar feeding Easy to obtain input match Disadvantages: Significant feedline radiation for thicker substrates For deep notches, pattern may show distortion. Advantages: Feed can be placed in desired location Easy to locate Low spurious radiation Disadvantages: Narrow bandwidth Easy to model Inner conductor causes impedance mismatch problems. Disadvantages: Requires multilayer fabrication Alignment is important for input match Advantages: Allows for planar feeding Feed-line radiation is isolated from patch radiation Higher bandwidth, since probe inductance restriction is eliminated for the substrate thickness, and a double-resonance can be created. Allows for use of different substrates to optimize antenna and feed-circuit performance Proximity (EMC) Coupling Advantages: Allows for planar feeding Less spurious radiation compared to microstrip feed The proximity coupling has the largest bandwidth (as high as 13 %), Disadvantages: Requires multilayer fabrication Alignment is important for input match Microstrip Antenna Analysis Analytical techniques · The transmission line model · The cavity model · The MNM Numerical techniques · The method of moments (MoM) . The finite-element method (FEM) · The spectral domain technique (SDT) · The finite-difference time domain (FDTD) method Transmission-Line Model It is the easiest of all available models It yields the least accurate results It gives good physical insight Basically the transmission-line model represents the microstrip antenna by two slots, separated by transmission line of length L. The discontinuity introduced by the rapid change in the line width at the junction between the feed line and patch radiates. No field variation along W and h field lines on Microstrip line Radiates in Broadside direction and Propagates quasi TEM wave. Example Design a rectangular microstrip antenna using a substrate (RT/ duroid 5880) with dielectric constant of 2.2, h = 0.1588 cm (0.0625 inches) so as to resonate at 10 GHz. Conductance Rectangular microstrip patch and its equivalent circuit transmission-line model. where for a slot of finite width W Since slot #2 is identical to slot #1, its equivalent admittance is Resonant Input Resistance The total admittance at slot #1 (input admittance) is obtained by transferring the admittance of slot #2 from the output terminals to input terminals using the admittance transformation equation of transmission lines If the reduction of the length is properly chosen (typically 0.48λ < L < 0.49λ), the transformed admittance of slot #2 becomes Broadband Techniques of MSA Modified shaped patches Planar multi-resonator configurations Multilayer configurations Stacked multi-resonator configurations Impedance matching networks Log periodic MSA configurations Ferrite based broad band MSA Improving Bandwidth U-Shaped Slot The introduction of a U-shaped slot can give a significant bandwidth (10%-40%). (This is due to a double resonance effect, with two different modes.) Improving Bandwidth Double U-Slot A 44% bandwidth was achieved. Improving Bandwidth Parasitic Patches Radiating Edges Gap Coupled Microstrip Antennas (REGCOMA). Non-Radiating Edges Gap Coupled Microstrip Antennas (NEGCOMA) Four-Edges Gap Coupled Microstrip Antennas (FEGCOMA) Improving Bandwidth Direct-Coupled Patches Radiating Edges Direct Coupled Microstrip Antennas (REDCOMA). Non-Radiating Edges Direct Coupled Microstrip Antennas (NEDCOMA) Four-Edges Direct Coupled Microstrip Antennas (FEDCOMA) Microstrip Antennas Linear array (1-D corporate feed) 22 array 2-D 8X8 corporate-fed array 4 8 corporate-fed / series-fed array UNIT-V Concept and benefits of smart antennas, Types of smart antennas, Beam forming techniques, Smart antenna methods, Algorithms. Unit IV Aperture and Modern Antennas Reflector Antennas – Introduction, Flat Sheet and Corner Reflectors, Paraboloidal Reflectors – Geometry, Pattern Characteristics, Feed Methods, Reflector Types – Related Features, Illustrative Problems. Horn Antennas – Types, Fermat’s Principle, Radiation from sectoral and pyramidal horns, Design Considerations of Pyramidal Horns, Reconfigurable antenna, Active antenna, Dielectric antennas, Electronic band gap structure and applications. Retro reflector G =2.15dBi for dipole . Here, shape of wave front is like a sheet of paper. There is no direct source to generate collimated beam. How to generate collimated beam ? What should be shape of reflector? Parabolic reflector generates collimated beam Parabolic reflector converts spherical waves originated from radiator at focus of parabola into a plane wave across the mouth or aperture of parabola Rays which do not strike the reflector appear as sidelobes. Minimized by source shield. Rays parallel to axis converge at focus. Others don’t due to path length difference. Types of parabolic reflectors Parabolic cylinder / Cylindrical parabolic reflector Generated by moving parabolic contour parallel to itself. Focal line – provide large aperture blockage Converts cylindrical to plane wavefront Rectangular aperture Source- dipole, Linear array BSA Mechanically simpler Generates fan beam Pill box is short parabolic cylinder enclosed by a plate and fed by coaxial feed. Importance of f/D ratio Small f/ D ratio ---Deep dish Feed is closer and can be small Difficult to support and move mechanically Spill over is less Non uniform illumination - low efficiency. Feeding Techniques a) b) c) d) Axial feed Offset feed Cassegrain feed Gregorian feed. a) Axial feed Blockage Increased sidelobes Impedance mismatch Transmission line losses b) Offset feed Horn is an antenna that consists of a flaring metal, shaped like a horn. What are Reconfigurable Antennas Traditional Antennas Designed for single predefined mission - fixed parameters (frequency, radiation pattern, polarization, and gain). Reconfigurable Antenna It is the antenna capable of dynamically modifying its fundamental operating characteristics like polarization, frequency and radiation pattern, to adapt to changing system requirements and environmental conditions. *It becomes most active part of communication link. NEED FOR RECONFIGURABLE ANTENNAS Single Element and Array Scenario Modern wireless communication systems demand Multifunctional capabilities High performance in transmission and reception Desirable features like minimum weight, low cost, low profile Traditional antenna Smart / Adaptable antenna Reconfigurable Antenna Reconfiguration mechanism lies in the antenna Support more than one wireless standard with good isolation Improves communication link quality and capacity Needs low front end processing Classification of Reconfigurable Antennas Reconfigurable Parameter Polarization switching Linear Polarization(LP) Circular Polarization(CP) LP, CP combinations Discrete tuning Continuous tuning Single frequency Multiple frequency Narrow and Wide band *Redirect, change or distribute CURRENT Shape Direction Gain Interesting RECONFIGURATION TECHNIQUES Easy to integrate 239 Linear behavior of switch No switching Elements Liquid crystal/Ferrite CHALLENGES Needs careful analysis and design - Parameter linkage Conceptual design functionality deviates - Modeling complexity Single geometry - Multiple operating modes Reconfiguration mechanism - Actuation requirements & effects Reliability of reconfiguration mechanisms Huge computational resources. Applications of Reconfigurable Antennas Cognitive Radio System Primary User - Owns the spectrum Secondary User – Uses idle spectrum Key Requirements of Cognitive Radio System Isolation between ports Substrate space Omnidirectional radiation pattern UWB Antenna reconfigurability process Ultrathin, Light weight Laptop Mobile Concept PC Utilizing Reconfigurable Antenna and Switching System Multiband frequency reconfigurable antenna – Cellular and WLAN bands Diversity board – connects RF modules and antennas Microcontroller - controls switches on antenna and diversity switch logic Field Programmable Gate Arrays (FPGAs) or Arduino Boards Satellite Communication Requirement – High gain – Deployable antennas Reconfigurable Deployable Helical Antenna Deployable reflector Antenna 240MHz to 450MHz Gold-molybdenum mesh Dielectric Resonator Antennas DRAs rely on radiating resonators that can transform guided waves into unguided waves Aperture coupling is applicable to DRA of any shape Aperture behaves as magnetic current parallel to slot It excites magnetic fields in DRA Feed below ground plane No spurious radiations Cancels reactive component of slot Impedance match Moving DRA with respect to slot. Coupling level Impedance tuning Slot coupling becomes bulky below L-band Aperture fed Rectangular DRA The probe can also be embedded within DRA Coupling Adjustment Probe height Probe location Mode of operation - Probe location with respect to DRA Adjacent TE110 Center- TE011 Advantages: Provides good coupling and hence efficiency useful at lower frequency No external impedance matching network is needed. Microstrip coupling excites magnetic fields in DRA – short horizontal magnetic dipole mode Coupling lateral position of ms line wrt DRA dielectric constant of DRA Fabricated DRA Unit V Wave propagation Ground wave propagation. Space and surface waves, Tropospheric refraction and reflection. Sky wave propagation – Virtual height, critical frequency, Maximum usable frequency – Skip distance, Fading , Multi hop propagation. Wave propagation Wave propagation is the behavior of the wave when transmitted/ propagated from one point to other on earth through the atmosphere. In free space wave propagates in TEM mode Wave polarization is maintained as generated while propagating in free space. Ground wave or surface wave mode is used upto 2 MHz. This mode exists when Tx and Rx antenna are near to earth. Factors affecting the surface wave are Frequency of operation Earth surface(surface irregularities, , conductivity) Tilt in wavefront of the wave Frequency of operation Surface waves follow the contour of the earth becau Diffraction: Spreading out of waves as they pass through an aperture or around objects. Factors affecting Diffraction Wavelength: Wavelength of the incident wave plays an important role in determining the magnitude of diffraction For the same aperture size: Wave having a longer wavelength will be diffracted more than shorter wavelength. Size of aperture: For same wavelength smaller aperture size diffracts to a much larger extent as compared to the aperture with a larger size. Size of Object: The surface wave curves or bend around object if object size is smaller than its wavelength. *High frequencies will not be diffracted by objects but absorbed. For low frequencies earth appears small and diffraction results in beyond horizon propagation. Large distance travel possible with low frequency and high power transmitters. Earth Surface Irregularities Or Effect Of Earth As surface travels over ground its induces voltage and it depends on earths and Equivalent circuit of earth. The induced voltage takes surface wave energy and contributes to attenuation Attenuation depends on electrical properties of terrain. Effect of wave polarization. The best type of surface is one that has good electrical conductivity. Tilt in Wavefront As the wave propagates over the earth, it tilts over more and more. Eventually, at some distance from Tx, the wave “lies down and dies.” wave front which is travelling out from the antenna is slowed slightly near the ground due to the refractive index being higher than air. This has the effect of tilting the wave front forward so that the bottom stays in contact with the ground. Maximum range of such a transmitter depends on its frequency as well as its power. Thus, in the VLF band, increasing the transmitting power works. This remedy will not work near the top of the MF range, since propagation is now definitely limited by tilt. Field strength at a distance Radiation from an antenna by means of Ground Wave Propagation gives rise to a field strength at a distance, which may be calculated by use of Maxwell’s equations. The signal received by receiving antenna when placed at this point **For large d, the reduction of field strength due to ground and atmospheric absorption reduces V. Space Wave Propagation Ionospheric Wave Propagation 50 Ionospheric Wave Propagation Ionosphere acts as a reflecting surface for waves from 2 to 30MHz. Sky wave of suitable frequency can cover any distance round earth. D layer disappears at night.... the E and F layers bounce the waves back to the earth. This explains why radio stations adjust their power output at sunset and sunrise. A path calculated on the basis of a constant height of the F2 layer will, if it crosses the terminator, undershoot and miss the receiving area as shown the F layer over the target is lower than the F2 layer over the transmitter. n<1 sinI sin r i r f>fc the wave escapes into atmosphere fpfc Secant Law As i fmuf Depends on N for Curved Earth Surface????
