Antenna wave propagation ppt

antenna & wave propagation question paper with answer and antennas and propagation for body-centric wireless communications
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Published Date:13-07-2017
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Antennas and Propagation updated: 09/29/2014 Wireless Communication Systems Our focus Multiplexer Modulator Converter Electromagnetic Energy De De Converter Electromagnetic modulator multiplexer Energy Antenna Characteristics o  Radiation patterns o  Radiated power o  Half-power beam width of the antenna o  Antenna position, shape, and length o  Antenna gain with respect to an ideal case radiating electromagne tic energy Remember: into space Antenna Properties o  An antenna is an electrical conductor (transducer) or system of conductors n  They carry time-varying currents and, consequently, accelerating electrons n  à A Transmission Antenna radiates electromagnetic energy into space n  à A Reception Antenna collects electromagnetic energy from space collecting electromagnet ic energy from space Remember: Waves and Propagation - Demo collecting Anntena electromagnet carry time- radiating ic energy from varying electromagne space currents tic energy into space sim=Radio_Waves_and_Electromagnetic_Fields Remember: Antenna Properties Reciprocal Devices o  Most antennas are reciprocal devices, n  That is they are exhibiting the same radiation pattern for transmission as for reception o  When operating in the receiving mode, the antenna captures the incident wave n  Only that component of the wave whose electric field matches the antenna polarization state is detected o  In two-way communication, the same antenna can be used for transmission and reception n  Antenna characteristics are the same for transmitting or receiving electromagnetic energy o  The antenna can receive on one frequency and transmit on another Polarization Start With Basics: o  We know: n  Q (static charges) à E field– capacitor example1 n  I (moving charges) à H field– compass example2 n  Thus, in the presence of time-varying current à we will obtain interdependent EM fields o  A time-varying current (I) along a wire generates rings of Electromagnetic field (B) around the wire o  Similarly the current passing through a coil generates Electromagnetic field in the Z axis 1 2 capacitance.html index.html Simple Experiments An induced magnetic field 1 Galvanometer needle moves Faraday confirmed that a moving magnetic field 2 is necessary in order for electromagnetic Electromagnetic induction to occur 3 field (B) Faraday’s Law: Electromotive force (voltage) induced by time-varying magnetic flux: Maxwell Equations Relationships between o  Gauss’s Law charges, current, o  Faraday’s Law electrostatic, electromagnetic, o  Gauss’s Law for Magnetism electromotive force o  Ampere’s Law Maxwell’s Equations – Free Space Set o  We assume there are no charges in free space and thus, = 0 Time-varying E and H cannot exist independently If dE/dt non-zeroà dD/dt is non-zero àCurl of H is non-zero à D is non-zero (Amp. Law) If H is a function of time à E must exist (Faraday’s Law) Interrelating magnetic and electric fields Our Focus: 2 d =(2l )/λ far_field Far-Field Approximation 1. In close proximity to a radiating source, the wave is far-field range spherical in shape, but at a far distance, it becomes approximately a plane wave as seen by a receiving antenna. 2. The far-field approximation simplifies the math. 3. The distance beyond which the far-field approximation is Very similar to throwing a valid is called the far-field range stone in water (will be defined later). What is the power radiated? A far field approximation o  Assuming the alternating current travels in Z directionà radiated power must be in Z o  Antenna patterns are represented in a spherical coordinate system o  Thus, variables R, θ, φ à n  range, n  zenith angle (elevation), n  azimuth angle shilmay23fin.html Hertzian Dipole Antenna o  Using Ampere’s Law o  But how is the current I(r) distributed on the antenna? o  One way to approximate this rather difficult problem is to use thin-wire dipole antenna approximation n  Dipole because we have two poles (wires) n  aL o  We only consider the case when L is very very short (Hertzian Dipole) n  Infinitesimally short n  Uniform current distribution Hertzian Dipole (Differential Antenna) Radiation Properties o  Very thin, short (lλ/50) linear conductor o  Observation point is somewhere in the space lHertzian Dipole (Differential Antenna) Radiation Properties Note that: ω= 2π f; η = µ /ε =120π; 0 0 0 k=ω /c= 2π f /c= 2π /λ; ε  is the free space permittivity (about 1) K=wave number lλ /50; µ  is magnetic Permeability ' η is the intrinsic impedance in Ohm R≈ R =Γ;Hertzian Dipole (Differential Antenna) Radiation Properties A/m V/m In this case: 1/Rà radiation field components 1/R2àinduction field components 1/R3àelecrostatic field components Far-field à only radiation field à Only Eθ and Hφ will be significant R= Range θ=Zenith (elevation) – side view φ=Azimuth – top view Hertzian Dipole (Differential Antenna) Radiation Properties A/m V/m Radiated Power Flux Density Electric and Magnetic Intensity Fields (E 0) R E(0, Eθ,0) H(0,y, Hφ) P(P ,0,z) R Average Power flux Density Cross product of E and H Normalized Radiation Intensity(F) Normalized Radiation Intensity àHow much radiation in each direction? Elevation Pattern (side view) So=Smax= Max. Power Density R= Range θ=Zenith (elevation) – side view φ=Azimuth – top view Azimuth Pattern (top view)

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