161_1_Lecture_14

161_1_Lecture_14 - EE161 El EE161 Electromagnetic Waves...

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EE161 Electromagnetic Waves all 2010 Fall, 2010 Instructor: Dr. Shenheng Xu Electrical Engineering Dept., UCLA gg p , © Prof. Y. Ethan Wang
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ecture 1 Lecture 14 • Antenna Radiation Resistance ipole Antennas • Dipole Antennas • Large Aperture Antenna • Rectangular Aperture with Uniform Field Distribution
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Beam Dimensions Pattern Solid Angle is defined as:  ) , ( d F p 4 For isotropic (omni-directional) antennas, 4 1 ) , ( p F Meaning of solid angle p for a directive antenna is equivalent to an antenna with a uniform radiation pattern within a cone of solid angle p
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Directivity Directivity is defined as the ratio of its maximum radiation density to the average density, e.g. S ax S R S ax 2 ax 4 P d av S D max rad rad P R P max 2 max 4 / 2 4 R S rad av (total radiated power is sum 2  4 max 2 ) , ( d F S R P rad As we also have, of the power radiated in every direction) p rad av d F S R S R R P S S S D  4 ) , ( 4 4 / 4 max 2 max 2 max max Thus,  ) , ( d F p Solid angle For isotropic case, 1 4 iso p D 4 ) , ( ) , ( max F D D For arbitrary direction, one may also define directivity for each directions as, Original definition f D is for D of D is for D max
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Antenna Gain How much a directive antenna helps in improving the received (or radiated) signal strength, in comparison to an ideal, lossless omni-directional antenna? in P S R G max 2 4 Antenna Gain is defined as Gain: Directivity considering loss, Dielectric loss, conductor ss mismatch If we define radiation efficiency in rad r P P loss, mismatch … rad P S R D max 2 4 D G r   Previously, we have 1 r G G dB log 10 D D B log 10 Directivity and Gain in Log scale, In reality, we always dB dBi, unit using isotropic reference have G < D dBd, unit using half-wave dipole reference
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Antenna Impedance A I jX R Z Antenna impedance: A I Equivalent Circuit A A A A X , represents the energy stored in near field A Z Current at the terminal A I 2 2 rad rad P R Ignore the reactive part, we have Radiation resistance Radiation resistance A R rad R ohmic R 2 2 ) ( 2 2 rad in ohmic ohmic P P P R A Ohmic resistance Antenna 2 2 2 1 1 1 A A I I otal input power is, resistance 2 2 2 A ohmic A rad ohmic rad A A in I R I R P P I R P rad rad rad rad R R P P Total input power is, A ohmic rad ohmic rad in r R R R P P P
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Radiation Resistance Example Find out the radiation resistance for a Hertzian dipole!
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161_1_Lecture_14 - EE161 El EE161 Electromagnetic Waves...

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