Lect32AntennaCircuits

Lect32AntennaCircuits - ECE 3030 Electromagnetic Fields and...

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1 ECE 3030 Electromagnetic Fields and Waves Fall 2009 Lecture 32 2009/11/11 Antenna Circuit Properties Transmitting Antennas Receiving Antennas 1 Swartz 06/5/25 Electromagnetic Fields and Waves – Fall 2009 Lecture 32: Instructor: Dr. Wesley E. Swartz Thevenin Equivalents Antenna Theorem z + - V th E ++ -- Z o P P rad P rad P rad P rad P = P inc -P ref Circuit Properties of Transmitting Antennas (1) What does an antenna look like to a driving circuit? It appears simply as a load impedance. d Z o     y x z I z r J ˆ x z I P = Net time-average power going into the antenna Z o I Z A P = P inc ref P inc P ref 3 Swartz 06/5/25 Electromagnetic Fields and Waves – Fall 2009 Lecture 32: Z o P Poynting theorem:   t r E t r J t t r W t r S , , , , Time-average form for time-harmonic signals: t r E t r J t r S , , , P rad P rad P rad P rad Note the distinction between P and P inc and P rad P = P inc ref Circuit Properties of Transmitting Antennas (2) Time-average form for time- harmonic signals: dv t , r E t , r J a d t , r S   Net power dissipated Net power going in 2 1 Z o P P rad P rad P rad P rad P = P inc ref 4 Swartz 06/5/25 Electromagnetic Fields and Waves – Fall 2009 Lecture 32: Z o I Z A P diss rad R I P P 2 But: rad rad R I P 2 2 1 So we get: rad diss R R I P 2 2 1 But from the circuit model:  A Z I P Re 2 1 2 diss rad A R R Z Re Circuit Properties of Transmitting Antennas (3) Not all the power P going into an antenna is converted into radiated power P Z o I Z A P Z o I P jX A R A A diss rad A A A jX R R jX R Z P = P inc ref P = P inc ref 5 Swartz 06/5/25 Electromagnetic Fields and Waves – Fall 2009 Lecture 32: power rad Some is reflected by any reactive part due to net inductance or capacitace of the antenna structure. Some is dissipated due to the less-than-infinite conductivity of real metals (i.e. non-zero R diss ). Definition of Antenna Gain for real antennas: 2 4 ˆ , , r P r t r S G 2 4 ˆ , , r P r t r S G rad NOTE: it is not: Circuit Properties of Transmitting Antennas (4) Because not all the power P going into an antenna is converted into Z o I Z A P Z o I P jX A R A A diss rad A A A jX R R jX R Z P = P inc ref P = P inc ref 6 Swartz 06/5/25 Electromagnetic Fields and Waves – Fall 2009 Lecture 32: radiated power P rad – the antenna efficiency is defined by: diss rad rad rad rad R R R P P   rad d d G  4 sin , 2 00  Transmitting and Receiving Antennas Need to determine the following: From the net power P going to the antenna, how much is the total radiated power P rad .
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This note was uploaded on 11/26/2009 for the course ECE 3030 at Cornell.

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Lect32AntennaCircuits - ECE 3030 Electromagnetic Fields and...

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