ELEG 648 Radiation/Antennas I. Mark Mirotznik, Ph.D. Associate Professor The University of Delaware

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1 ELEG 648 Radiation/Antennas I Mark Mirotznik Ph.D. Associate Professor The University of Delaware

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3 A jk rr ' e ' r J r dv ' 4 r r '

4 F If we have magnetic sources jk rr ' e ' r M r dv ' 4 r r '

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8 Field Regions Far-field Fraunhofer region D R 1 R Reactive near-field region R D 3 Radiating near-field Fresnel region R D

9 Radiation Pattern Lobes

10 Radiation Intensity tot Prad Prad r r sin d d 0 0 define U r Prad r This assumes we are in the far field and the power varies as 1/r^ P U tot rad ave U sin d d U d 4 tot Prad U d The average radiation intensity for a given antenna represents the radiation intensity of a point source or isotropic antenna that produces the same amount of radiate power as the actual antenna.

11 Radiation Intensity max * 1 ~ 1 ~ ~ 1 U U U E E r U E E E H E r P rad

12 Directive Gain 10log ] [ max max D db D directivity P U D D P U P U U U D tot rad o tot rad tot rad ave Directive in db

13 Antenna Gain G U 4 P input DIRECTIVITY POWER DENSITY IN A CERTAIN DIRECTION DIVIDED BY THE TOTAL POWER RADIATED GAIN IF ANTENNA HAS OHMIC LOSS THEN GAIN < DIRECTIVITY POWER DENSITY IN A CERTAIN DIRECTION DIVIDED BY THE TOTAL INPUT POWER TO THE ANTENNA TERMINALS FEED POINTS

14 Antenna Gain Sources of Antenna System Loss 1. losses due to impedance mismatches. losses due to the transmission line 3. conductive and dielectric losses in the antenna 4. losses due to polarization mismatches According to IEEE standards the antenna gain does not include losses due to impedance or polarization mismatches. Therefore the antenna gain only accounts for dielectric and conductive losses found in the antenna itself. However Balanis and others have included impedance mismatch as part of the antenna gain. The antenna gain relates to the directivity through a coefficient called the radiation efficiency e t conduction losses dielectric losses impedance mismatch G e D e e e e D t Polarization losses p r c d e t 1

15 Overall Antenna Efficiency The overall antenna efficiency is a coefficient that accounts for all the different losses present in an antenna system. e e e e e e p r c d cd e p et e e e r c d e p e polarization mismatches r reflection efficiency impedance conduction losses dielectric losses conductor & dielectric losses e cd mismatch

16 Typical Antenna System for Transmission

17 Reflection Efficiency The reflection efficiency through a reflection coefficient G at the input or feed to the antenna. e r G Z Z 1 G input Z Z output input input Z Z antenna generator generator generator input output impedance impedance

18 Radiation Resistance The radiation resistance is one of the few parameters that is relatively straight forward to calculate. R rad P I total rad o 4 U d I o

19 Antenna Radiation Efficiency Conduction and dielectric losses of an antenna are very difficult to separate and are usually lumped together to form the e cd efficiency. Let R cd represent the actual losses due to conduction and dielectric heating. Then the efficiency is given as e cd P P rad total P rad P rad P ohmic For wire antennas without insulation there is no dielectric losses only conductor losses from the metal antenna. For those cases we can approximate R cd by: R cd l o b where b is the radius of the wire is the angular frequency is the conductivity of the metal and l is the antenna length R cd R rad R rad

20 Antenna Radiation Efficiency For wire antennas without insulation there is no dielectric losses only conductor losses from the metal antenna. For those cases we can approximate R cd by: R cd l o b where b is the radius of the wire is the angular frequency is the conductivity of the metal and l is the antenna length

21 Polarization Loss Factor Assume the polarization of the electric field incident on an antenna is directed in the w direction ~ E inc Also assume the polarization of perferred by the antenna is directed in the a direction ~ E a The polarization loss factor is defined as PLF w * a cos p a w E E a inc tells us the amount of incident power lost by any mismatches in polarization between the incident field and the antenna.

22 Hertzian Dipole

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25 P Il 3 R rad rad 1 I 1 3 R rad I l R rad Radiation Resistance

26 3 max sin 3 3 sin 3 4 sin 3 sin 4 sin 4 4 D D Il Il D kil kil r kile j r U E E r U P U D o jkr rad Directivity Directivity and Effective Aperture Directive gain

27 Finite Length Dipole

28 Finite Length Dipole

29 Finite Length Dipole

30 Finite Length Dipole

31 Finite Length Dipole

32 l=1.5

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35 Finite Length Dipole Does the wire thickness matter? real part of input impedance imaginary part of input impedance