Lecture 31 Wire Antennas In this lecture you will learn Generation of radiation by real wire antennas Short dipole antennas Half wave dipole antennas Three half wave dipole antennas Small wire loop antennas magnetic dipole antennas ECE 303 Fall 2006 Farhan Rana Cornell University Center Fed Wire Antenna Consider the following wire antenna fed via a transmission line z r r J r z I z x y I Zo x d If one is interested in radiation far fields only then assume r r d 2 r r r r r r J r I z j k rr z z A r o r r e j k r r dv z o r e dz 4 r r 4 r z z Far field approximation r r also called the Fraunhoffer Aff r z o I z e j k r r z z dz 4 r approximation z o j k r d 2 j k z cos e dz I z e 4 r d 2 Recall that r x sin cos y sin sin z cos ECE 303 Fall 2006 Farhan Rana Cornell University 1 Center Fed Short Dipole Wire Antenna I Short dipole wire antenna fed via a transmission line z z Short dipole d Make an assumption for the current distribution on the antenna a triangular distribution d 2 2 I Zo I x d I z r r J r z I z x y d 2 d 2 r r Aff r z o e j k r I z e j k z cos dz 4 r d 2 z o j k r d 2 e I z dz 4 r d 2 z o Ideff j k r e 4 r d 2 Where I d eff Since z d 2 A Hertzian dipole like solution I z dz I d 2 d 2 deff d 2 ECE 303 Fall 2006 Farhan Rana Cornell University Center Fed Short Dipole Wire Antenna II Short dipole wire antenna fed via a transmission line z z Short dipole d 2 d 2 I Zo d r r J r z I z x y I x I z d 2 The radiation from a short dipole looks like that from a Hertzian dipole except that d is replaced by deff r r j o k Ideff Eff r sin e j k r 4 r r r j k Ideff Hff r sin e j k r 4 r r r r r r r 1 S r t Re Eff r Hff r 2 r o k Ideff 2 4 r 2 sin2 2 r r Prad S r t r r 2 sin d d 0 0 o k Ideff 12 2 ECE 303 Fall 2006 Farhan Rana Cornell University 2 Center Fed Short Dipole Wire Antenna III Short dipole wire antenna fed via a transmission line z z Short dipole d 2 d 2 I Zo d r r J r z I z x y I x I z d 2 Antenna Gain 0 For a short dipole the gain is G r r S r t r Prad 4 r 2 30 p 0 3 sin2 2 60 90 Antenna Radiation Pattern p G sin2 G max 120 150 180 ECE 303 Fall 2006 Farhan Rana Cornell University Center Fed Half Wave Dipole Wire Antenna I Half wave dipole antenna fed via a transmission line z z Half wave dipole d d 2 2 I Zo d Make an assumption for the current distribution on the antenna a sinusoidal distribution x I z r r J r z I z x y d 2 I z I cos k z d 2 r r Aff r z o e j k r I z e j k z cos dz 4 r d 2 z o j k r 4 j k z cos e dz I cos k z e 4 r 4 o I e j k r z 2k r cos cos 2 sin2 ECE 303 Fall 2006 Farhan Rana Cornell University 3 Center Fed Half Wave Dipole Wire Antenna II Half wave dipole wire antenna fed via a transmission line z z Half wave dipole d d 2 2 I Zo d x I z r r J r z I z x y I z I cos k z d 2 cos cos r r o I 2 j k r Aff r z e 2k r sin2 This implies cos cos r r j o I j k r 2 Eff r e 2 r sin cos cos r r jI j k r 2 Hff r e 2 r sin ECE 303 Fall 2006 Farhan Rana Cornell University Center Fed Half Wave Dipole Wire Antenna III Half wave dipole wire antenna fed via a transmission line z Half wave dipole d 0 p 0 Hertzian dipole 30 2 60 I Zo d x 90 r r J r z I z x y 120 Half wave dipole This total power radiated is 2 r r Prad S r t r r 2 sin d d 0 0 2 0 0 o I 2 2 r 1 22 o 2 I 4 2 cos2 cos r 2 sin d d 2 sin2 Rrad Prad I 2 2 73 150 180 cos2 cos 2 G 1 64 sin2 cos2 cos 2 p sin2 ECE 303 Fall 2006 Farhan Rana Cornell University 4 Center Fed Three Half Wave Dipole Wire Antenna I z z d 2 Three half wave dipole d 3 2 I Zo Make an assumption for the current distribution on the antenna a sinusoidal distribution x d I z I z I cos k z r r J r z I z x y d 2 r r o j k r d 2 j k z cos Aff r z e dz I z e 4 r d 2 z o j k r 3 4 j k z cos e dz I cos k z e 4 r 3 4 o I e j k r z 2k r 3 cos cos 2 sin2 ECE 303 Fall 2006 Farhan Rana Cornell University Center Fed Three Half Wave Dipole Wire Antenna II z d p 0 z 3 2 0 30 d 2 60 I d x I z 90 I z I cos k z 120 d 2 150 3 cos cos r r j I 2 j k r o Eff r e 2 r sin 3 cos cos r r j I 2 Hff r e j k r 2 r sin 180 3 cos2 cos 2 p sin2 ECE 303 Fall 2006 Farhan Rana Cornell University 5 Home Made Dipole Antennas A 1 5 GHz home made dipole antenna for Wireless LAN with a co axial SMA RF feed Buddipole TM A 16 ft dipole for 1 50 MHz radio ECE 303 Fall 2006 Farhan Rana Cornell University Radars for Upper Atmosphere Research 49 92 MHz incoherent scatter radar at the Peru Observatory The radar has an array of 18 432 half wave …