School on Digital and Multimedia Communications Using Terrestrial

School on Digital and Multimedia Communications Using Terrestrial and Satellite Radio Links The Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 12 February – 2 March 2001 Antenna Fundamentals (3) R. Struzak ryszard. struzak@ties. itu. int 15 Feb 2001 Property of R. Struzak 1

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Summary Slide • • Transmission vs. Reception Polarization More Complex Antennas Antenna Arrays, Adaptive Antennas 15 Feb 2001 Property of R. Struzak 3

Polarization 15 Feb 2001 Property of R. Struzak 4

Ex Ey Polarization ellipse M N 15 Feb 2001 • The two linear far-field components radiated by the horizontal and the vertical antenna sum up to a resultant elliptically polarized wave • The polarization ellipse is defined by its axial ratio N/M (ellipticity), tilt angle and sense of rotation Property of R. Struzak 5

Polarization states LHC UPPER HEMISPHERE: ELLIPTIC POLARIZATION LEFT_HANDED SENSE LATTITUDE: REPRESENTS AXIAL RATIO EQUATOR: LINEAR POLARIZATION LOWER HEMISPHERE: ELLIPTIC POLARIZATION RIGHT_HANDED SENSE 450 LINEAR POLES REPRESENT CIRCULAR POLARIZATIONS 15 Feb 2001 (Poincaré sphere) RHC Property of R. Struzak LONGITUDE: REPRESENTS TILT ANGLE 6

Antenna Polarization • The polarization of an antenna in a specific direction is defined to be the polarization of the wave produced by the antenna at a great distance 15 Feb 2001 Property of R. Struzak 7

Polarization Efficiency (1) • The power received by an antenna from a particular direction is maximal if the polarization of the incident wave has: – the same axial ratio – the same sense of polarization – the same spatial orientation as the polarization of the antenna in that direction. 15 Feb 2001 Property of R. Struzak 8

Polarization Efficiency (2) • When the polarization of the incident wave is different from the polarization of the receiving antenna, then a loss due to polarization mismatch occurs Polarization efficiency = = (power actually received) / (power that would be received if the polarization of the incident wave were matched to the receiving polarization of the antenna) 15 Feb 2001 Property of R. Struzak 9

Polarization Efficiency (3) LCH A: POLARIZATION OF RECEIVING ANTENNA W: POLARIZATION OF INCIDENT WAVE W 2 A H Polarization efficiency = cos 2 450 LINEAR RCH 15 Feb 2001 Property of R. Struzak 10

Circularly-Polarized Antenna y x Ixcos( t+ x) 15 Feb 2001 • Radio wave of any polarization can be Iycos( t+ y) obtained by superposition of 2 linearly-polarized waves produced by 2 crossed dipoles and by controlling the amplituderatio and phase-difference of their excitations. Property of R. Struzak 11

More Complex Antennas 15 Feb 2001 Property of R. Struzak 12

Antenna Over Ground: Image Theory • Perfect ground = perfectly conducting plane surface • Tangential electrical field component = 0 – vertical components: the same direction – horizontal components: opposite directions • The field (above the ground) is the same if the ground is replaced by the antenna image 15 Feb 2001 Property of R. Struzak + - 13

2 Antennas • 2 identical antennas r rr – Excitation: I 1 = I, I 2 =Iej r • Ant#1 field-strength: E’= C*D( , ) • Ant#2 field-strength: 2 E” = C*D( , )*ej ( r+ ) • E = E’ + E” 15 Feb 2001 Property of R. Struzak d 1 r = d*cos 14

Antenna Array Factor (AAF) • Resultant field-strength E = E’ + E” • E = C*D( , )*[1+ej ( r+ )] = C*D( , )*AAF( , ) Pattern multiplication • |AAF( , )|2 = Antenna array factor = Gain of array of isotropic antennas 15 Feb 2001 Property of R. Struzak 15

2 Antenna Array Factor (1) • AAF( ) = 1+ej ( r+ ) ; ( r+ ) = x • AAF( ) = 1+ejx = 2[(1/2)(e-jx/2 +ejx/2)]ejx/2 = 2 cos(x/2)ejx/2 • |AAF( )| = 2 cos(x/2) = 2 cos[ (d/2)cos + /2) = 2 cos[( d/ )cos + /2] • |AAF( )|2 Antenna Array Factor 15 Feb 2001 Property of R. Struzak 16

2 Antenna Array Factor (2) • |AAF( )|2 = {2 cos[( d/ )cos + /2]}2 • Gain: Max{|AAF( )|2} = 4 (6 d. Bi) when ( d/ )cos + /2 = 0, , …, k • Nulls: when ( d/ )cos + /2 = /2, …, (k + 1) /2 • Relative gain = |AAF( )|2 / Max{|AAF( )|2} 15 Feb 2001 Property of R. Struzak 17

Demonstration (Simulation) Array 2 ant This program simulates radiation pattern of 2 antenna-array factor. It produces 2 D diagrams showing how the radiation lobes maximums and minimums depends on the antennas distance and excitation phases and magnitudes 15 Feb 2001 Property of R. Struzak 18

Antenna Arrays 15 Feb 2001 Property of R. Struzak 19

Yagi-Uda Arrays • Only one antennaelement fed • Other elements unexcited (parasitic) • Non-identical elements • Non-identical distances Directors Reflector 15 Feb 2001 Property of R. Struzak Driver 20

Linear Array of n Antennas • equally spaced • F = 1+ejx+ej 2 x+ej 3 x+…+ej(N-1)x antennas in line = (1 -ej. Nx) / (1 -ejx) • currents of equal magnitude • |F| = |(1 -ej. Nx) / (1 -ejx)| • constant phase = [sin(Nx/2) / sin(x/2)] difference between = F( ) array factor adjacent antennas • numbered from 0 to (n-1) • x/2 = ( d/ )cos + /2 15 Feb 2001 Property of R. Struzak 21

Demonstration (Simulation) Array_Nan This program simulates radiation pattern of N - antenna-array factor. It produces 2 D diagrams showing how the radiation lobes maximums and minimums depends on the antenna distance increment and on excitation phase and magnitude functions 15 Feb 2001 Property of R. Struzak 22

Mutual Impedance Array of antennas V 1 = I 1 Z 11+I 2 Z 12+…+In. Z 1 n V 2 = I 1 Z 12+I 2 Z 22+…+In. Z 2 n. -…… Vn = I 1 Z 1 n+I 2 Z 2 n+…In. Znn Z 1 input = V 1/I 1= Z 11+(I 2/I 1)Z 12+…+(In/I 1)Z 1 n The input impedance depends on mutual impedance (coupling) with other antennas and on relative currents 15 Feb 2001 Property of R. Struzak 23

Example: Impedance of Dipole ~73 /2 ~300 < /4 15 Feb 2001 Property of R. Struzak 24

Phased Arrays • Array of N antennas in a linear or spatial configuration • The amplitude and phase excitation of each individual antenna controlled electronically (“software-defined”) – Diode phase shifters – Ferrite phase shifters • Inertia-less beam-forming and scanning ( sec) with fixed physical structure 15 Feb 2001 Property of R. Struzak 25

Antenna Arrays: Benefits • Possibilities to control – – – Direction of maximum radiation Directions (positions) of nulls Beam-width Directivity Levels of sidelobes using standard antennas (or antenna collections) independently of their radiation patterns • Antenna elements can be distributed along straight lines, arcs, squares, circles, etc. 15 Feb 2001 Property of R. Struzak 26

Beam Steering Beam direction d 3 2 • Beam. Equi-phase steering wave front using phase = [(2 / )d sin ] shifters at Radiating each elements radiating Phase 0 shifters element Power distribution 15 Feb 2001 Property of R. Struzak 27

4 -Bit Phase-Shifter (Example) Input Bit #3 Bit #4 00 or 22. 50 00 or 450 Bit #1 Bit #2 00 or 900 00 or 1800 Output Steering/ Beam-forming Circuitry 15 Feb 2001 Property of R. Struzak 28

Switched-Line Phase Bit Delay line Input Output Diode switch 2 delay lines and 4 diodes per bit 15 Feb 2001 Property of R. Struzak 29

Switching Diode Circuit PIN diode Tuning element b a a: RF short-circuited in forward bias b: RF short-circuited in reverse bias 15 Feb 2001 Property of R. Struzak 30

Adaptive “Intelligent” Antennas 15 Feb 2001 Property of R. Struzak 31

Adaptive (“Intelligent”)Antennas • • • Array of N antennas in a linear or spatial configuration Used for receiving signals from desired sources and suppress incident signals from undesired sources The amplitude and phase excitation of each individual antenna controlled electronically (“software-defined”) The weight-determining algorithm uses a-priori and/ or measured information The weight and summing circuits can operate at the RF or at an intermediate frequency 15 Feb 2001 1 w 1 w. N N Property of R. Struzak Weight-determining algorithm 32