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Antennas

Antennas. Hertzian dipole antenna. Heinrich Hertz (1857-1894). Schematic diagram of Hertz’ experiment. Propagation of electromagnetic wave. Electric field : red Magnetic field : blue. Reception of EM wave. current. V. Transmitting antenna. Receiving antenna.

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Antennas

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  1. Antennas

  2. Hertziandipole antenna Heinrich Hertz (1857-1894)

  3. Schematic diagram of Hertz’experiment

  4. Propagation of electromagnetic wave Electric field : red Magnetic field : blue

  5. Reception of EM wave current V Transmitting antenna Receiving antenna The charges on the receiving antenna move toward the antenna terminal, which causes voltage drop across them.

  6. Example – Radiation from current filament E H

  7. Example – Radiation from a dipole antenna

  8. Far field radiation from a dipole antenna

  9. Example – Radiation from current loop

  10. Radiation from a tapered transmission line

  11. Dipole antenna - resonance

  12. Example of resonance

  13. Advantage of a resonant circuit Generate higher voltage than input voltage. At resonance Maximum current level depends on internal resistance.

  14. How to generate time varying currents Alternating currents accelerate electrons which emit electromagnetic waves propagating in perpendicular direction Electronic circuit generate oscillating voltages Output voltage

  15. Antenna types

  16. Radiation from an infinitesimally small current segment Exact solution :

  17. Far field approximation Near field approximation Electrostatic solution Biot-Savart’s law Coulomb’s law

  18. Radiation pattern of an infinitesimally small current

  19. Gain and directivityof an antenna Isotropic pattern Omnidirectional pattern Directional pattern Directivity 정의 : (Efficiency) : Gain takes into account losses and reflections of the antenna.

  20. Friis equation transmitter receiver

  21. Example – half wavelength dipole antenna

  22. Array antenna

  23. Array factor Array factor : z-directed array

  24. x-directed array Array factor Top view

  25. Typical array configurations

  26. How to change currents on elementary antennas? Magnitudes and phases of currents on elementary antennas can be changed by amplifiers and phase shifters.

  27. Huygens principle

  28. Pattern synthesis Equi-phase surface Equi-phase surface

  29. Examples (1) Two element array (2) Two element array

  30. (3) Five element array 3dB Beamwidth Beam direction (4) Five element array (5) Five element array

  31. Sample MATLAB codes phi=0:0.01:2*pi; %0<phi<2*pi k=2*pi; d=0.5;% 0.5 lambda spacing. shi=k*d*cos(phi); alpha = pi*0.0; beta = exp(i*alpha); %Currents=[1,2*beta, 3*beta^2,2*beta^3,1*beta^4]; %Current excitations Currents=[1, 1*beta, 1*beta^2, 1*beta^3,1*beta^4]; %Current excitations E=freqz(Currents,1,shi); %E for different shi values E = DB(E)+30; % 최대값에서 30dB 범위까지 그림. E = (E>0.).*E; polar(phi,E); %Generating the radiation pattern

  32. N-element linear array antenna Uniform Array : Magnitudes of all currents are equal. Phases increase monotonically.

  33. Difference : • Universal Pattern is symmetric about y= p. • Width of main lobe decrease with N • Number of sidelobes = (N-2) • Widths of sidelobes = (2π/N) • Side lobe levels decrease with increasing N.

  34. Visible and invisible regions 1 visible region Array Factor의 특성 • Array factor has a period of 2p with respect to ψ. • Of universal pattern, the range covered by a circle with radius “kd” become visible range. • The rest region become invisible range Visible range of the linear array

  35. Grating Lobes Phenomenon 1 grating lobes major lobe They have the same strength ! visible region • If the visible range includes more than one peak levels of universal pattern, unwanted peaks are called grating lobes. • To avoid grating lobes, the following condition should be met. Example : , no grating lobe occurs , no grating lobe occurs

  36. Example : array antenna (77GHz) 표면 전류 분포 두께 0.127mm 비유전율 2.2 17mm 10mm 77GHz에서 array element들이 모두 동위상을 갖도록 설계함.

  37. Radiation pattern (77GHz) elementarypattern Radiation pattern of 8-element array

  38. Automotive radar antenna

  39. Analog beamforming : phase shifter

  40. Beamforming Approaches : Digital Beamformer (DBF) Interference or multipath signal direction Digital Signal Processing (Amplitude & Phase) LPF A/D LPF A/D LPF A/D Desired signal direction LPF A/D ~

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