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Antennas & Propagation Wu Qun

Antennas & Propagation Wu Qun. Overview of Lecture VII. Review of Lecture VI Frequency Independent Antennas Basics of Aperture Antennas Horn Antenna Slot Antenna Microstrip (Patch) Antenna Parabolic Antenna Antennas: Practical Considerations. Review. Wire Antennas.

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Antennas & Propagation Wu Qun

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  1. Antennas&PropagationWu Qun

  2. Overview of Lecture VII Review of Lecture VI Frequency Independent Antennas Basics of Aperture Antennas Horn Antenna Slot Antenna Microstrip (Patch) Antenna Parabolic Antenna Antennas: Practical Considerations

  3. Review

  4. Wire Antennas • Hertzian Dipole • Finite Length Dipole • Antenna Array • Uda-Yagi • Turnstile • Loop • Helix • Quadrifilar Helix

  5. VHF TV Receive Antenna Uda-Yagi Antenna Sheet Reflector Folded Dipole Driver Feeding Mast 5-6 Directors

  6. Helical Antenna Axial Mode Radiation (endfire) appears if: 3/4 < C/ < 4/3 • Narrow Mainbeam with minor sidelobes • HPBW  1/(Number of turns) • Circular Polarisation (orientation  helix orientation) • Wide Bandwidth • No coupling between elements • Supergain Endfire Array z y x Circumference C

  7. Frequency Independent Antennas

  8. Rumsey’s Principle All antenna characteristics so far were always scaled with respect to . Thus, changing  changes the characteristic. The impedance and pattern properties of an antenna will be frequency independent if the antenna shape is specified only in terms of angles and the antenna itself is infinite.

  9. Rumsey’s Principle Scaling through angles  self-scaling Infinite size  problem of realisation Current should decay fast Finite Bowtie Antenna

  10. Log-periodic toothed Antenna Effectively infinite  current decays fast Current decays fast  introduce discontinuities Discontinuities  destroy self-scaling nature Self-scaling nature  log-periodic toothed antenna Log-periodic sheet Log-periodic wire Characteristic will be repeated at (discrete) nf1.

  11. Log-periodic Dipole Array

  12. Spiral Antenna

  13. Fractal Antenna

  14. Aperture Antennas

  15. Huygen’s Principle Any wavefront can be considered to be the source of secondary waves that add to produce distant wavefronts. z P r’ en r J,  y x

  16. Aperture Plane Towards infinity Aperture Plane • E-field vanishes on the Hemisphere at infinity. • Total field is derived from the knowledge of the field on the aperture plane. Closing Hemisphere

  17. Rectangular Aperture y P r’ x b/2 r  z Polarisation in the far field is the same as in the aperture. -a/2 a

  18. Parameter Rectangular Aperture y-z plane: x-z plane:

  19. Circular Aperture y P r’ x r  z a Polarisation in the far field is the same as in the aperture. J1(x) is the first order Bessel Function of first kind.

  20. Parameter Circular Aperture y-z plane: x-z plane: Large Apertures:

  21. Directivity Rectangular Aperture: Definition Real Physical Area Circular Aperture: Thus, for the uniform rectangular and circular aperture the physical area is equal to the effective area. Non-uniform apertures or fields: Aperture Antennas: 30-90% … Aperture Efficiency Horn Antennas: 50%

  22. Horn Antennas

  23. TE10 Horn Antennas E-Plane sectoral horn H-Plane sectoral horn Pyramidal horn Excitation: TE10 mode Impedance Matching through flare Gradual Transmission with minimised reflection

  24. Specifications • Directive Radiator • Primary feed for parabolic reflectors • High gain, wide bandwidth and simple • Particularly used in microwave region (>1GHz) • Fan radiation patterns

  25. Slot Antennas

  26. z Slot Antennas -x L y w Bookers Principle:

  27. Slot on Waveguide Walls TE10 mode Radiation is maximum at maximal interrupted current Radiation No Radiation

  28. Applications • Slot Antennas are used in fast-moving vehicles. • The slot-length is usually /2 • Particularly used in microwave region (>1GHz)

  29. Microstrip (Patch) Antennas

  30. Patch Structure Patch Feed Substrate L t - - - - + + + + d + + + + - - - - r

  31. Rectangular Dipole Patch Shapes Circular Ring Elliptical • Analysing Methods • Transmission Line • Cavity • Maxwell Equations Triangular

  32. Application & Performance • It is applied where small antennas are required: •  aircrafts, mobiles, etc • 2. Due to shape variations they are versatile in polarisation, pattern, impedance, etc. • 3. They have a low efficiency, spurious feed radiation and a narrow bandwidth • 4. They usually operate in broadside regime • 5. /3 < L < /2 and 2 < r < 12

  33. Parabolic Reflector Antennas

  34. Uda-Yagi: 15dB • Helical Antenna: 15dB • Antenna Arrays high gains  many elements • Horn: high gains  large size Large Gains Complicated Feeding Aperture increasing Reflector Artificially increase size • (re-) transmitted waves are in phase • (re-) transmitted waves are as parallel as possible

  35. Parabolic Reflector Parallel and in-phase waves Parabolic Dish Feed r • Dish has to be 100% parabolic • Feeder shouldn’t block too much Non-uniform fields due to aperture blocking etc … Aperture Efficiency = 80%

  36. Used where high gains are required: •  Cosmic Radiation, etc. • Navigation Applications • Beam is slightly steerable • Deviation from perfect surface can be made <1mm • Diameters are usually 100m-300m

  37. Practical Considerations

  38. Practical Considerations - The Quality Factor Q - Electrically Small Antennas - Physically Small Antennas - Imperfect Ground

  39. Feeding

  40. ‘Exotic’ Antennas - Fractal Antennas - Light Antennas - Gravity Antennas Everything what propagates can be transmitted. Everything what can be transmitted can be received. - EM waves, sound, smell, light, gravity and maybe 6th sense -

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