Antenna

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Antenna

  1. 1. Prof. D. R. WiltonNotes 21Notes 21Introduction to AntennasIntroduction to AntennasECE 3317[Chapter 7]
  2. 2. Introduction to AntennasAntennasAn antenna is a device that is used to transmit and/orreceive an electromagnetic wave.Examples: Cell-phone antenna (transmit and receive) Wireless LAN antenna (transmit and receive) FM radio antenna (receive only) Satellite dish antenna (receive only) Satellite (possibly separate receive and transmit)The antenna itself can always transmit or receive, but ina given application it may be used for just one of thesefunctions.
  3. 3. Introduction to Antennas (cont.) For communication over long distances, to have lower loss Where waveguiding systems (e.g., transmission lines) areimpractical or inconvenient When it is desired to communicate with many users at onceAntennas are often used for a variety of reasons:Power loss from waveguiding system:Power loss from antenna broadcast:21/ r2 re α−rA B
  4. 4. Introduction to AntennasMain properties of antennas: Radiation pattern Directivity (how directional the beam is) Efficiency (power radiated relative to total input power) Polarization (linear, CP) Bandwidth (the useable frequency range) Input ImpedanceAn antenna is essentially a matching device betweena transmission line and a wave radiating into someregion of space
  5. 5. Introduction to Antennas (cont.)Reflector (dish) antenna Very high bandwidth Medium to high directivity (directivity determined by the size) Linear or CP polarization (depending on how it is fed) Works by focusing the incoming wave to a collection (feed) point
  6. 6. Introduction to Antennas (cont.)Dipole Wire Antenna Very simple Moderate bandwidth Low directivity Most commonly fed by a twinline transmission line Linear polarization (Eθ , assuming wire is along z axis) The antenna is resonant when the length is about one-half free-space wavelengthcurrent0 / 2L λ≈
  7. 7. Introduction to Antennas (cont.)Dipole Wire Antenna (cont.)
  8. 8. Introduction to Antennas (cont.)Monopole Wire AntennaThis is a variation of the dipole, using a ground plane instead of a second wire.hFeeding coax0 / 4h λ≈ Similar properties as dipole Mainly use for vertical polarization, with coaxial cable feeds
  9. 9. Introduction to Antennas (cont.)Monopole Wire Antenna (cont.)
  10. 10. Introduction to Antennas (cont.)Yagi AntennaThis is a variation of the dipole, using multiples wires (with one “reflector” andone or more “directors”. Low bandwidth Moderate to high directivity Commonly used as a UHF TV antennaProf. Yagi
  11. 11. Introduction to Antennas (cont.)Yagi Antenna (cont.)UHF YagiVHF Log-periodicUHF YagiUHF Yagi
  12. 12. Introduction to Antennas (cont.)Log-Periodic AntennaThis consists of multiple dipole antennas of varying lengths, connected together. High bandwidth Moderate directivity Commonly used as a VHF TV antenna
  13. 13. Introduction to Antennas (cont.)Log Periodic Antenna (cont.)
  14. 14. Introduction to Antennas (cont.)Typical Outdoor TV AntennaVHF Log-periodicUHF Yagi
  15. 15. Introduction to Antennas (cont.)Horn AntennaIt acts like a “loudspeaker” for electromagnetic waves. High bandwidth Moderate directivity Commonly used at microwave frequencies and above Often used as a feed for a reflector antenna
  16. 16. Introduction to Antennas (cont.)Horn Antenna (cont.)Arno A. Penzias and RobertW. Wilson used a large hornantenna to detect microwavesignals from the “big bang”(Nobel Prize, 1978).
  17. 17. Introduction to Antennas (cont.)Horn Antenna (cont.)This is a variety called the “hoghorn antenna (a combination of horn+reflector).
  18. 18. Introduction to Antennas (cont.)Microstrip (Patch) AntennaIt consists of a printed “patch” of metal that is on top of a grounded dielectric substrate. Low bandwidth Low directivity (unless used in an array) Low-profile (h can be made very small, at the expense of bandwidth) Can be made by etching Easily fed by microstrip line or coaxial cable Can be made conformable (mounted on a curved surface) Commonly used at microwave frequencies and above01/ 22drLλλε≈ =xyhLWxyhLWrεcurrent
  19. 19. Introduction to Antennas (cont.)Microstrip (Patch) Antenna (cont.)
  20. 20. Introduction to Antennas (cont.)Dielectric Resonator AntennaIt consists of a dielectric material (such as ceramic) on top of a groundeddielectric substrate. Moderate bandwidth Low directivity (unless used in an array) Commonly used at microwave frequencies and aboveThe dielectric resonatorantenna was invented byProf. Stuart Long of UH ECE!
  21. 21. Introduction to Antennas (cont.)Dielectric Resonator Antenna (cont.)GPS antenna

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