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Prof. D. R. Wilton

ECE 3317. Prof. D. R. Wilton. Notes 21 Introduction to Antennas . [Chapter 7]. Introduction to Antennas. Antennas. An antenna is a device that is used to transmit and/or receive an electromagnetic wave.

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Prof. D. R. Wilton

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  1. ECE 3317 Prof. D. R. Wilton Notes 21 Introduction to Antennas [Chapter 7]

  2. Introduction to Antennas Antennas An antenna is a device that is used to transmit and/or receive an electromagnetic wave. The antenna itself can always transmit or receive, but in a given application it may be used for just one of these functions. 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)

  3. Introduction to Antennas (cont.) Antennas are often used for a variety of reasons: • For communication over long distances, to have lower loss • Where waveguiding systems (e.g., transmission lines) are impractical or inconvenient • When it is desired to communicate with many users at once Power loss from antenna broadcast: Power loss from waveguiding system: r B A

  4. Introduction to Antennas Main 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 Impedance An antenna is essentially a matching device between a transmission line and a wave radiating into some region of space

  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. Introduction to Antennas (cont.) current Dipole Wire Antenna • Very simple • Moderate bandwidth • Low directivity • Most commonly fed by a twinline transmission line • Linear polarization (E, assuming wire is along z axis) • The antenna is resonant when the length is about one-half free-space wavelength

  7. Introduction to Antennas (cont.) Dipole Wire Antenna (cont.)

  8. Introduction to Antennas (cont.) h Feeding coax Monopole Wire Antenna This is a variation of the dipole, using a ground plane instead of a second wire. • Similar properties as dipole • Mainly use for vertical polarization, with coaxial cable feeds

  9. Introduction to Antennas (cont.) Monopole Wire Antenna (cont.)

  10. Introduction to Antennas (cont.) Yagi Antenna Prof. Yagi This is a variation of the dipole, using multiples wires (with one “reflector” and one or more “directors”. • Low bandwidth • Moderate to high directivity • Commonly used as a UHF TV antenna

  11. Introduction to Antennas (cont.) Yagi Antenna (cont.) UHF Yagi UHF Yagi VHF Log-periodic UHF Yagi

  12. Introduction to Antennas (cont.) Log-Periodic Antenna This consists of multiple dipole antennas of varying lengths, connected together. • High bandwidth • Moderate directivity • Commonly used as a VHF TV antenna

  13. Introduction to Antennas (cont.) Log Periodic Antenna (cont.)

  14. Introduction to Antennas (cont.) UHF Yagi VHF Log-periodic Typical Outdoor TV Antenna

  15. Introduction to Antennas (cont.) Horn Antenna It 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. Introduction to Antennas (cont.) Horn Antenna (cont.) Arno A. Penzias and Robert W. Wilson used a large horn antenna to detect microwave signals from the “big bang” (Nobel Prize, 1978).

  17. Introduction to Antennas (cont.) Horn Antenna (cont.) This is a variety called the “hoghorn antenna (a combination of horn+reflector).

  18. Introduction to Antennas (cont.) current Microstrip (Patch) Antenna It 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 above

  19. Introduction to Antennas (cont.) Microstrip (Patch) Antenna (cont.)

  20. Introduction to Antennas (cont.) Dielectric Resonator Antenna The dielectric resonator antenna was invented by Prof. Stuart Long of UH ECE! It consists of a dielectric material (such as ceramic) on top of a grounded dielectric substrate. • Moderate bandwidth • Low directivity (unless used in an array) • Commonly used at microwave frequencies and above

  21. Introduction to Antennas (cont.) Dielectric Resonator Antenna (cont.) GPS antenna

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