Prof. David R. Jackson Notes 21 Introduction to Antennas Introduction to Antennas ECE 3317 [Chapter 7]

Introduction to Antennas Antennas An antenna is a device that is used to transmit and/or receive 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) The antenna itself can always transmit or receive, but it maybe used for only one of these functions.

Introduction to Antennas (cont.)  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 Antennas are often used for a variety of reasons: Power loss from waveguiding system: Power loss from antenna broadcast: r A B

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)  Input Impedance  Bandwidth (the useable frequency range)

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

Introduction to Antennas (cont.) 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 current

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

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

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

Introduction to Antennas (cont.) Yagi Antenna 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 Prof. Yagi

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

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

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

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

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

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).

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

Introduction to Antennas (cont.) 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 current

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

Introduction to Antennas (cont.) Dielectric Resonator Antenna 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 The dielectric resonator antenna was invented by our very own Prof. Long!

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