Antennas

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Antennas

• Chapter 13

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• Antennas, to quote a friend, are one of life's
  eternal mysteries. "All I'm totally certain of is
  that any antenna is better than no antenna and
  the antenna should preferably erected as high and
  be as long as is possible or desirable".

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Objectives

• Define properties of antenna reciprocity and
  polarization
• Explain antenna radiation
• Exain induction field
• Explain radiation pattern
• Explain gain
• Explain radiation resistance

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• Describe the physical and electrical
  characteristics of common antenna types and
  arrays
• Differentiate between beam width and bandwidth

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Terms

• Reciprocityan antenna ability to transfer energy
  from the atmosphere to its receiver with the same
  efficiency with which it transfers energy from
  the transmitter into the atmosphere
• Polarizationthe directio of the electric field
  of a given electromagnetic radiated signal

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• Radiation fieldradiation that surroundds an
  antenna but does not collapse its field back into
  the antenna
• Induction fieldradiation that surrounds an
  antenna and collapses its field back into the
  antenna
• Near fieldregion less than 2D/? from the antenna
• Far fieldregion greater than 2D/ ?

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• Radiation patterndiagram indicating the
  intensity of radiation from a transmitting
  antenna or the response of a receiving antenna as
  a function of direction
• Omnidirectionalspherical radiation pattern
• Directionalenergy is concentrated in certain
  directions at the expense of lower energy in
  other directions

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• Beam widththe angular separtation between the
  half-power points on an antennas radiation
  pattern
• Antenna gaina measure of how much mor power in
  dB an antenna will radiate in a certain direction
  with respect to that which would be radiated by a
  reference antenna (a source point)

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• Radiation resistancethe portion of an antennas
  input impedance that results in power radiated
  into space
• Corona dischargeluminous discharge of energy by
  an antenna from ionization of the air around the
  surface of the conductor
• Feed linetransmission line that transfers energy
  from the generator to the antenna

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• Delta matchan impedance matching device that
  spreads the transmission line as it approaches
  the antenna
• Vertical antennausually a ¼ wavelength grounded
  antenna
• Image antennasimulated ?/4 antenna resulting
  from the earths conductivity with a vertical
  antenna

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• Counterpoisereflecting surface of a vertical
  antenna if the actual earth ground cannot be
  used. A flat structure of wire or screen placed
  a short distance above ground with at least a
  quarter wavelength radius
• Loading coila series inductance used to tune out
  the capacitive appearance of an antenna

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Basic Antenna Theory

• Basic dipoles
• Loops and Verticals

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An intro to antenna
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Another view
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A DISCUSSION OF ANTENNA THEORY
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Some topics in antenna theory
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Arrays

• A group oa antennas or antenna elecments arranged
  to provide the desired directional
  characteristics

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Parasitic

• One or more of the elements in an array is not
  electrically connected

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Reflector

• The parasitic element that effectively reflects
  energy from the driven element

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YagiUda Antenna

• Director The parasitic element that effectively
  directs energy to the driven element
• Lobes small amounts of radiation shown on a
  radiation pattern. Not desirable
• Front to back ratio the difference in antenna
  gain in dB from the forward to the reverse
  direction

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Driven Collinear

• Driven arraymultielement antenna in which all
  the elements are excited through a transmission
  line
• Collinear arrayany combination of ½ wavelength
  elements in which all the elements are excited by
  a connected transmission line

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Broadside Array

• Elements are arranged vertically one over the
  other

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Phased Array

• Combination of antennas in which there is control
  of the phase and power of the signal applied at
  each antenna resulting in a wide variety of
  possible radiation patterns

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Vertical Array

• Vertically arranged antennas lined up in a row
  with equal spacing between themAM antennas

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Special types

• Log-periodic xx
• Small loop
• Ferrite loop
• Slot
• Folded dipole
• J-pole

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Summary

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