Antenna Types

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Antenna type determines the propagation pattern or shape of the radio waves transmitted.

Omnidirectional

• most wireless radios use omnidirectional vertical rod-type antennae
  • receives and sends signals in all directions equally
  • ideally be ceiling mounted for best coverage
    • above head height
    • minimizes interference from obstructing furniture
    • allows clear line of sight
    • but too high (25 ft) will reduce signal strength
      • especially for stations directly below the AP
  • propagation pattern is shaped like a torus (donut)
    • radiates more powerfully in horizontal plane than vertical

Unidirectional

• unidirectional antenna focuses signal in a single direction to a particular area
  • both sender and receiver must use unidirectional antenna
    • otherwise can receive signals but not respond
  • form factor types
    • yagi (bar with fins)
      • common for outdoor use
    • parabolic (dish or grid)
      • looks like a rectangular dome with grid-like openings
      • expensive up-front costs and maintenance
  • useful for point-to-point wireless bridge connections
• gain is the increase in signal strength obtained by focusing the signal
  • measured in decibel isotropic (dBi)
  • beamwidth is the amount of directionality
    • measured in degrees
    • 10-degree antennae are highly directional
      • requires more exact alignment

Polarization

Polarization is the orientation of the wave propagating from an antenna.

• to maximize signal strength
  • transmission and reception antennae should use the same polarization
    • important for unidirectional antennae in point to point link
• dual-polarized antennae can be installed in either direction
  • best way to support mobile devices