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What Is An Electrically Small Antenna (ESA)

Electrically Small Antenna (ESA):
  • The term "electrically small antenna” (ESA) does not necessarily refer to a physically small antenna.

  • "SMALL" means electrical size in relation to wavelength - not physical size.

  • A full size UHF antenna, though very small, is not an ESA.

  • An ESA is smaller than the minimum size required to be resonant at a given frequency; that is, generally about 1/2 wavelength for free space antennas, or about 1/4 wavelength for antennas that use the ground as the other half.

  • A minimal 1/4 wavelength resonant antenna can also be described as being 90 electrical degrees in length.

  • ESAs will generally need some kind of "loading" to make the antenna look electrically longer to bring it to resonance at the required frequency.

  • ESAs tend to have:
    - a very low radiation resistance
    - a high Q value
    - a narrow bandwidth
    - a capacitive reactance
    - a lower efficiency than full size antennas.

  • These characteristics are not set in stone and are more reflective of a raw unaltered shortened antenna. Practical small antennas will have modifications applied to increase efficiency and widen the bandwidth.