The decibel and antennas

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Information on the dB relating to antennas.

The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power) relative to a specified or implied reference level. Its logarithmic nature allows very large or very small ratios to be represented by a convenient number, in a similar manner to scientific notation. Being essentially a ratio, it is a dimensionless unit. Decibels are useful for a wide variety of measurements in acoustics, physics, electronics and other disciplines.

The dB is used to express relative or absolute values.

In antenna work the dBd, dBi and dBic are used to suggest the gain of an antenna with respect to some known base and are, thus, an absolute reference.

dB(isotropic) the forward gain of an antenna compared to an idealized isotropic antenna, which uniformly distributes energy in all directions.
dB(dipole) the forward gain of an antenna compared to a half-wave dipole antenna. 0 dBd is about 2 dBi.
dB(isometric circular) power measurement relative to a circularly polarized isometric antenna.

One way to spot a suspicious antenna manufacturer is to look over their literature for any claims to the gain of their product.

If they claim gain of, say, 10dB that's your clue they:

  • don't know what they are talking about
  • have a disconnect between their engineering and sales department
  • are trying to inflate their numbers by taking a gain of x dBi and making you think it is x dBd, thus adding about 2dB to the number they promote.


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