• Andrew Campbell

What's a decibel?

A decibel is a logarithmic unit used to express a ratio between two values. The decibel is used to conveniently represent very large or small values and allow for simple mathematical calculations that would otherwise be difficult for most humans.

Types of Decibels


There are various forms of decibel units. Pretty much any ratio of two values can be made in decibel form. For instance, I can say I drank 3 dB of my glass of water. In other words, I drank half my water.


A decibel is a logarithmic unit used to express a ratio between two values. The decibel is used to conveniently represent very large or small values and allow for simple mathematical calculations that would otherwise be difficult for most humans.


Some commonly used in Wi-Fi are:

  • dBm - Power relative to 1 milliwatt (mW).

  • dBi - Decibels relative to isotropic radiator.

  • dBd - Decibels relative to a dipole antenna. A dipole antenna has a gain of 0 dBd. An isotropic radiator has a gain of -2.15 dBd.

What is dBm?


dBm is decibels relative to 1 milliwatt and mW is a milliwatt (one thousandth of a watt), i.e. 0 dB = 1 mW.


A watt is a unit of power named after the Scottish engineer James Watt that is used to express the rate of energy conversion or transfer with respect to time. It can be calculated relative to an object’s velocity as Joules per Second (J/S) or in terms of electromagnetism as the rate at which work is done when one Ampere of current flows through an electrical potential difference (Volt), aka Watt = Volts x Amperes.


Wi-Fi Engineers often use dBm when referring to signal strength. For instance, transmit power on an Access Point (AP) could be set in dBm or a received signal strength measured at a client may be reported in dBm.

What is dBi?


dBi is decibels relative to isotropic radiator. It is used to define the gain of an antenna system relative to an isotropic radiator.


An isotropic radiator is an antenna (transmitter/receiver) that produces (or receives) electromagnetic signals in all directions with equal intensity.

The isotropic radiator is used primarily as a laboratory reference with the behavior and performance of more complex antennas is measured against it. For example, a dipole antenna has signal output and sensitivity approximately 2.15 decibels greater, in its focused directions, than the output and sensitivity of an isotropic antenna.


A dipole antenna has a gain of 2.15 dBi and an isotropic radiator has a gain of 0 dBi.

What is dBd?


dBd is decibels relative to a dipole antenna. It is used to define the gain of an antenna system relative to an dipole antenna.


The dipole antenna is what's called a balanced antenna, because it is bilaterally symmetrical. Meaning both sides of the antenna radiation pattern are equal. When using dBd, the antenna gain of a particular antenna is being compared to an ideal dipole antenna.


A dipole antenna has a gain of 0 dBd and an isotropic radiator has a gain of -2.15 dBd.

Logarithmic vs. Linear Units


As we stated, decibels are logarithmic units which are used to show relationships between two values. Whereas, milliwatts are linear units. Meaning they are absolute values, i.e. what most people are used to when dealing with math.


So, why do engineers use logarithmic units when working with RF communications? It's to make your life difficult... actually no... it's really intended to make things easier for people to understand the differences between two very large or very small values.


Let's imagine a scenario where a laptop measures a signal strength of -50 dBm on one side of a wall then -53 dBm on the opposite side of the same wall.



Let's take the same scenario, but this time we'll use mWs instead of dBm.



So, why dBm over mW?

  • It is easier to work with the relativistic log values than the absolute values. As you can see above, it is easier to write -53 dBm than 0.0000005 mW.

  • You are less likely to make mistakes when using dBm. Look closely at the bullet above. I accidentally wrote an additional "0" when writing out the mW value. It should be "0.000005" and not "0.0000005."

  • Solving math equations with dB is easier than mW, in most cases.


Read our section on Decibel Math for more detailed information about converting between dB (dBm) and linear (mW) units.

© 2018 by Syranova

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