dBd vs dBi: Understanding Antenna Gain Units

This page explains the difference between dBd and dBi, units commonly used to express the directional gain of an antenna. Antenna manufacturers use these units, dBd and dBi, to specify how well an antenna focuses radio waves in a particular direction. Let’s dive into the details.

What is dBd?

dBd is a unit used to measure the directional gain of an antenna compared to a dipole antenna.

The last letter ‘d’ in ‘dBd’ refers to “dipole.” Essentially, it tells you how much better the antenna performs compared to a standard dipole antenna.

What is dBi?

dBi is a unit used to measure the directional gain of an antenna compared to an isotropic radiator.

The last letter ‘i’ in ‘dBi’ refers to “isotropic.” An isotropic radiator is a theoretical antenna that radiates power equally in all directions.

Both dBd and dBi represent values calculated relative to the input power, indicating the directional output power of the antenna.

The Relationship Between dBd and dBi

The key is that a dipole antenna itself has a gain of 2.14 dBi (over an isotropic radiator). This is an important benchmark.

Therefore, an antenna with a gain of 7 dBd will have a gain of 9.14 dBi. Both 7 dBd and 9.14 dBi represent the same antenna gain, just expressed in different units.

Here’s the formula that describes the relationship:

$0 \text{ dBd} = 2.14 \text{ dBi}$

• To convert dBd to dBi: Add 2.14.
• To convert dBi to dBd: Subtract 2.14.

Example:

Let’s say you have an antenna with a gain of 10 dBd. To find the equivalent dBi gain:

$10 \text{ dBd} + 2.14 = 12.14 \text{ dBi}$