dB vs dBi in Relation to Cell Phone Signal Boosters

When talking about cell phone signal boosters, we often look at the gain of the signal booster. There is however another important measurement that is worth looking at and understanding. I’m talking about an antenna's gain, also called power gain.

This is an important performance measurement which measures the antenna's electrical efficiency combined with its directivity. In receiving antennas, the gain describes how well an antenna can convert radio waves that are received from a specific direction into electrical power. In transmitting antennas, the gain describes how well an antenna can convert input power into radio waves that are sent in a specific direction.

If a direction is not specified, gain refers to the peak value of the gain, i.e. the gain in the direction of the main lobe of antenna.

The gain of an antenna is normally defined as the ratio of the power generated by the antenna from a source on the beam axis of antenna to the power generated by a hypothetical lossless isotropic antenna. An isotropic antenna is one that is equally sensitive to signals from any direction.

When looking at antenna gains at radio frequencies which are used by signal boosters, this relative gain is expressed in dBi, i.e. dBi defines the gain of an antenna system relative to an isotropic radiator.

dBi is an abbreviation for "decibels relative to isotropic."

While dB is a relative number of the amount of increase or decrease in signal, dBi defines the gain of an antenna system relative to an isotropic radiator.

The specification for dBi is based on the decibel, a logarithm measure of relative power. As an example, an antenna X produces an electromagnetic field of intensity I_X microwatts per square meter (I_X µW/m²) in its preferred direction at a point a certain distance away. An isotropic antenna Z produces an electromagnetic field of intensity I_Z µW/m² at the same distance. Antenna X’s gain G can be calculated in dBi as:

G = 10 log ₁₀ (I_X / I_Z)

Using this formula, we can calculate that a dipole antenna typically has a gain of 2.15 dBi.

An Isotropic antenna has a 0 dB power rating, i.e. it has no gain or loss when compared to itself.

In the cell phone signal booster industry, dBi as the amount of power an antenna can send to, or receive from a specific direction.

dBi is an important number when looking at the outside signal strength and determining which antenna will be able to pick up enough of that signal to send to a signal booster.

dBi is also useful for determining which indoor antenna is most suitable for getting the indoor coverage area we want from an amplifier system. For example, an antenna specification may state something like this in reference to dB and dBi:

Max Gain: 7.5 dBi. Non-Circularity: ±4.0 dB.

Although this topic is more complex than what I can describe in this short post, I hope this has given you a basic understanding of what these terms refer to.

Read about differences between dB vs dBm.

What if you do not have enough signal to use cell phone signal booster?

If you cannot garner minimum signal strength using your omnidirectional or unidirectional antenna (survey kit), a cellphone booster, bi-directional amplifier (BDA) or RF repeater will not work. In such instances, you may submit location details for a indoor cellular coverage solution that may include a system such as a Small Cell, Microcell, Femtocell or active distributed antenna system (Active DAS).