dBi (Decibel-Isotropic)

The decibel isotropic (dBi) is a unit that measures antenna gain relative to an ideal isotropic radiator. An isotropic antenna radiates power uniformly in all directions and serves as a theoretical reference with a gain of 0 dBi. Antenna gain expressed in dBi indicates how much more power is radiated in the antenna’s strongest direction compared to this idealized reference.

How dBi is Calculated

Antenna gain in dBi is defined as:

G(dBi) = 10 × log₁₀(P₁ / P₀)

Where:

• P₁ = Power density in the main radiation direction

• P₀ = Power density of an isotropic antenna with equal input power

Gain also depends on antenna directivity (D) and efficiency (η):

G = η × D

Where:

• G = Antenna gain (linear)

• η = Efficiency (between 0 and 1)

• D = Directivity

These formulas are essential in antenna design and system performance modeling.

Examples of Antenna Gain in dBi

• Wi-Fi Router Antennas:
  Typically 2–5 dBi for balanced indoor coverage.

• Directional Yagi Antennas:
  Around 10–15 dBi, focusing energy in a narrow beam.

• Satellite Dish Antennas:
  Often exceed 30 dBi, enabling high-gain, long-range communication.

Such values guide antenna selection based on required coverage area and link budget constraints.

Importance in System Design

Higher dBi values do not increase the total transmitted power but focus it into a narrower beam, extending range in specific directions while reducing coverage elsewhere. This trade-off makes dBi a critical factor in:

• Wireless network planning (Wi-Fi, LTE, 5G)

• Satellite and radio communication

• Radar, telemetry, and point-to-point systems

Unlike dBd, which references a physical dipole antenna, dBi uses a standardized isotropic model. This universal reference simplifies performance comparison and system modeling across applications and manufacturers.