Watt to dBi Calculator (with Example)

This tool converts RF Transmitter Output Power in Watt to Antenna gain in dBi.

Enter the following:

The tool will calculate the Antenna Gain in dBi required to hit this target EIRP (Effective Isotropic Radiated Power).

🔄 dBi to Watt Calculator

Formula

GAnt  = EIRP + LC – PTx

where

  • GAnt  is the antenna gain in dBi
  • EIRP is the Effective Isotropic Radiated Power in dBm
  • LC is the loss due to antenna cables and connectors expressed in dB
  • PTx is the transmitter power in dBm

In this formula subtract one dBm value from another (EIRP-PTx) and the result is a value in deciBel (dB).

This dB value can be added to another dB value (LC) to find the antenna gain in dB, except that it’s relative to Isotropic (remember that we used EIRP) and hence it’s dBi.

Example Calculation

If the transmit power is 1 Watt, cable loss insertion loss 1 dB and EIRP of 33 dBm, then the required antenna gain is 4 dBi.

Using a higher gain antenna will result in a higher value of EIRP. A higher value of EIRP can exceed the FCC limit and that’s something an RF designer always has to look out for.

Background

Watt is a unit of power. It is commonly used in RF systems to represent the power in or out of a circuit.

An antenna’s dBi value is a measure of its gain compared to a reference antenna. The i in dBi indicates that the reference antenna is isotropic – an ideal radiator that emits signal equally in all directions. An antenna with a higher dBi value has more gain than an antenna with a lower dBi value.

When choosing an antenna for a particular application, it is important to consider its gain.

💡 A directional antenna with high gain can be a good choice for applications where you need to maximize the signal strength in a specific direction.

An example is when you want to aim a radio signal in the direction of a known receiver location. The trade off is that directional antennas can make it more difficult to receive signals from other directions, so they are not always the best choice for every application.

Antennas for cell phone boosters and TVs are often directional to maximize signal from a fixed, known cell tower or TV station.

Antennas in your cell phone on the other hand, are omni-directional as the signal can be received from any direction.

Effective Isotropic Radiated Power or EIRP is the power that would have to be radiated by an isotropic antenna to provide the same signal level as the actual source antenna in the direction of the antenna’s strongest beam.

Antenna Cable Loss depends on the type of cable (e.g. LMR 400), its length and the frequency of operation.

Example Calculation

If the output power from a transmitter is 1 Watt, and the target EIRP is +33 dBm or 2 Watt, an antenna gain of 3 dBi is required. This assumes no cable loss.

11 dBi Directional Antenna

Applications

An RF systems engineer might use the antenna gain and the transmitter power along with other parameters to understand and calculate the range of the radio system. The calculator can be used to explore various trade offs between transmitter power, antenna gains, frequency of operation and receiver sensitivity.

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