Use this calculator to find the max output power of a transmitter in Watt from the antenna gain in dBi. For this calculation, Effective Isotropic Radiated Power (EIRP) is also required.
Watt to dBi Formula
PTx = EIRP + LC – GAnt
How to use the calculator
Enter the following quantities:
- EIRP in dBm
- LC or the total of transmitter and RF adapter cable losses in dB
- GAnt Antenna Gain in dBi
The tool will provide the power PTx at the output of the transmitter in dBm. dBm is then converted to Watt.
The equation above is an example where dB (gains and losses) can be subtracted from dBm to give the final result in dBm
Example Application
The maximum EIRP for a LoRa device operating in the European 868 MHz ISM band is +16 dBm. The maximum antenna gain is specified to be 2.15 dBi.
Using these numbers and a cable loss of 2 dB, the maximum output power from the transmitter is restricted to 0.038 Watt. These power restrictions are in place by spectrum regulatory agencies to prevent RF interference to other systems operating at the same frequency.
Background
This calculator can be used by RF Designers and Systems Engineers.
RF designers can use this tool to design the transmit chain of a radio and ensure among other technical specifications that it complies with government regulation for power.
A Systems engineer uses the EIRP information in their planning tools which includes among other things, a path loss calculator and receiver sensitivity to calculate the range.