dBi vs dBm – what’s the difference?

The difference between dBi and dBm lies in what they measure: dBi measures antenna gain, while dBm measures power levels.

dBi

This is a unit of measure that indicates the gain of an antenna compared to an isotropic radiator (a theoretical antenna that evenly distributes energy in all directions). The i in dBi stands for isotropic

dBi values show how much an antenna can focus energy in a particular direction compared to the isotropic radiator. A higher dBi value indicates a higher gain, meaning the antenna can send or receive signals more effectively in specific directions.

The picture below shows visually the difference between a 2 dBi vs 9 dBi antenna. The 9 dBi is more focused and signals emitted from this will reach further.

This measurement is particularly useful when comparing the efficiency of different antennas.

dBm

This unit measures power levels in decibels relative to 1 milliwatt. The m in dBm stands for milliwatt.

dBm quantifies absolute power levels, allowing for direct comparison of the power output or received by different devices regardless of their type or configuration.

For example, a signal strength of 0 dBm corresponds to a power level of 1 milliwatt, while 30 dBm corresponds to 1 watt. dBm is often used to specify the power level of transmitters, receivers, and other RF components, as well as to measure signal strength in wireless networks.

Can you calculate dBm from dBi?

Converting dBi to dBm isn’t a direct conversion in the way one might convert meters to miles, because they represent different concepts.

In a wireless transmitter system, dBm represents the power at the transmitter output P_Tx and the Effective Isotropic Radiated Power (EIRP). dBi is used to represent transmit antenna gain. The Cable Loss is represented as a number in dB.

The conversion is given by the equation

P_Tx = EIRP + L_C – G_Ant

• Calculate dBm from dBi
• Calculate dBi from dBm

Summary

While dBi is used to compare the directional or gain characteristics of antennas, dBm is used to measure absolute power levels of signals. Both are important in the design, analysis, and troubleshooting of wireless communication systems, but they serve different purposes.

Related Posts

• dBm vs dBc – what’s the difference?
• dBm vs dB – what’s the difference?
• The dBi value of an antenna can be calculated from the Antenna Factor and Frequency of operation