dBi, dBm and mW?

The terms dBi, dBm and mW are all measurements used when talking about wireless communications. The terms dBm and mW are used when talking about the output power of a transmitter or wireless interface. The term dBi is used when talking about the Gain of an antenna.

dBm and mW

The terms dBm and mW are the two ways of expressing the output power of a wireless interface or transmitter. Many manufactures quote the output power of there card in milliwatts (mW) but others quote the output powers in dBm.

Both of there methods are correct ways of expressing the output power, but how do we compare between the two.

Example: Converting mW to dBm

For this example we will look at the Ubiquiti XR2. This card is quoted as having a maximum power output of 600mW. To convert this into dBm you use the following formula:

Power Output (dBm) = 10*log(Power Output in mW)

10*log(600) = 27.78dBm

Which agrees with the manufactures datasheet

Example: Converting dBm to mW

For this example we will look at the Ubiquiti XR2 . This card is quoted as having a maximum power output of 28dBm. To convert this into dBm you use the following formula:

mW = 10^(dBm/10)

10^(28/10) = 630mW

It can be seen that this value is a bit high, this shows the importance of precision when dealing with power outputs. A very slight change can have a large effect of the end result.

Lets use the value for the output power of the XR2 calculated in the previous example

10^(27.78/10) = 599.79mW

This value agrees with the manufactures specifications.

What is dBi?

The term dBi (decibel isotropic)is used to express the gain of an antenna in comparison with an isotropic antenna. An isotropic antenna is a theoretical antenna which broadcasts in every direction. An isotropic antenna has a gain of 0dBm.

The directionality of the antenna is inversely proportional to the gain of hte antenna, therefore the higher the dBm of the antenna the more directional it is. This can be seen when comparing the following antennas:

Kenbotong 19dBi 2.4GHz Grid- Has a Horizontal beamwidth of: 19º

Kenbotong 24dBi 2.4GHz Grid – Has a Horizontal beamwidth of: 10º

Common Conversions

The following conversion are the common power outputs given by various wireless card manufactures.

  • 100mW = 20dBm
  • 200mW = 23dBm
  • 300mW  = 24.7dBm
  • 320mW = 25dBm (Mikrotik R52Hn)
  • 350mW = 25.4dBm (Mikrotik R52H)
  • 400mW = 26dBm (Ubiquiti SR2)
  • 600mW = 27.8dBm (Ubiquiti XR2)
  • 700mW = 28.5dBm (Ubiquiti SR9)
  • 1333mW = 31.2dBm

Comments are closed.

created By ooyes.net