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# Introduction to dB math and Effective Isotropic Radiated Power (EIRP)

Wireless networks are typically created using different components from different manufacturers. These manufacturers use different measurement values to rate their products. For example, wireless access points are typically rated in milliwatts and the antennas that connect to them are rated in decibels. This is not a big deal except when you consider that the maximum transmit power in North America is 1000mW or 30dBm and the maximum **EIRP (Effective Isotropic Radiated Power),** or power transmitted from the end of the antenna, is 36dBm or 4 watts.

## Understanding decibel (dB)

According to Wikipedia, “a decibel (dB) is one tenth of a bel (B), i.e., 1B = 10dB. The bel represents a ratio between two power quantities of 10:1, and a ratio between two field quantities of √10:1. A *field quantity* is a quantity such as voltage, current, sound pressure, electric field strength, velocity and charge density, the square of which in linear systems is proportional to power. A *power quantity* is a power or a quantity directly proportional to power, e.g., energy density, acoustic intensity and luminous intensity.”

Just to make things more difficult, sometimes this all has to be converted to milliwatts. A milliwatt is a unit of power equal to one thousandth of a watt. So in laymen’s terms what does this all mean?

## Determining the EIRP value for a radio antenna combination

To determine what the EIRP will be for any radio antenna combination, all you need to know is a few things.

*The definition of db values so you know what ratio to work with.*

dBi – decibel compared to an isotropic antenna

dBm – decibel compared to 1 milliwatt

dBd – decibel compared to a 2.2 dBi Dipole

dBx – decibel compared to x

*The dBm to milliwatt ratio chart with standard dBm to milliwatt values:*

0 dBm |
1 mW |

3 dBm |
2 mW |

6 dBm |
4 mW |

10 dBm |
10 mW |

20 dBm |
100 mW |

30 dBm |
1000 mW |

1000 mW |
4 Watts |

Using the information stated above, it becomes rather easy to calculate the EIRP value of the diagram below.

30dBm + (-9dBm) = 21dBm EIRP

## Why is this important?

For a wireless network it is absolutely crucial that you do your math right, any errors at this stage could result in a complete failure while deploying a wireless network.

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