Definition of Decibel

This topic describes the decibel (dB) calculation.

Definition of Decibel

Decibel (dB)

• Ratio of one value to another

• dBi = Antenna gain based on isotropic antenna

[dB] = 10 log10 (Ratio)

0 dB

1:1

10 dB

10:1

+3 dB

Multiply by 2

-3 dB

Divide by 2

+10 dB

Multiply by 10

-10 dB

Divide by 10

13 dB = 10 + 3

20 = 10 * 2

20 dB = 10 + 10

100 = 10 * 10

17 dB = 20 -3

50 = 100/2

Antennas and RF power measurements use units based on decibels.

A decibel (dB) is the ratio between two signal levels. This measurement is named after Alexander Graham Bell. Descriptions of the different types of decibel measurements follow.

■ dB Milliwatt (dBm): A signal strength or power level. 0 dBm is defined as 1 mW (milliwatt) of power into a terminating load such as an antenna or power meter. Small signals are negative numbers (for instance -83 dBm).

■ dB Isotropic (dBi): The gain a given antenna has over a theoretical isotropic (point source) antenna. Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and system operating margins.

These values were all estimated using 0 dBm = 1 mW as a starting point.

— Subtract 10 dB = divide power by 10 Example:

0 dBm = 1 mW, and 14 dBm = 25 mW 0 dBm = 1 mW, therefore 10 dBm =10 mW, therefore 20 dBm = 100 mW, subtracting 3 dB (17 dBm = 50 mW) subtract 3 more (14 dBm = 25mW)

6-134 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Effective Isotropic Radiated Power

This topic describes Effective Isotropic Radiated Power (EIRP).

Effective Isotropic Radiated Power

Transmit power is rated in dBm or mW.

Power coming off an antenna is Effective Isotropic Radiated Power (EIRP).

Transmit power is rated in dBm or mW.

Power coming off an antenna is Effective Isotropic Radiated Power (EIRP).

FCC and ETSI use EIRP for power limits in regulations for 2.4-GHz and 5-GHz WLANs.

EIRP [dBm] = Power [dBm] - cable_loss [db] + antenna_gain [dBi]

EIRP is defined as the effective power in front of the antenna. The EIRP of a transmitter is the power that the transmitter would appear to have if the transmitter were an isotropic radiator (that is, if the antenna radiated equally in all directions).

By virtue of the gain of a radio antenna (or dish), a beam is formed that preferentially transmits the energy in one direction. The EIRP is estimated by adding the gain (of the antenna) and the transmitter power (of the radio). Transmit power is rated in dBm or mW:

EIRP = transmitter power + antenna gain - cable loss

When using radio equipment, there are limits on the output of the system. These limits are given as EIRP and must not be exceeded. Different countries have different standards. Check with authorities in the country of installation to determine maximum EIRP.

EIRP is what the FCC and European Telecommunications Standards Institute (ETSI) uses for power limits in regulations for 2.4-GHz and 5-GHz WLANs.

© 2006 Cisco Systems, Inc. Wireless LANs 6-135

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