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How do I obtain the dB gain/loss of cables and antennas?

I'm new to wireless field and have a question regarding RF math. How do I obtain the dB gain/loss of cables and antennas?

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Cables and cable connectors induce signal loss, while antennas are designed to introduce signal gain. These products cite a loss or gain rating in product specs. A loss of -3 dB cuts power (mW) in half, while a gain of +3 dB doubles power. A -10 dB loss reduces power to one tenth, while a +10 dB gain increase power ten times. The actual formula is ( Pdbm = 10logPnW ), but using 3's and 10's can approximate signal gain/loss without a logarithmic calculator.

For example, suppose you have an AP that outputs 100 mW. The first -3 dB connector cuts power in half (50 mW). The second -3 dB connector cuts power in half again (25 mW). A +6 dBi antenna doubles power twice (25 x 2 x 2 = 100 mW). Cable loss, typically rated in dB per 100 feet, must also be factored in -- as must any amps, attenuators, and splitters in the path between the AP and antenna. The result of this equation is known as Equivalent Isotropic Radiated Power (EIRP): the power output at the antenna. Remember that there is also Freespace Loss introduced as the signal propagates through the air between the sender and receiver.

To learn more about RF math, refer to the CWNA Study Guide, available from Planet3Wireless. A SignalLoss page with helpful conversion and example links can also be found at SeattleWireless.

This was first published in January 2004

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