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Power lunch

When you are planning a network installation and determining the allowable length of cable runs, you may need to determine the loss, in dB, of the cable that you will be installing. You may also need to know the power-transmission characteristics of your access to the local telephone company.

There is a simple formula for power gain or loss. This is the ratio of the output power to the input power of any piece of electronic equipment, whether it's an amplifier, a passive network or a piece of cable, which is really one of the simplest kinds of passive networks available.

Power calculations don't make a lot of difference for low-frequency power-distribution applications, or for DC power supplies, because the loss over cable systems is negligible for small distances. But when you get to high frequencies, such as those used for today's networking installations, it does make a difference, because the cable starts acting like a transmission line, rather than just as a piece of wire.

Power gain or loss in dB, or decibels, is defined by the following expression:

P = 10 log10 (Pout/Pin)

The leading 10 makes the expression come out in decibels (10 bels) rather than in bels. The log is the common base-10 logarithm, which you can figure out on a good scientific calculator, or you can use tables of logarithms if you can find them. Power is determined by measurement.

David Gabel is the executive technology editor of
This was last published in November 2000

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