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Table 2.

Table 2.

The important thing to notice is that when the power gain ratio is larger than I- which means that the power after is larger than the power before—the number of decibels is positive. On the other hand, if the power gain ratio is smaller than 1—which means that the power after is smaller than before -the number of dB is negative. Therefore a positive number of dB means a gain, whereas a negative number of dB is loss. (By the way, -3 dB represents a power gain ratio of 0.5, and so many people refer to -3 dB as "half power.")

For example, we said earlier that a I km length of foam Kti-8 cable would have about 40 dB los^ at 50 MHz. That translates to -40 dB (negative because it is a loss), so the power gain ratio is 0.0001, That means that the power after the cable is only 0.0001 times as large as the power going in. If you sent I watt into the cable, only 0.0001 watt would come out after just one kilometer, or about 3300 feet.

Compare that with glass fiber, which has a loss of 0.5 dB (which is really -0 5 dB) per kilometer. In six kilometers (about 4 miles), this adds up lo -3 dB, and the above table tells us that is a gain ratio of 0.5. This means that the power coming out of a 4-mile length of glass fiber is half of what went in. This is an astounding number—just visualize how clear u piece of window ghtss would have to be if it were 4 miles thick, yet half of the light going in would still come out the other end!

Besides comparing powers, decibels can also be used to compare voltages. But in this ease the equation has a 20 instead of a 10 in front of the log. and it becomes dB = 20 log m Voltage after/ Voltage before.

This makes all the dli values twice as large, and gives us the following table:

Voltage Gain Ratio

db

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