Figure 118. Transmission line with 50 percent power loss.

4-22. SOLVING THE FORMULA FOR DB

b. In the first example you had a power

GAIN

loss of 50 percent and this gave a loss of 3 db. Then,

in the second example, you had a gain of twice as

a. Figure 119 shows a repeater that

much power and this gave you a gain of 3 db. This

amplifies the input power to twice its original value.

brings out two important facts that you should

You see that the input power to the repeater is 1 mw

remember:

and the output power is 2 mw. You can find out

how much db gain this repeater provides by using

(1) A LOSS OF 3 DB ALWAYS

the formula:

REPRESENTS A 50-PERCENT

POWER LOSS. It doesn't matter

how much power is involved.

When you lose half the power you

always have a loss of 3 db.

(2) A GAIN OF 3 DB ALWAYS

REPRESENTS A GAIN OF

TWICE AS MUCH POWER.

Again the amount of power

You know the log of 2 is .3010. Therefore you

involved doesn't mater. Whenever

have:

you gain twice as much power, you

have a gain of 3 db.

db = 10 x .3010

This time you have a gain so you use a plus sign to

express the final result:

+3.01 db or approximately +3 db.

Figure 119. Repeater providing power gain.

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