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
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
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
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
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.