SECTION V. DECIBELS
4-18.
INTRODUCTION
Note:
This formula is used both for db loss and db
gain. The rule to follow is to always let P1
a. In operating certain types of equipment
equal the larger amount of power. You'll
you'll be measuring power losses and gains. A unit
know you have a loss (-db) when the input
called the decibel (db) simplifies your task because
power is greater than the output power.
instead of having to calculate losses and gains that
Similarly, you'll know you have a gain (+db)
range anywhere from .000001 watt up to .004 watt,
when the output power is greater than the
you use meters that express power losses and gains
input power.
in terms of minus db's and plus db's. With this
method there are no complex decimal calculations to
4-21. SOLVING THE FORMULA FOR DB
perform.
LOSS
b. Before you can use the decibel, you
A transmission line is shown in figure 118.
have to know something about it. That's the aim of
The input power to the line is 1 milliwatt (mw) and
this section -- to tell you what the decibel is -- how
the output power is .5 mw. It is easy to see that this
the decibel is derived -- and, most important, how
line causes a power loss of 50 percent. But what is
you'll use the decibel in your daily work.
the db loss? You can use the formula to find out
4-19.
WHAT IS THE DECIBEL?
a. The decibel (db) is a transmission
measuring unit used to express power loss and gain.
When used to express lose, a minus sign is placed
before db like this: -10 db. When used to express
Dividing 1 by .5 you get 2 as the result.
gain, a plus sign is placed before db like this: +10
This gives:
db.
db =10 x log 2
b. The db does not express exact amounts
like the inch, the pound, or the gallon. The db does
Now look at Table III to find the log of 2. The log is
not tell you how much power you have. Instead, the
.3010 so you have:
db tells you the ratio of power in a circuit.
db = 10 x .3010
c. In other words, the db compares the
output power of a circuit to the input power. If there
Since you're dealing with a power loss, you use a
is less output power than input power, then you have
minus sign to express the final answer:
a db loss. If there is more output power than input
power, then you have a db gain.
-3.01 db or approximately -3 db
4-20.
HOW YOU COMPUTE DB LOSS AND
TABLE III
DB GAIN
LOGARITHMS
Db losses and gains are computed using the
following formula:
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