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: