LEARNING EVENT 36: MEASURING POWER LOSSES AND GAINS
a. In operating certain types of equipment you will be measuring power losses and gains. A unit called the
decibel (dB) simplifies your task because instead of having to calculate losses and gains that range anywhere
from .000001 watt up to .004 watt, you use meters that express power losses and gains in terms of minus dB(s)
and plus dB(s). With this method, there are no complex decimal calculations to perform.
b. Before you can use the decibel, you have to know something about it. That's the aim of this learning
event--to tell you what the decibel is, how the decibel is derived, and, most important, how you will use the
decibel in your daily work.
2. What is the decibel?
a. The dB is a transmission measuring unit used to express power loss and gain. When used to express
loss, a minus sign is placed before dB like this: -10 dB. When used to express gain, a plus sign is placed before
dB like this: +10 dB.
b. The dB does not express exact amounts like the inch, the pound, or the gallon. The dB does not tell you
how much power you have. Instead, the dB tells you the ratio of power in a circuit.
c. In other words, the dB compares the output power of a circuit to the input power. If there is less output
power than input power, then you have a dB loss. If there is more output power than input power, then you
have a dB gain.
3. How you compute dB loss and dB gain.
DB losses and gains are computed using the follow formula2:
Number of dB = 10 x log of P1 (larger power)
P2 (smaller power)
This formula is used both for dB loss and dB gain. The rule to flow is to always let P1 equal the larger
amount of power. You will know you have a loss (-dB) when the input power is greater than the output power.
Similarly, you will know you have a gain (+dB) when the output power is greater than the input power.