If you hold any class of amateur radio license, you’ve read and studied about the decibel. However you’ve come to grips with the term “decibel” or managed to choose the correct answer when asked about it on an exam, do you really understand it? Don’t be ashamed to say “no”. You’re not alone in doing so! Without getting too involved in mathematics, let’s talk about the “decibel” in easy to understand language.
The decibel is simply one tenth of a bel (deci means ten). The bel is so named in honor of its “inventor”, Alexander Graham Bell. Bell wanted a way to measure changes in loudness, in relative terms rather than specific ones. In other words, in the early days of telephony, Bell wanted a simple way to measure the relative gain or loss of sound level. Thus, the bel as a unit of measurement of gain or loss of volume, was born.
So for Alexander Graham Bell, the bel as well as the decibel is simply a ratio of change in volume, either up or down. Since the decibel measures a more minute change in power, intensity or volume than the bel, we’ll concentrate on the decibel more closely than the bel in this series of articles.
What’s the difference between a 5 watt transmitted signal and a 500 watt one? The answer is that 500 watts is 100 times 5 watts, which can be expressed as a 20 db power gain. How do we get that figure (20 db?)
There’s a formula for that, which is db=10log(P1/P2). So, the definition of db is simply the base logarithm of the ratio of two power values. Don’t let that formula scare you. There’s an easier way to understand the decibel, and we’ll get to it soon. Why do we use logarithms instead of the voltage or power units themselves? The answer is that whenever we encounter large numerical ranges (such as the ratio of power of a transmitter versus the voltage measured at the antenna input of a receiver, it is more convenient to use the logarithm of the numbers rather than the numbers themselves. If you don’t want to bother with formulas in order to convert actual values to the logarithm of those values, you can often determine db gain by easier means. If the higher power has ten times the power of the first, that’s 10 db gain. If it has 100 times the power, that’s 20 db gain. If it has 1000 times the power, that’s 30 db gain……and so on. You see, gain is expressed in powers of ten. Ten times 10 is 100. Ten multiplied by itself three times is 1000. It’s important to understand the decibel because it is used extensively in amateur radio. Antenna gain, transmitted power, receiver sensitivity and a myriad of other measurements are expressed in terms of decibels. Once you get used to “relative gain or loss” expressed in decibels, you’ll come to fully understand why they’re used so extensively in amateur radio.
Rick Fearns
K6VE
