3/20/2021 0 Comments Saved By The Bell Pattern
But like I said, no one uses bels, so multiply the answer by 10 to get decibels (10dB).Actually, the real measure is a bel, but you almost never see that in practice.If you are versed in metric, you wont be surprised to learn a decibel is 110 of a bel.
Sometimes in electronics, we deal with really large ratios, so the decibel is logarithmic to cope with this. Doubling the number of decibels doesnt double the ratio, as you will soon see. Its all about logarithms, and this ends up being extremely useful when measuring something like antenna or amplifier gain. The reason is that human ears and eyes have a logarithmic response to those quantities. That is to say, you can hear a whisper or a space shuttle launch. This is also why potentiometers made for volume controls have a logarithmic taper. A linear pot would seem off because, for example, a tenth of a turn at one extreme will affect the apparent volume much more than a tenth of a turn at the other extreme. Originally, the unit used to measure loss in telephone and telegraph cables was miles of standard cable (MSC). There was an elaborate definition of what a standard cable was and 1 MSC was the amount of loss of a predefined signal over a mile of this cable. It caught on, and we still use it today to measure gain and loss (loss shows up as a negative number). Thats a total gain of about 148 times ( log(148)2.17 bels21.7 dB see the section below for help with these calculations). However, it is difficult (and perhaps not too useful) to measure the power output since it isnt really a two terminal device. Instead, antennas are measured by their gain over an isotropic antenna (an unbuildable antenna that is a single point in space that radiates the input power evenly in all directions). To avoid confusion, you almost always see an antenna gain written as 3 dBi (for isotropic) or 3 dBd (for dipole). For example, a dipoles gain is 1.64:1 (2.15 dBi) peak. But in some directions, it is much less. You can see the radiation pattern for a dipole in the figure to the right. There are also some handy math facts that youll need to know if you want to do more than just memorize the decibel formula. Adding two logs is like multiplying the two original numbers and taking the log. But you can also find the answer by taking the log of 20 (about 1.3) and multiplying by 9. If you just want to remember a rule of thumb, the formula (see below) with the 10 in it is for power and the one with 20 in it is for voltage or current ratios. So if you put in 1 watt and get out 10 watts, thats a gain of 10X and thats 1 bel.
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