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Overheard Scanner Features, Part II by Ken Reiss |
 More sophisticated receivers often include an RF gain control in addition to, or instead of, an attenuator. This reduces the amplification in the front end of the receiver and can help reduce interference. Of course, it also reduces sensitivity to weak signals, so you don’t want to use it unless necessary. |
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This month we’re continuing our series on scanner terms and features, and we’ll be tackling a couple of tough ones. Look through the subheads, and if you already know all the terms you can have the month off, we’ll see you next time. In the meantime, let’s jump right in where we left off. Just What Is dB? The term dB is commonly used but often misunderstood so, while it’s not really a scanner feature per se, I thought it might be appropriate to include. If you’re reading this paragraph, then it means that good ol’ “What the heck did he say anyway?” Harold agreed with me and left it in. The “d” in dB stands for deci, or one tenth. One tenth of a Bel is what the dB measures. So what exactly is a Bel? A Bel is a measure of gain or efficiency expressed in a funny notation. If something is 10 times better than something else, or can amplify or produce 10 times more signal than something else, it would be 1 Bel better or higher. Something one hundred times better would be 2 Bels, so each zero adds one to the count, so to speak. Mathematically inclined people will recognize this as a logarithmic scale and that’s exactly what it is. Now to confuse things a bit. Since we don’t normally get as much as 10 times the amplification, a smaller, more graduated scale is in order, and that’s where the decibel comes into play. Ten dB is equal to 1 Bel, or 10 times the amplification. Twenty dB would be 2 Bels, or one hundred times the amplification. By using decibels, we can get some easy-to-remember numbers that are meaningful. For instance, 3 dB is about two times the efficiency; or, if it’s a loss, -3 dB would be about 1/2. Six dB is about four times the gain or efficiency. So really all we’re saying is that one thing is better or worse than another by some factor. Adding a 6-db attenuator would cut the signal to about 1/4 of what it was originally. An antenna with 3 dB gain would, in theory, be able to present twice the strength of signal to the radio as one without gain.
All things considered, that seems relatively
simple. But not so fast, because there’s a problem. The problem is the
standard for measurement. In order to get a gain measurement of one thing
over another, there must be two things to compare. But what was the other
thing compared to when you see published radio specs? That’s where things
get a bit tricky. There are some standards, but they’re not always adhered
to. By comparing to some other “adjusted” standard, you can make the dB
figures much more impressive. |
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