what the heck is a "DB"?

[hookedon6]

we hear them used all the time. "buzz" words like: s-units, attenuation, velocity factor, gain, dB, dBi (ect).

please, don't just cut and paste a tech post from another site, this thread (hopefully) will require some thinking and result in a better understanding of rf theory and practical applications.

just about everything in radio somehow relates to radiated power; be it antenna gain, external amplifers, coax vs hardline vs ladderline.
we have all made decisions in these areas. but what have we based those decisions on?

what the heck is a "dB"?

 

[scout943]

Decibel

DB Decibel used to measure the loudness of sound or strength of a signal. A DB COOPER is the guy that jumped out of a 727 Aircraft with a bag full of money!!

 

[W5LZ]

A 'decibel' is a way of comparing two quantities of the same thing. Volts/volts, amps/amps, quarts to quarts, pints to pints, and so on and so forth. It's based on a logarithmic (base 10) scale rather than a 'straight' mathematical scale. Originally it was used to compare sounds heard by ear. It can make hugely different quantities easier to 'relate' to each other, simpler number so to speak. For instance, if you measure something by the pound, and you want to compare a pound of dirt in relation to a pound of atoms, the dB of the number of atoms making up a pound of dirt would be a (-) value since there's a huge number of atoms compared to just one pound of dirt. That dB number would be quite large, 'negatively' speaking, but still a number the average 'mind' can still comprehend, sort of. That's a really terrible example! But you sort of can see the relationship/value of a logarithmic scale instead of a straight 2 time, 50 time, or 10 trillion times this and that. I'm afraid that's not helping any, is it? What the hell, I can't say I really understand (or even like dB's). Sort of a complex way of making a possibly humongus difference easier to relate to? Forget it, if you ever get a very simple way of understanding it, let me know!
- 'Doc


I can use logarithms. But it's not something I 'think' in terms of. I can 'relate' to what a dB of this-n-that is, but it doesn't mean I understand the quantity envolved, really. It's getting to the point I wish I'd never heard of the @#$ things. I'm lazy! You are trying to make me think! Shame on you!

 

[Beetle]

A decibel (dB) is 1/10 of a Bel. That's not much help, so -
The decibel is a means of defining the difference between two quantities based on an agreed-upon reference. It's non-linear, but rather logarithmic, and it can be used to compare sound levels, electrical/RF power levels, the stage gain of amplifiers, and (of course) antenna performance.

As I stated earlier, there HAS to be a standard or reference on which measurements are based. For antennas, the two most common are dBd (which uses a half-wave dipole as the reference), and dBi (which uses an "isotropic" antenna, which is purely an imaginary construct that radiates equally in all directions in three dimensions. Think of a glowing spark at the center of a basketball.)

 

[dudmuck]

you got some good answers here.

For power (watts):
+3dB is doubling your power
+6dB is doubling it twice
+10dB is ten times the power

-10dB is one tenth the power
-6dB is one-fourth the power
-3dB is one half the power

and so on, +20dB is 100x the power, and +30dB is a thousand times, etc.
(+) dB is gain
(-) dB is attenuation

 

[Moleculo]

For power (watts):
+3dB is doubling your power
+6dB is doubling it twice
+10dB is ten times the power

-10dB is one tenth the power
-6dB is one-fourth the power
-3dB is one half the power

and so on, +20dB is 100x the power, and +30dB is a thousand times, etc.

This is a good practical application of how we can use the decibel. For example, if we say that an amplifier has -30db attenuation on the 2nd harmonic, that means that a 1KW amplifier will be putting out 1 watt on the 2nd harmonic of whatever frequency you're operating on. (1000watts/1000 = 1) 1 watt of the right type of signal is enough to wipe out some types of RF communications. Now, if we can increase that to -40db on the 2nd harmonic, it will be 10,000 times less, or 1/10th of a watt. That's a big difference!