decibels for coax, huh?

I know what it means to go 25 mph (or km, sort of) versus 100 MPH (or knots, if I was a sea captain) I even have a idiot level grasp of ohms Ω for my 4Ω speakers (they like power and do better with power) but, I have no idea what it means to loose .5db versus 100db on coaxial cables. I thought db was for decibels? How is that a cable signal(?) measure? Wait, does that mean, the loss you incur with splitters is mostly all about audio? But, with DTV over coax, how is that possible?
What is the scale? (The decibel scale for volume with AV gear is annoyingly hard to use versus one like 0-10 on a dial.) If this is the measure for coax, what is the maximum? (Perhaps in this case, for human use and consumer electronics capability versus NASA use.) What is a **db** in this case and why does my antenna or coax *have* db? Saying you loose db means nothing to people who are not electrical engineers. Can someone explain it? Pretend I am 7 years old. I am 7 years old… actually…

3 Replies to “decibels for coax, huh?”

  1. Db is used to measure the power of a signal. It’s most commonly used in audio, but works for any type of signal. The information in the video portion of tv is very similar to the audio portion, but your tv is smart enough to know the difference between picture information and sound information. All electronics have a minimum signal to noise ratio they need before they can tell the background noise from actual information. Random radio stations and cell phone calls and all other electronics put random signals into the air that get absorbed into the wire randomly. If the DB falls low enough in a tv signal, instead of a picture and audio you’ll just get white snow or a black screen because the tv decoder can’t tell what’s random noise and whats picture or audio information.

    Because of the logrithmic scaling, every 3 Db is double the power. So a signal thats 20 db is twice as strong as a 17db signal. If your operating near the noise floor, eliminating a few connections could pretty easily double the signal strength, effectively turning up the volume that’s getting delivered into your tv.

    If you’re stereo has a db setting for audio you can think of it like this. You’re ear has a signal to noise ratio, eventually you cant hear things but instead just have silence. As your radio is playing music start turning down the volume. Once you can’t hear the music anymore, see what the radio says the volume is, that’s the minimum power required for your ear to distinguish between something and nothing.

    If your coax says it has 3db/100 feet, it means the resistance in the wire causes half of the signal to disappear every 100 feet. Basically its telling you how much the wire is turning down the volume before it gets to your tv. turn it down too much and your tv cant hear or see anything, so you just get a black screen.

  2. All coax cable has loss. The term is “Attenuation”,
    and it is measured in Decibels per 100 feet. One
    Decibel is about 25%. It’s a logarithmic scale
    (based on the power to which 10 is raised to equal
    the factor expressed in the ratio) where:
    1 dB = x 1.26
    3 dB = x 2
    6 dB = x 4
    10 dB = x 10
    20 dB = x 100
    30 dB = X 1000
    We use Decibels to express a ratio between two
    values. In this case, we will look at the
    attenuation in typical coax cables, where we are
    looking at the ratio of the power that goes in one
    end to the power out the other end of 100 feet of
    cable. Fifty feet would exhibit half this much
    loss.
    The attenuation of coax cable increases with
    frequency. So the values below are listed for 30
    MHz (HF), 50 (6 meters) 150 (2-meters), 220 (1.25
    meters) and 450 MHz (70 centimeters)
    Coax attenuation per 100 feet (dB)
    Frequency RG58 RG8/LMR400
    30 MHz 2.5 dB 0.7 dB
    50 MHz 3.1 dB 0.9 dB
    150 MHz 6.2 dB 1.5 dB
    220 MHz 7.4 dB 1.8 dB
    450 MHz 10.6 dB 2.7 dB

    ….continue to [full PDF](https://www.qsl.net/w7rdr/documents/coax.pdf)

  3. In practice you can ignore the reasons that people think it is a good idea to measure what comes out of a thing with respect to what goes into a thing in such a weird way as dB. If your cable has less dB *loss* then that is good. If your antenna has more dB *gain* than that is also good. If your amplifier has more dB *gain* than that is good too in most cases.

    Gain is just the opposite of loss. You could say that your really high performance antenna had -10 dB of loss if you wanted to be different for no reason. If the same way you could say your crappy cable had -10 dB of gain.

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