How to read signal strength in dbm's

Archived from groups: alt.internet.wireless (More info?)

Hello,
I googled around and can't find anything on this. Anyway, I see people
online mention that they are getting a -87dbm signal strength. As a
amateur radio operator I know about db's and antenna gain, but I use
S-Units for signal strength, not dbm's.
So, is -100dbm better than -1dbm?? How does that scale work? I am not
used to better signals having a negative number.

Thanks,
Scott
7 answers Last reply
More about read signal strength
  1. Archived from groups: alt.internet.wireless (More info?)

    /dev/scott0 <outsideworld@scottbrown.us> wrote in message news:<10h0tggh112et49@corp.supernews.com>...

    > So, is -100dbm better than -1dbm?? How does that scale work? I am not
    > used to better signals having a negative number.

    -1 dBm is significantly better than -100 dBm.

    Absolute power of a signal is measured in wattage. The bel or decibel
    system can only describe relative power- a gain of 3 dB means your
    signal is 2 times as strong as it was before, but the dB scale doesn't
    define where you're starting from or what your 'zero' is. So, we
    specify dBm, indicating that our scale is relative to 1 milliWatt of
    power. 0 dBm = 1 mW.

    The reason you see negative values is that you're representing small
    but positive numbers, on a logarithmic scale. In logarithms, the value
    indicated represents an exponent... for example, under a log 10 scale,
    a value of -2 represents 10 to the -2 power, which equals 0.01.
    Likewise, a negative dBm means that you're applying a negative
    exponent in your power calculations; 0 dBm equals 1 mW of power, so
    -10 dBm equates to 0.1 mW, -20 dBm equates to 0.01 mW, and so forth.
    It's a lot easier, and more useful in some calculations, to describe a
    weak signal as -100 dBm as opposed to 0.0000000001 mW.
  2. Archived from groups: alt.internet.wireless (More info?)

    In article <10h0tggh112et49@corp.supernews.com>, /dev/scott0
    <outsideworld@scottbrown.us> writes
    >Hello,
    > I googled around and can't find anything on this. Anyway, I see people
    >online mention that they are getting a -87dbm signal strength. As a
    >amateur radio operator I know about db's and antenna gain, but I use
    >S-Units for signal strength, not dbm's.
    > So, is -100dbm better than -1dbm?? How does that scale work? I am not
    >used to better signals having a negative number.
    >
    >Thanks,
    >Scott

    Without a lecture on the subject a dBm is a simple reference level and
    then you can go up in level say 10 dBm, meaning 10 dB UP from 0 dBm or
    down from that say -10 dBm so -80 dBm is 70 dB lower than -10dBm its all
    "relative" levels as such!. So -1dBm is a "higher" level than -100dBm!

    Go to the Jack Daniel's site www.rfsolutions.com and download his RF
    calculator which runs under excel and you can do lots of useful
    conversions i.e. dBm to dBw etc!.....
    --
    Tony Sayer
  3. Archived from groups: alt.internet.wireless (More info?)

    On Tue, 03 Aug 2004 22:37:17 -0700, /dev/scott0
    <outsideworld@scottbrown.us> wrote:

    >I googled around and can't find anything on this. Anyway, I see people
    >online mention that they are getting a -87dbm signal strength.

    0dbm is equal to 1 milliwatt into 50 ohms.
    -3dBm is half a milliwatt
    -6dBm is 1/4 of a milliwatt
    -9dBm is 1/8 of a milliwatt.
    For every 3dB below 0dBm, you get half your power.

    >As a
    >amateur radio operator I know about db's and antenna gain, but I use
    >S-Units for signal strength, not dbm's.

    Gaaak. Turn in your ham license and reread the study guides. S-Units
    are an ancient anacronism left over from the stone age of thermionic
    valve (tube) receivers. The characteristic curve between receiver
    input signal level and indicated AGC (automagic gain control) DC level
    was anything but linear or even consistant. Someone decided to
    establish a reference signal level. S-9 magically became 50
    microvolts because either Collins of Hammarlund (I forgot which)
    managed to make receivers that would consistantly read S-9 with 50 uv
    of RF. I dunno what frequency was used for testing. It's anything
    but a standard and suffers from serious errors (and guesswork).

    See:
    http://www.ac6v.com/sunit.htm
    for more details and why you should avoid using S-units.

    Most of todays cell phones have genuine logarithmic amplifiers to
    detect IF signal strength (before limiting). The output is fairly
    linear (well, fairly logarithmic) and can be used as an accurate
    measure of signal stength (assuming you don't attach an external RF
    amplifier stage). If you hack your cell phone into the test mode,
    you'll get a display of receive signal in -dBm (without the - sign).
    Anyway, -dBm and sometimes uv are the standard for receive signal
    strength.

    Sometimes cell phones and 802.11 boxes use RSSI (receive signal
    strength indicator) instead of -dBm. The scale is usually 0-100 and
    there is usually no direct relationship to actual receive signal other
    than perhaps a lookup table in the data sheet. This is where the
    %signal and %s/N display on 802.11 boxes are generated. It's not as
    useful, but much easier to understand.

    Nobody but hams use S-units.

    > So, is -100dbm better than -1dbm?? How does that scale work? I am not
    >used to better signals having a negative number.

    Better for what? -100dBm is a MUCH lower signal level than -1dBm. If
    you're measuring receiver sensitivity, -100dBm is much better. If
    you're measuring 3rd order IM overload characteristics, -1dBm is
    better.


    --
    Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    150 Felker St #D http://www.LearnByDestroying.com
    Santa Cruz CA 95060 AE6KS 831-336-2558
  4. Archived from groups: alt.internet.wireless (More info?)

    Okay, first you say to "Turn in your ham license and reread the study
    guides"
    > Gaaak. Turn in your ham license and reread the study guides. S-Units
    > are an ancient anacronism left over from the stone age of thermionic
    > valve (tube) receivers. The characteristic curve between receiver
    > input signal level and indicated AGC (automagic gain control) DC level
    > was anything but linear or even consistant. Someone decided to
    > establish a reference signal level. S-9 magically became 50
    > microvolts because either Collins of Hammarlund (I forgot which)
    > managed to make receivers that would consistantly read S-9 with 50 uv
    > of RF. I dunno what frequency was used for testing. It's anything
    > but a standard and suffers from serious errors (and guesswork).

    Then you say: "Nobody but hams use S-units" If nobody but hams use
    s-units than what good is the study guide? The only part I remember
    about DB is in the antenna gain section, but hey, that was 5 years ago
    and if you you don't use you lose it.
    Every radio I ever had (Icom, Yaesu, Kenwood, and Alinco) all showed
    signal strength in S-Units. Therefore, that is all I had to know about
    till now.
    > Nobody but hams use S-units.
    >

    --Scott
  5. Archived from groups: alt.internet.wireless (More info?)

    On Thu, 05 Aug 2004 06:43:08 -0700, /dev/scott0
    <outsideworld@scottbrown.us> wrote:

    >Then you say: "Nobody but hams use S-units" If nobody but hams use
    >s-units than what good is the study guide?

    Well, I have my own opinions about the validity of the question pool.
    For a long time, the question pool was not being properly updated.
    You couldn't buy a commercial tube radio, but all the questions
    related to tubes, dynamotors, Marconi antennas, and such. I
    contributed some input on the question pool in the distant past, and
    am gratified to see that the questions are currently being updated and
    repaired. Unfortunately, most ham radios still have signal strength
    meters calibrated in S-Units. Therefore, the questions remain
    relevent. My guess(tm) is that the manufacturers use S-Units to save
    them the effort of properly calibrating the meter.

    >The only part I remember
    >about DB is in the antenna gain section, but hey, that was 5 years ago
    >and if you you don't use you lose it.

    Ok, you're forgiven. Open book, insert face, read, think, etc.

    >Every radio I ever had (Icom, Yaesu, Kenwood, and Alinco) all showed
    >signal strength in S-Units. Therefore, that is all I had to know about
    >till now.

    You have embarked on the road to technological nirvana by taking a
    detour. I suggest you abandon the dark side and immerse yourself in
    decibels, for that is the way to the enlightenment, where your
    presense among those that know shall be made welcome in the common
    tongue of design, which be decibels, not S-Units. You must abandon
    your worship of the S-Unit, for it only leads to an amateurish dead
    end. However, the road to technological nirvana is a rough road and
    full of miscalculations. The journey is not an easy one as it is
    written that one must suffer before enlightenment. It will be a much
    easier journey if you speak the common tongue, and that be decibels.
    Learn the common tongue and you shall be known as technically literate
    and astute. Continue to workship the cult of the S-Unit and it can
    only lead to perhaps some DX and the absurdity of the 5-9-9 signal
    report.


    --
    Jeff Liebermann jeffl@comix.santa-cruz.ca.us
    150 Felker St #D http://www.LearnByDestroying.com
    Santa Cruz CA 95060 AE6KS 831-336-2558
  6. Hi!! Could you please explain to me, how I can make the equivalence from negative dBm to positive dBm. I wanna use my computer to measure the SLP at my church and I donwloaded this free program http://minorshill.co.uk/pc2/testgen.html which is called audio test, but I don´t know how to interpret the scale because it shows negative values. Thanks in advance. God bless you.
  7. You don't need to convert anything. You just need to recognize that it's a continuous scale that contains both negative and positive numbers.

    -99 -98 -97 ... -2 -1 0 +1 +2 +3 ...

    Just like anything else involving such a scale, -1 is greater (louder) than -99.

    That's why the absolute numbers don't really matter. It's their relative values that do. When your AV receiver (for example) shows you a volume knob of low to medium to high (or some other scale that’s not dBm), it’s completely arbitrary. The AV manufacturer just doesn’t want to confuse the customer, so he hides the true, underlying values (dBm). Of course, the audiophile hates this, he WANTS the raw data.
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